The War on Truth: The Triumph of Newtonianism.

Battle o’ the specula; Orion 180mm  f/15 Maksutov Cassegrain versus the SkywWatcher 204mm f/6 Newtonian. The latter proved superior on all test targets.















*** New testimonies recently added to the end of the article.


For sheer brute force light gathering ability, Newtonian reflectors rate a best buy. No other type of telescope will give you as large an aperture for the money……For the sake of discussion, I have divided Newtonians into two groups based on focal ratio. Those with focal ratios less than f/6 have very deeply curved mirrors, and so are referred to here as ‘deep dish’ Newtonians. Reflectors with focal ratios of f/6 and greater will be called ‘shallow dish’ telescopes.

Pardon my bias, but shallow dish reflectors are my favorite type of telescope. They are capable of delivering clear views of the Moon, the Sun and the other members of the solar system, as well as thousands of deep sky objects. Shallow dish reflectors with apertures between 3 inches(80mm) and 8 inches( 203mm) are usually small enough to be moved from home to observing site and quickly set up with little trouble………..Most experienced amateurs agree that shallow dish reflectors are tough to beat. In fact, an optimised Newtonian reflector can deliver views of the Moon and the planets that eclipse those through a catadioptric telescope and compare favorably with a refractor of similar size, but at a fraction of the refractor’s cost. Although the commercial telescope market now offers a wide variety of superb refractors, it has yet to embrace the long focus reflector fully.

From Star Ware, 4th Edition ( 2007), by Philip S. Harrington pp 32−33.


To savor stargazing we need to strike a balance between the time, energy, and expense devoted to this activity and what we channel  into other necessary human tasks. A contented evening of stargazing comes with this balance. ‘In medio stat virtus’, or, as this Latin rendition of Aristotle’s maxim has been translated into English, “All things in moderation.’

Otto Rushe Piechowski

Sky & Telescope February 1993, pp 5

The trend towards larger and larger reflectors is indeed exciting, and I can understand the need to keep them short focus( typically f/4 to f/5). But why are so many small ones made with these focal ratios? Such telescopes bring out the worst in the Newtonian design. The 6 inch f/8s and 8 inch f/7s common many years ago, were much better and more versatile reflectors than many commercially available today.

Alan French

Sky & Telescope, November 1993 pp 4.

Newtonian reflecting telescopes are great telescopes for observing Jupiter.

John W. McAnally, from Jupiter and How to Observe It, pp 152.

Indeed a high quality Newtonian reflector is a very powerful instrument, fully capable of superb performance in viewing the planets when the optics are kept clean and properly aligned. They have been amng the favorite instruments of serious planetary observers for many decades.

Julius L. Benton, Jr. from Saturn and How to Observe It, pp 57.

Newtonian reflector telescopes, apart from their complete freedom from chromatic aberration, can be made with much shorter focal ratios than refractors, usually between f/6 and f/8 so that even an 8 inch reflector is portable and easily affordable by most amateurs. An 8 inch refractor would be financially out of the question for the vast majority of amateurs  and would need permanent housing in a large observatory. As a choice for planetary observation, then, there is a lot to be said for the Newtonian reflector in the 6 to 10 inch aperture range.

Fred W. Price, from The Planet Observer’s Handbook 2nd Edition, pp 41.

It is true that such reflectors are considerably less expensive than are refractors of the same size. This does not mean that they are not as good; in fact, Newtonian reflectors are more widely used by more experienced observers than any other type.

David H. Levy, from Guide to the Night Sky, pp 61

The simplest, cheapest and overall the most efficient design you could choose to use as a richfield telescope is the Newtonian reflector. As ever, an aperture of 100 to 200mm and a focal ratio of f/4 to f/6 is most appropriate.

Nick James & Gerald North, from Observing Comets, pp 57.

It is not proposed to enter upon the controversial topic of reflectors versus refractors. If does not grudge the extra attention to keep a reflector in perfect adjustment, its performance in revealing planetary detail will equal that of a refractor of the same aperture, particularly if it is mounted with an open, lattice work tube, when a further improvement may be derived from the employment of an electric fan  to keep the column of air above the mirror well mixed.Moreover it has practically negligible chromatic aberration, whereas colour estimates made with a refractor are exceedingly unreliable.

Bertrand Peek, from The Planet Jupiter;The Observers Handbook, pp  37.

What is “over-expensive”? It depends on your point of view. A 106mm Takahashi flat field quadruplet refractor can be had, without mount, for only a little more than I paid for a high-end 12.5″ dobsonian Newtonian with state-of-the-art optics. If astro-imaging is your forte, the Tak will be the better choice. Don’t forget a high-end mount to go with it. For visual, though? Get real. The Tak will be severely limited–a well-machined, and beautifully-performing SMALL scope that can’t see much. If you buy your scopes for visual use, the Tak isn’t just over-expensive, it’s ridiculous. If you buy your scopes for photography, though, that Tak is an incredible choice.

Don Pensack from post no. 235 in an online a discussion on Why is Takahashi Overly Expensive?

Apochomats are expensive, particularly in the larger (repeated word ‘Larger’ omitted) apertures. Object the commonly available designs, reflectors are free of chromatic aberration since mirrors reflect all the colors identically. This means that a good quality reflector with a large aperture can be free of false color and yet provide the resolution and fine scale contrast while still being affordable. These Scopes have issues of their own..

Jon Isaac, from  post no. 14 in an online discussion entitled, Can An Apochromatic Refractor Use More Powerful Eyepieces than an Achromatic Refractor Of The Same Size?

So What telescope Should I Spend with My Money On?

All in all, if you can afford it, and if you have the room to house it permanently in some sort of observatory (perhaps a run off shed), I would say go for a Newtonian reflector of 10 to 14 inches (254 to 356mm) aperture and as large a focal ratio as you can reasonably accommodate……..If you can’t afford a 10 inch then go for a smaller Newtonian reflector. Remember this type of telescope is the cheapest of any but please do not compromise on quality for the sake of size. My second choice for an instrument intended for visual observation of the Moon would be a refractor of at least 5 inches (127mm) aperture…

Gerald North, from Observing the Moon; the Modern Observer’s Guide (2007), pp 52


A first-rate 130-mm Newtonian is roughly equivalent to a first-rate 102-mm refractor for planetary observing, but superior to the refractor for all other purposes.

Tony Flanders, former Associate Editor, Sky & Telescope, in an online discussion ( post no. 1837) of the Astronomers Without Borders One Sky Newtonian.

The Newtonian reflector is a popular choice. Money wise, they are cost effective, and most importantly, you can obtain a large aperture telescope  for a reasonable sum of money…..It is true the Newtonians can come with long tubes if a longer focal length and high focal ratio is required…..Although you will hear it said high focal lengths and ratios( example f/7 etc) are desirable for planetary work, telescopes with a focal ratio of f/5 can be very satisfactory.

Paul Abel, BBC Sky at Night Presenter, from his book, Visual Lunar and Planetary Astronomy (2013), pp 14/5.

The best view of Jupiter I ever had was at Peach State with a 12.5″ Portaball on an equatorial platform at 567x (10/10/2010). There was so much detail I tried to find a larger scope in use (no luck, alas). We could see albedo features on Ganymede and Callisto. Amazing what you can see on a night of exceptional seeing. Even when the seeing is not at its best, I find there are often times on any night when a larger scope has an advantage.

Alan French, from an online discussion (post no.14) entitled Large Telescopes and Jupiter.

Guan Sheng Optical (GSO) uses high-volume, state-of-the-art, high quality manufacturing and test lines. GSO guarantees diffraction limited performance, but their mirrors typically have a mirror surface quality of 1/16 wave RMS at least, and often better. This very smooth mirror surface results in excellent optical performance with practically no light scatter, while Antares Optics secondary mirrors are 1/15 P-V or better and come Zygo tested;

Rob Teeter, founder of Teeter’

Amateur astronomers and telescope makers have debated from time immemorial the advantages and disadvantages of different telescope designs. In particular, mountains of hard copy and electronic articles are available on the merits of refracting and reflecting telescopes, more recently, apochromatic refractors vs. Newtonian reflectors. This debate has become rather rancorous (Newtonian telescopes as APO “killers” comes to mind.) and unscientific, to say the least. And when all is said and done, in a discourse without loaded words and acrimony, a discussion devolves to one concerning perfect optics. And isn’t this what we all want or wish we had?

From Ed Turco’s online article: The Definitive Newtonian Reflector.

Many people shy away from Newtonians because they have exposed optics that get damp and deteriorate. In this respect a good closed tube Newtonian wins hands down over an open tube one, as the tube keeps dew at bay. For the best planetary resolution, a telescope must, must, must must be precisely collimated!!! Also, the optics should be able to cool down quickly by fan cooling. Mirrors thicker than 40mm have serious cooldown problems unless fan cooling is employed. My 254mm f/6.3 Orion Optics Newtonian (plus mirror fan)is the best planetary telescope I have ever used. Some planetary observers line their tubes with cork too, to reduce currents. Many good planetary telescopes are comprehensively ruined by being in a huge unventillated dome with a narrow slit, a concrete floor and a metal dome.

Martin Mobberley, Author, from his webpage: Telescopes.

See also his review of the same instrument here.

Much has been written on the subject of the central obstruction and its impact on fine planetary detail. It remains a hot-button topic in on-line forums, and yet, despite the intensity of the debate, obstruction effects are well understood and fairly simple to quantify. The issue was most elegantly summarized by William Zmek in the July 1993 issue of Sky & Telescope magazine. Zmek’s rule of thumb states that if you subtract the diameter of the obstruction from the objective, you have the equivalent unobstructed instrument when it comes to contrast resolution. In other words, an 8-inch reflector with a 1.5-inch obstruction has the potential to resolve the same low-contrast planetary detail as an unobstructed 6.5-inch scope.

Gary Seronik, former Sky & Telescope Contributing Editor and Author, from his personal website.

I have the 12″ Orion as well (Truss, but the same thing otherwise). I have seen two other instances where optical quality on this model was excellent, and my own has perhaps the best mirror I have seen in a mass market dob.Star test with [2]3% obstruction shows an identical secondary shadow breakout, and the mirror had no zones or turned edge, and the smoothness was quite good. In other words, by anyone’s measure, the mirror in my own sample is a really fine mirror. If yours is in the same category for quality, be prepared for some awesome planetary views. The 12″ easily put up better planetary views than my 6″ Astro-Physics Apo and my C14. Views with 31mm Nagler and MPCC Coma Corrector are wonderful (I use the MPCC because it is 1x and while maybe not quite as sharp as the Paracorr, that 1x is important when trying to get the largest field possible out of today’s very expensive wide field eyepieces. I had fantastic views with my 12″ and it is my favorite larger aperture telescope ever. It does everything with excellence.

Ed Moreno, from an online thread entitled, First Light with my XT12G.


I tested the scope over a four week period, and found the optics to be very good, to excellent. I star tested the scope extensively, and found the mirror to be extremely smooth, with the slightest under correction.. I couldn’t put a figure on it, GS advertise 1/12 wave. Planetary images were rock hard. I tested the scope against my friend’s 12.5 Inch premium Dobsonian, which sports a “Swayze” mirror, and we couldn’t split the images over four different nights. On one particular night of excellent seeing, I had the magnification up to 500x, without any image breakdown.Our local club Telescope guy ran a series of tests on the optics, the main one being a Double pass Ronchi test, against a certified optical flat. He informed me the mirror was very well corrected, and very smooth. He didn’t believe the price I paid for the entire scope.

In summary, this scope has been a huge surprise. My experiences with “light Buckets” previously were not great. My intention was to use this scope for deep sky observation only, as I already have a Zambuto equipped premium scope for Planetary work, but it’s a lot more than that. Since purchasing my scope, I have looked through fourteen of these instruments at our club nights, and the images in all of them are almost identical. Guan Sheng seem to be producing a great mass-produced scope.

Con Stoitsis
Comet Section
Astronomical Society of Victoria Inc
Melbourne, Australia

From a review of the Guan Sheng 12″ f/5 Dob (essentially identical to the author’s instrument pictured above). Source here.

I purchased a 15” Obsession from owner David Kriege on January 12, 2007 and took delivery in the first week of March of the same year. Since that time it has been my principle telescope for visual use. For comparison, I have owned two Takahashis (FS-102 and TOA 130), three Televues (TV85; NP127; TV85 again), a WO 10th anniversary 80mm, an 8” Celestron SC with Starbright coatings and a 6” Orion dobsonian.Despite the demonstrated affinity I have for apochromatic refractors, I had been to enough star parties to learn that aperture ultimately wins…………Those interested in purchasing a larger dobsonian may wonder what you see in a 15” scope. Having compared views with everything from 60mm refractors to a 30” dobsonian, I can honestly say “more than enough to keep you busy for a long, long time”. Globular clusters really seem to take life at about 12” and galaxies are already more than nondescript smudges by 15”. Of course local conditions make a huge difference. I have had nights with a 5” refractor that gave the 15” a run for its money. I have been at star parties looking at a galaxy in the 15”, then wandered next door to a 24” expecting to be utterly blown away by the difference and then been surprised by how little there was. That said, on any given night, the 15” tends to beat the socks off my small refractor nearly every night across the board, from planets to wisps.

From a review of the 15″ Obsession Dobsonian by Rene Gauge. See here for more details.

The Oldham optics on this Dobsonian are superb. A number of tests were carried out, the results of which are outlined below. The round airy discs of bright stars appear perfect. Faint stars are fine razor sharp points of light. On nights of good seeing I have been able to see faint diffraction rings around stars. Images snap into focus even at high power.

After extensive star testing, I could detect no major defects. There was no sign of any astigmatism, even close to focus. The test revealed near perfect correction and very smooth & high contrast optics. Given that the primary mirror is large and fast, and the star test is particularly sensitive, I consider these results to be very good. A particularly impressive aspect is the ability of the optics to handle very high powers. The results of the magnification roll-off test were excellent. First light revealed that the mirror could easily handle forty times per inch of aperture on planets. In good seeing conditions Saturn would reveal a high level of detail and remain sharp at magnifications in excess of 600X, and Jupiter showed no sign of image breakdown at 507X. Under no circumstances have I ever seen Jupiter soften at powers less than 450X. I have even enjoyed good views of the moon at 888X. I have also shared my experiences with a number of experts around the world who believe as I do that the optics are performing to a high specification.

Nick Koiza, from his review of a David Lukehurst 16″ f/4.4 Dobsonian detailed here.

Here’s what everyone wants to know. The primary mirror, as with all Zambuto-equipped Starmasters I’ve seen, is a jaw-dropper. Since quoting data on the mirror is often misleading and can cause flame wars, I’ve chosen to leave these out of this review. What I will say is that the Zambuto mirrors have an extremely smooth optical surface, with a near-perfect star test. Running through focus, the Fresnel rings are identical on both sides of focus and evenly illuminated. I can find no zones, no turned edge, and no astigmatism. There is perhaps a “slight” under-correction, however it’s often not even noticeable to me, which could indicate that I might have tested while the mirror was still slightly out of thermal equilibrium. If it is there, it’s very, very slight & I couldn’t begin to guess by what fraction of a wave.

However, that smooth surface correlates to what I believe to be the most important aspect of visual observing, and that is contrast. In a word – exceptional. The scope shows even the most subtle differences in illumination. There is definitely an emotional response with Carl’s mirrors, an underlying feeling that’s hard to describe, except to say sitting at the eyepiece is more like observing from space rather than the ground. The extremely fine details seen are amazing, and sometimes I seem to subconsciously pick up things that I don’t notice when looking through other instruments, only to go back to my scope & find that I wasn’t dreaming!

Jupiter usually shows 10 bands and massive amounts of detail within the belts & GRS, as well as the festoons & barges. And being able to see this kind of detail routinely at 400x, and many nights up to 600x is definitely like looking at a photograph. Polar regions & surface detail are visible on Io. Saturn shows the Crepe ring every night, as well as the Encke minima without fail.. Despite the short focal length it is a killer planetary scope. When Mars was last at opposition before the dust storms, picking out surface details was as easy as looking at our moon. In addition both polar caps were easily seen. Phobos & Deimos were also seen. On the planets I rarely use filters, so most of the views described above were natural.

No matter the conditions, the Trapezium easily breaks into 6 components, even at very low magnifications. The detail level seen in M42/43 is far better than any photo I’ve ever seen, from low-power views that show the entire nebula, to using a binoviewer at 500x on the Trapezium vicinity that reveals details in the nebula, which are reminiscent of the structure seen in cumulo-nimbus clouds.

With globular clusters, “resolved” takes on a new meaning & the scope provides “in your face” visual observing! Obviously M13 & Omega are completely resolved. One of my favorite globulars is M92 because of the super-dense core that seems to go on forever. One night I decided to push the scope to what our group likes to call “silly power” & view M92 at 700x. WOW – the core showed a tremendous amount of resolution, but again it’s so dense I couldn’t quite get it to go all the way. M13 at this power was like looking right through it to the other side. And I can’t forget the extra-galactic clusters, G1 & G2 in M31. G1 actually started show resolution at about 500x. Another extra-galactic object is NGC 604, the giant HII region in M33, about 2.5 MLY away. At 700x I can see much structure & filaments within the nebula. Seeing that in real time is spectacular.

Most galaxies within the local group show quite a bit of structure, knotting, & dust lanes. Those more distant objects do reveal detail well above being mere smudges. Many Virgo galaxies show spiraling. Every component of Stephan’s Quintet is always visible, and much easier to see than in many other scopes of similar aperture I’ve used. NGC 7331 is stunning. Closer neighbors like M51, M31, M33, M81/82, M104, NGC 4565 & NGC 891 take on photographic qualities in the eyepiece. I’ve been able to determine the rotational direction of a galaxy that NSOG stated was over 500 MLY distant. The scope is also quite capable of hunting the faint Abell planetary nebulae & galaxy clusters, and with an H-Beta filter, the Horsehead is a snap with direct vision. These are just a few of my experiences at the eyepiece. In short, once the mirror has reached thermal equilibrium, it’s like having a 14.5″ f/4.3 APO. Tight, pinpoint stars sharp across the FOV, and a nice “snap” to focus – there’s no mistaking it. The low f/ratio provides a nice wide field, at least for a scope of this size. I can get 1.4* with the 31 Nagler & using the Paracorr which boosts the focal length from 1584mm to 1822mm. (1.6* without it) Not a bad FOV for a 14.5″ mirror.

From a very happy and experienced owner of a 14.5″f/4.3 Starmaster Dobsonian. Details here.


Like every Sky-Watcher scope I’ve tested, this one arrived perfectly collimated out-of-the-box, and has held its collimation over the period I’ve been testing it. This speaks highly of the mechanical integrity of the scope, and alleviates the beginner’s greatest anxiety about Newtonians. No doubt the scope will require collimation at some point, but if it can make it to Canada to from China without losing collimation, it should be pretty stable.

I tested the telescope under the stars on four different nights, exploring a wide range of objects. Well, actually, one night and three mornings, as I was unable to resist the lure of using this scope on my old favourites, Jupiter and Saturn, currently in the predawn sky. I also spent time looking at the Moon, Mars, and Venus, favourite double stars like Epsilon Lyrae (split easily at 120x) and Rigel, and deep sky showpieces like the Ring Nebula and the Orion Nebula. All were well shown, as one would expect in a good quality 150mm scope. The supplied eyepieces, 25mm and 10mm “Super” modified achromats with 50° fields, performed quite well, yielding magnifications of 48x and 120x. This scope showed that it can handle much higher powers easily; I found myself using a 6mm eyepiece (200x) on the Moon and planets most of the time. Fans of deep sky objects will probably want to add a 2″ eyepiece to take in the wide field of view this scope is capable of.

As Terry Dickinson says in NightWatch, “There may not be a perfect telescope for the beginner, but the closest thing to it is the 6-inch Dobsonian-mounted Newtonian reflector.” The Sky-Watcher 150mm is an excellent example of this breed, at a very attractive price. My wife and I usually donate a telescope to our favourite charity to auction off each year, and this year this scope is our choice. Highly recommended!

Source here.

I put together one of his 6″ f8 telescope kits with the help of my kids. It’s still one of my favorite scopes. These days, I lend it to people who have shown an interest in astronomy, but can’t afford a telescope.

Barry Fernelius, from this online thread discussing Stargazer Steve’s kit ‘scopes.

Changing technologies have meant that amateur telescope making has largely been replaced  by the purchase of accessible high quality commercially produced instruments, but the Western world’s passion for the night sky is as strong as ever, and long may it continue.

Dr. Allan Chapman, from an essay entitled: The First Astronomical Societies, Astronomy Now, January 2018, pp 49.


Optical quality matters, but these days it’s usually not the main problem. Most of the commercial mirrors I’ve evaluated in the past 10 years have been pretty good — a few have even been excellent. That’s not to say there aren’t duds out there, but if your telescope isn’t performing, the items that top this list are more likely to be the reason.

Gary Seronik, from an online article entitled: Five Reflector Performance Killers.

In this department of astronomy, the names of Herschel, South, Struve, Dawes, Dembowski, Burnham, and others are honourably associated and it is notable that refracting-telescopes have accomplished nearly the whole of the work. But reflectors are little less capable, though their powers seem to have been rarely employed in this field. Mr. Tarrant has lately secured a large number of accurate measures with a 10-inch reflector by Calver, and if care is taken to secure correct adjustment of the mirrors, there is no reason why this form of instrument should not be nearly as effective as its rival.

W. F. Denning, from Telescopic Work for Starlight Evenings (1891), pp 290-291.

Since about a month this telescope is parked in my garage.
It is a Newtonian of 158mm aperture and a focus of 1240mm.The primary and secondary mirrors were made by David Hinds of UK.The telescope was home-made by my friend Tavi aka Erwin.Because he have other commitments, I was offered to give it a ride. I used the oculars seen in the second picture, from left to right: HM 6mm, Galilei – 50 mm, Galilei – 9mm , Baader Classic 6mm Ortho. This are the double.multiple stars observed on 23rd of December. All stars were split except 52 Ori where I believe to see a black space between main star and companion but not 100% sure.I was very pleased to see very well E and F stars in Trapezium.Good telescope, excellent optics, reliable mount……..I continued the testing on 26 of December when I made observations of 36 And, Delta Gem , Theta Aur and Eta Ori. All eas(i)ly split ,beautiful views……………..On 14 of January I targeted 7 Tauri double star. Unfortunately on the rare occasions when the stars were visible, the sky was hazy but still seeing was not better than 5 to 6 Pickering. Like with 52 Ori ,also at 7 Tau double star the Hind telescope showed at 248 x the two touching Airy discs but no black space bewteen them. I have high hopes in good seeing this telescope will split 0.7” double stars.

Mircea Pteancu from an online thread entitled: Double Stars in the Hind 158mm x 1240mm.

Visual report on the 12.5” f/6.5 Teeter Dob with Mike Lockwood mirror.

My eyepiece ‘fleet’ with the 12.5″:

31mm Nagler 67x

24 mm Explore Sci 86x

17.3mm Delos 119x

12mm Delos 172x

8mm Delos 258x

5mm Nagler 413x

6-3mm Nagler Zoom 344x to 688x

The first two nights (Tuesday and Wednesday) of observation were very foggy and absolutely dew-drenched – the most dew I have ever seen. Both nights the main mirror dewed up just after midnight – the joys of a thin 1.1” mirror which tracks ambient temperature very well, I suppose….

Along with the dew was some of the best atmospheric steadiness I have ever experienced. I would place the seeing at 9 to 9.5 (pickering) out of 10. With a 3mm eyepiece (688x) on a 4th mag star near the zenith, the full diffraction pattern was stable and almost unmoving. Unfortunately the transparency was mediocre and, towards midnight, increasingly poor….

The third night (Thursday) was very transparent and drier, with much more manageable dew but the seeing was extremely poor. The close pairs of Epsilon Lyrae were two touching fuzzballs (the night before you could have driven a HumVee through the blackness between…)

I looked at a bunch of double stars the first night… I used my Nagler 6mm – 3mm zoom which gives magnifications from 344x to 688x. Close pairs seen were:

STF 186: sep. 0.8”, mags 6.79/6.84
wide, dark sky split. The dark space was equal in width to the central discs of both components. very delicate first rings were present at all times…

A 1504: sep. 0.6”, mags 8.84/8.92
darkline split. Central discs ‘kissing’… first rings were pretty much too faint to see…

BU 525: sep. 0.5”, mags 7.45/7.47
very deep notch. a black or grey line seemed visible at times..

STF 346 AB sep.0.5” mags 6.19/6.21 This is triple star 52 Arietis. The ABxC pair is at 5” separation… Very nice to see three stars here. The AB pair was a deep notch, again with fleeting glimpses of a line between…

Dave Cotterell, Madoc, Ontario, from an online thread entitled: 12.5″ f/6.5 Teeter Dob with Lockwood Glass.

This report details my visual and photographic observations of some sub-arcsecond double stars that have been the subject of a few CN threads the past few months.  This document is necessarily heavy on technical details to support those who may wish to independently evaluate the results.

Visual observations were made with a 15-inch f/4.5 Dobsonian reflector setting atop an equatorial platform.  All observations were made between September 26th and October 26th of this year with a Paracorr Type I lens (setting no. 1) in the optical train.  In all cases, Pentax lenses were used to achieve the following magnifications:  ‘low’ (5XW; 398x), ‘moderate’ (3.5XW; 569x), and ‘high’ (2.5XO; 798x).

Imaging was accomplished using an ASI 178MC cooled camera [AVI files; mono mode] in an optical train consisting of a Paracorr Type I lens (setting no. 5) and a 5x Powermate.  The plate scale for imaging was previously determined to be 0.0553 +/- 0.001 “/pixel using calibration stars (n = 10) and 0.0553 +/- 0.002 “/pixel using a diffraction grating with monochromatic red light (n = 8).  Sharpcap 2.8 was used as the image capture software.  Fine focus was achieved using a Bhatinov mask [All-Pro, Spike-a brand] modified to fit over the Obsession UTA.  Separation values were determined using REDUC.  Images were stacked and processed using Registax with final presentation formatting in Gimp.

Bu 720, 72 Pegasi
magnitudes:  5.7, 6.1
position angle:  105 degrees
separation:  0.575” (orbital elements estimate); 0.505” (last precise measure; 2015)

The separation data are not in good agreement for this object.  This is, therefore, a good candidate for quantitative scrutiny.

At 398x the object vacillated on the border between elongated and just resolved to two golden-orange disks of similar magnitude in the correct position angle; 569x proved sufficient to show the stars as different magnitude and clearly resolved (but not yet split); a final increase in magnitude to 798x showed the pair as split, again with a golden-orange color and a small difference in magnitude.  The ease of resolution at modest magnification led me to think the larger separation value [0.575”] was more accurate for Bu 720.

Bu 720 was easily imaged using an exposure of 10 ms [gain = 320].  Four movies were made and separation was measured by three methods using REDUC:  cross correlation of the top 5% of frames using S4 filter; simple measure of a Registax composite; and simple measure of a composite generated in REDUC.  There was good agreement across these methods, giving a measured separation of 0.61”.

STT 20AB, 66 Piscium
magnitudes:  6.1, 7.2
position angle:  176 degrees
separation:  0.598” (orbital elements estimate); 0.59” (last precise measure; 2015)

Good agreement between WDS listed separation values.  Should be able to split at moderate magnification.

Low magnification (398x) shows two white stars that are clearly resolved and are oriented in the position angle as stated in the WDS.  Moderate magnification (569x) shows that the components possess dissimilar magnitudes; the pair was barely split about 20% of the time at this power.

STT 20AB was imaged using an exposure of 12 ms [gain = 400].  Four movies were made and separation was measured by two methods using REDUC: simple measure of a Registax composite; and simple measure of a composite generated in REDUC.  There was good agreement across these methods, giving a measured separation of 0.59”.  REDUC Correlation methods were not useful with this target for producing separation values because of the faintness of the secondary.

16 Vulpeculae, STT 395
magnitudes:  5.8, 6.2
position angle:  127 degrees
separation:  0.849” (orbital elements estimate); 0.81” (last precise measure; 2015)

This target possesses a wide discrepancy between WDS values and was discussed at some length in a prior CN thread.

This object was observed as split using an 8-inch reflector at 340x (3.5XW lens).

16 VUL was imaged using an exposure of 14.5 ms [gain = 450].  Four movies were made and separation was measured by three methods using REDUC:  auto correlation of the top 5% of frames using S4 filter; stacked REDUC reductions; and simple measure of a composite generated in REDUC.  There was good agreement across these methods, giving a measured separation of 0.771 +/- 0.006”.  Previous REDUC autocorrelation measurements of this system using a 2x Powermate [plate scale = 0.143 “/pixel] gave a measure of 0.78 +/- 0.02”.


Mark McPhee, Austin, Texas, from an online thread entitled: Examination of Some Sub-Arcsecond Doubles: Bu 720, STT 495, Bu172AB, STT 20AB, and 16 VUL


Currently, my only objections to the short f/ratios becoming common are that the depth of focus is very short, making focus variability in mediocre seeing a bit more of a problem than in longer f/ratios, and that most eyepieces don’t perform as well at the edges at f/3 as they do at f/5, even when both are Paracorred with the latest Paracorr II at the correct setting. But, that being said, I would still unhesitatingly choose a fast f/ratio at the really large sizes of scope simply because it’s easier and safer to stand on the 3rd step of a step ladder than it is to stand on the tenth (!), and I’ve done that in a 36″ f/5.
Along the way of large scope progress have come better cells, thinner mirrors, better fans, and better collimation tools. Put those all together, and the performance level of the large scopes seems now to be only limited to the mirror qualities, and there are makers of large mirrors now who put the same quality into their mirrors as some of the better makers who stop with much smaller sizes.
I truly wish many of you had seen the poor quality large scopes over the years that I have seen. If you had, you’d realize how we truly live in the Golden Age of Astronomy right now.
Don Pensack, from a thread entitled: Large/Fast Newtonian Mirrors and Quality.
I thought I wanted a 140 class APO. The image and fantasy of it has been kicking around in my mind for most of the year. They’re such nice looking telescopes – what a telescope is supposed to look like. At a major star party a few weeks ago, I found a beautiful 140 – a very well-known top quality model mounted atop a big G11 and I asked the owner if he would please show me M15 in it. He was proud to do so and we looked. He raved and I was silent. I thanked him and walked back to my premium 16″ Dob. Looked at M15 in the 16 and raved to myself saying I’m so glad I have the 16. For the same amount of setup and money, what a difference!
Peter Natscher, Central Coast, California, from an online thread entitled; How much increase in aperture to see a difference?
 Ps. Peter is the proud owner of a couple of large premium Dobs and an Astrophysics 175 EDF apochromatic refractor.

yea sometimes (like this evening) I ask myself too: “why you silly id… buy all this expensive apo stuff???” :)

Cloudy for the whole day – this evening reported to be one of the most interesting of the year – Io & Europa before Jupiter – together with their shadows and crowned by the GRS. Before 1,5h I saw a break in the clouds and to be fast I just grabed my 8″ GSO Dob and took it out. After 30min cooling another break off – I could see so much detail in the bands, GRS shining in a bright red, both shadows, Europa just beginning to leave the planets disc and – man – I could swear to see a round structure in the band that could have been Io… – best experience for a looong time! :)

So go get a 8″ f6 Dob – Houston out

Donadani, Germany, from an online thread entitled; How much increase in aperture to see a difference?



Hello Peter [Natscher],

Hope you are well.

An 8 inch Portaball with a Zambuto mirror on a tracking platform is going to show you much more than a 130mm telescope, even a Starfire 130EDF. Both of these will be sit down and observe telescopes. If you were not going to get a tracking platform, I would go the refractor route. You will want tracking for dedicated planetary or lunar observing. Shoot, you could roll the dice and get a Sky-Watcher 8″ f/6 collapsible Go-To dobsonian. I have the non motorized version and it has a great mirror, however, your mileage may vary on the chinese mirrors.

Tony M, from an online thread entitled: 5″ Refractor or 8″ Newtonian for Lunar/Planetary Observing.

I would take a high-quality 8″ f/6 on a GEM over a 5″ refractor. The GEM will negate all the disadvantages of the Dob. My old Meade 826 easily gives me a sharp, detailed Mars at 400x (conditions willing). I wouldn’t trade it for any 5″ scope ever made.

Rick Woods, from an online thread entitled; 5″ Refractor or 8″ Newtonian for Lunar/Planetary Observing.

I may be too late to add my .02, but it should be valuable.

I have owned a AP130EDF and the GT model. I now own an 8″ f/5.5 Portaball.

My conclusion: The Portaball rips the APs to shreds. A perfect 8″ mirror over a perfect 5″ lens…no comparison, obviously assuming collimation and thermal equilibrium. Especially since the Portaballs are constructed with fans and other ergonomics to help with the thermal adjustment. And I’m solely talking about visual planetary, since that is almost exclusively what I do.

Markab,  Kansas City, from an online thread entitled: 5″ Refractor or 8″ Newtonian for Lunar/Planetary Observing.

One should never lean too heavily on sketchers. Although the vast majority are genuine, some go to an imaging website, study a particular image, and fake a sketch. In this way, they can make a 4 inch peashooter telescope look better than a Hubble image. I’d put more faith in the unlying eye of a CCD camera than any sketch, however elaborate.

Mr. Hardglass.

A major reason refractors give images with higher overall contrast than reflectors is that objective lenses may scatter only approximately 2% of the light passing through them. This is why I believe that the high refelectivity coatings that are now applied to many astronomical mirror surfaces are so important. With 95% reflectivity, not only will they give somewhat brighter images but they will also greatly reduce the amount of scattered light, so improving the overall contrast. A high reflectivity coating is well worth having even at an additional cost; not only will the telescope perform better but a second advantage is that the mirror surface allows far less moisture to penentrate and is likely to last perhaps 25 years before it has to be recoated. I have a 10 year old Newtonian whose mirror was one of the first to be given a high refelctivity coating and it still looks like new………The overall design of the telescope will affect the overall contrast as well. It is impossible to beat a well designed refractor, but Newtonian telescopes, where one observes across the tube assembly to the far wall, are almost as good.

Ian Morison, from An Amateur’s Guide to Observing and Imaging the Heavens (2014) pp 10.


A Newtonian for All Round Use

A very good compromise in designing a 200mm Newtonian is to use a focal ratio of f/6, and many such telescopes are sold with this basic specification. The focal length will thus be 1200mm, and the field of view when a low power, wide field, 2 inch eyepiece is used will be approximately 2 degrees. The secondary mirror willprobably be approximately 50mm in diameter, and this would give a percentage of obstruction of 25% and provide full illumination over the central 25mm diameter region of up to 46mm diameter field of view. This is a good compromise, but some manufacturers, such as Orion Optics in the UK allow the purchaser to choose other secondary diameters should he or she wish to optimise the telescope towards planetary (approximately 36mm) or widefield imaging (approximately 60mm). One could even purchase two flats for use depending on the type of observations planned for a given night!

Ian Morison, from An Amateur’s Guide to Observing and Imaging the Heavens (2014) pp 64.


Yes, I have active cooling for my 10″ Dob. For that scope, a fan in a baffle below the primary works fine. I have an EdgeHD 8″, a SW120ED and a 10″ solid tube Dob, and I know how Jupiter looks in all three. No 7″ APO in the stable, but aperture does count for something. Some of my best views of Jupiter have been through the 10″ Dob during early morning. All of my best views of Jupiter have been through the 10″!

Sarkikos, from an online review of the Celestron EdgeHD 8 inch SCT.


Making a good 8 inch Newtonian by Toshimi Taki (Japan).

I think I can speak with at least a little credibility on this subject, having owned numerous large refractors:
TeleVue 140
One of only two Christen 6″ f/15 folded Triplets
Takahashi FC-125
Takahashi FCT-150
8″ Alvan Clark
and having used several other premium 8″, 10″ and 12″ refractors.
As a recovering “refractor-holic” I still crave their look, fit and finish. And inch-for-inch they can’t be beat. But an 8″ or larger refractor w/mount is a true BEAST to own unless you can leave it permanently mounted. Topping it off there’s that little thing called “COST”!
Once we started having master mirror makers like Carl Zambuto turning out mirrors that were simply without compromise, optically – the paradigm shifted!
A premium 12.5″ reflector today will simply blow away that beautiful old 8″ Clark. My present 14.5″ Ed Stevens mirror produces planetary images that are superior to anything I ever saw in a 10″ Zeiss triplet and pretty darn close to the 12″! And don’t even get me started on comparisons with my 28″ Starstructure w/Steve Kennedy mirror! This scope is fully driven and features GOTO, yet I can set it up, take it down and transport it (easily) all by myself. Try to picture what a 20″ refractor would look like and cost…that’s about what it would take to begin to match the performance!
Would I love to have a giant TMB set up in a dome behind my house? Sure! But not for ‘performance’ reasons.

Mike Harvey (Florida), from an online thread entitled: Refractor Versus Reflectors.

My 6″ refractor [AP 155] will show the arms of M51 from a dark site, but fairly subtly. It’s far behind what my 10″ Dob can do in that area.The inability of the refractor to show much spiral detail in galaxies is probably its greatest drawback as a deep sky instrument.

Joe Bergeron, (Upstate New York), from an online thread entitled: Refractor Versus Reflectors.

My advice to everyone wanting better planetary views is is to always spend money on a better instrument, or make their instrument better than it is.   An 8″ f/6 reflector with a high quality mirror is one of the best planetary scopes money can buy.  Period.  Don’t be duped into thinking a 6″ MCT is going to be better. Physics simply do[es]n’t permit it.

Ed Moreno, from an online thread entitled:8 inch f/6 Dob versus 6 inch Orion Mak on the Moon and Planets.

I’ve tried a single curved spider in my 8 f7.6, but went back to the straight 4 vane after a year. Didn’t notice even a tiny hint of more fine detail on Jupiter with the curved vane.

Planet Earth, from an online thread entitled: Curved Spider Vanes?

The idea of curved-vane spiders isn’t new — the concept has been around for a long time and several designs were detailed in the May 1985 issue (page 458) of S&T. For telescopes up to 12-inch aperture, a curved spider can be a good alternative to traditional 3- or 4-vane spiders. With larger scopes, the diagonal mirror typically becomes big and heavy enough that the greater rigidity offered by conventional spiders or more robust curved ones may be required. I’ve successfully used the design described here in numerous telescopes, including my 12¾-inch. My single-curve spider has the added benefit of being simpler than the ones in the 1985 article, and therefore easier to build.

Gary Seronik, from his online article: How to Build a Curved Vane Seconday Mirror Holder.

This is essentially an aesthetic issue. If you don’t like spikes, then go ahead and get a curved vane spider. It does eliminate the spikes. You will see an even glow around bright objects like Jupiter or Venus, and nothing around everything else……..If the spikes don’t bother you, then stick with a straight vane.

Jarad, from an online thread entitled: Curved Spider Vanes?

I have put a 6″ APO up against a mass market 8″ f/6 reflector and I can tell you that the 6″ APO overall is a better performer. It is sharper everywhere in the field, had better planetary contrast, and came SURPRISINLY close in terms of deep sky (Globular and Galaxy) performance.

But this didn’t have much to do with the fact that it was a refractor vs a reflector, but rather more to do with the fact that is is a virtually PERFECT refractor up against a telescope with optics that were only “Fairly good” optically.

But.. IF you were to put a TOP QUALITY mirror in your scope, along with the highest quality diagonal you could find, you would find that on axis, it would indeed take refractor very close in size to yours to give a better visual image at the center of the field.

Ed Moreno, from an online thread entitled: Refractor Equivalent to a 8″ Reflector.

A good 7.1″ refractor is very close to a good 8″ reflector on M13. For planetary resolution most of the time the 7.1″ refractor beats the good 8″ reflector. But, they can be very close on a good night.

Rich N (San Francisco Bay Area), from an online thread entitled: Refractor Equivalent to a 8″ Reflector.

As I mentioned earlier in this thread my experience is that a 7.1″ refractor is very close to an 8″ Newt.

I’ve had my AP 180EDT f/9 APO side by side with a friend’s well made 8″ Newt a number of times. My refractor is more consistant in giving high contrast, high resolution images but on the right night that Newt can give some fine planetary images. For deep sky the views are very similar.

Rich N(San Francisco Bay Area), from an online thread entitled:Refractor Equivalent to a 8″ Reflector.


I’m not talking about local seeing from night to night. I’m suggesting that if you set up a high end APO and a high end Newt of roughly the same size (maybe a 180mm APO and 8″ Newt) side by side over several nights, the APO (refractor) will more often show better high res, high contrast, planetary detail.

Rich N(San Francisco Bay Area), from an online thread entitled:Refractor Equivalent to a 8″ Reflector.

No, a well crafted 8″ reflector with a Spooner f/7 mirror will totally outclass not only 5 or 6″ achros, but 5 or 6″ apos as well. And look at the original posters question again. He was wondering if an 80mm refractor would equal a 6″ reflector or a 100mm refractor would equal an 8″ reflector. No, and it’s not really close. And if you can tie your shoe you can collimate a reflector and clean the mirror once a year.

And my 6″ $250 Orion 6″ f/8 Dob has run totally neck and neck with my buddy’s Tak 102 on more than a few nights on the moon and planets. And it’s beaten the 4″ apo on some nights. Same result with a TV102 and a Vixen Fluorite 102. And it beats my TV85 and his Tak 78 100% of the time. Myths die hard.

Quest Do Not Delete, from an online thread entitled.Refractor Equivalent to a 8″ Reflector.

I routinely compare 6″ APO to 7″ and 8″ Zambuto reflectors – see my sig for specifics. I am a dedicated planetary observer and really enjoy such comparisons.

Basically, I’ve found that the 8″ Z-mirror when cooled will be ever so slightly better then the 6″ APO. And the 6″ APO will edge out the 7″ reflector. The 6″ APO, an FS152, is a doublet and does not focus all colors to a common point as a matter of design. The blue is thrown way out of focus so the image has a slightly warmer look to it then that of the reflectors. I have often wondered if a triplet was compared to a the 8″ reflector how it would perform. Probably the same as the doublet as resolution is primarily a function of aperture.

The differences are quite minor in good seeing. But when the atmosphere is unsteady, I prefer the views in the FS152 over everything.

Peleuba(North of Baltimore, MD), from an online thread entitled: Refractor Equivalent to a 8″ Reflector.

I have a nice C6-R with a Chromacorr, and a nice 8″ f8 newt with a very fine Raycraft Primary, Protostar secondary, and flocked interior. I can’t recall offhand the size of the secondary, but it’s scaled for my 2″ EP’s, so it’e bigger than is optimal for planets.

Aestheticly, I find the refractor better, especially on evenings of dropping temps. I know the refractor has to cool too, but the views seem to be sharper sooner, and I never see tube currents.

The newt does reach deeper, though. But, I generally like to see sharper than deeper for the kinds of things I use the refractor for (planets and clusters). When depth is the concern, I opt for more aperture.

Under ideal conditions, I’m sure my 8″ newt will out-perform the refractor. But, conditions are rarely ideal, and so the edge goes to the refractor– tends towards better sharpness and better contrast due, I think, primarily to the lack of tube currents and cooling time.

Apples to Oranges, though.

Kerry R (Mid West Coast, Michigan), from an online thread entitled: Refractor Equivalent to an 8″ Reflector.

I will go toe to toe with any 7″ or 8″ APO with my 8″ F8 Newt and my Dan Joyce 8″ f/6.7 Newt. In the end I paid very litte for both Newts, and the 8″ APO owner has paid big time for the 8″ APO and it’s mount.

Had my share of 5″ to 7″ APO’s and they just dont cut it for such a high price. They do give that super fine snap to focus image, but my well built 8″ Newts can pretty much do the same thing and cost 50x less.

CHASLX200 (Tampa, Florida), from an online thread entitled; Refractor Equivalent to an 8″ Reflector.

8″ f/6. You’ll get a little better contrast with an unobstructed scope and imperceptibly brighter image because of light loss on a reflective surface. You won’t have coma on a refractive system, but you won’t have the huge amounts of CA with a Newt that you would have with an 8″ f/6 refractor.

Aperture, aperture, aperture. That’s what matters. A high quality, thermal controlled reflective system will give up very little to a similar size refractor.

Deep 13 (NE Ohio), from an online thread entitled: Refractor Equivalent to an 8″ Reflector.


Recognizing that everyone has their own opinions and preferences, here is my opinion and preference based upon 45 years of telescope ownership which by inventory would include something like about 40+ mounted telescopes and numerous guide scopes that would add about 15 or so more to the mix. I have had all types of scopes including both apo and achro refractors, maksutovs, schmidt-cass, newtonian reflectors (both equatorially mounted and dobsonian). While different telescopes are suited for different purposes, my overall preference is the refractor for a number of reasons, in particular the apo refractor of which I have owned 8 different apos. I also enjoy the classic long focal length achro refractor and have extensive use and ownership experience with Unitron refractors………………From my personal experience and in general for most applications my rule of thumb would be that a Newtonian would have to “out-aperture” a refractor by about 33% to be roughly equivalent for most applications. Accordingly the statement that a 6″ refractor is equivalent to an 8″ reflector is for the most part, pretty valid.

Barry Simon ( New Orleans, LA), from an online thread entitled; Refractor Equivalent to an 8″ Newtonian.


My guess is that a great 5 inch apo refractor would equal an average 8 inch mass produced mirror and it would take a great 6 to 7 inch refractor to equal a great 8 inch reflector. A generic 10 to 12 inch mirror will beat or equal any refractor under 8 inches. These guesses are based on real world observing at most objects. There are always exception.

Wade A. Johnson (North Central Iowa), from an online thread entitled; Refractor Equivalent to an 8″ Newtonian.

I have done the side-by-side during a couple of well attended starparties. My well collimated and optically very good C8 performed extremely similar to a very good Meade 6″ APO on Jup[i]ter on a night of exeptional seeing. The refractor edged it out with better contrast. No surprise. Compared to a home made 8″ Newtonian with a small 20% obstruction the images were almost impossible to tell apart.

So, my answer to the original question is: 6″ refractor.

I think we are finally well past the myth of a 4″ refractor being “sharper” or “showing more” than an 8″ reflector.

Contrailmaker, from an online thread entitled: Refractor Equivalent to an 8″ Reflector.


This old and tired conversation just seems to drag on and on, using the same old arguments that fly in the face of both common sense and physics. Drop it…let’s all agree that a 6″ Apo costing $7000 is a bit better than an 8″ F5.9 reflector costing less than $1000.
Which one would a sane person on a budget buy?

Covey32 (Georgia, USA), from an online thread entitled: Refractor Equivalent to an 8″ Reflector.

I would politely disagree to this. I looked through an excellent 5″ apo (my old Tak TOA-130S) and a very good 8″ Newt (8″ f/7) at Jupiter and Saturn. My Tak was soundly beaten by the views through the 8″ Newt.

Alvin Huey (Greater Sacramento) from an online thread entitled: Refractor Equivalent to an 8″ Reflector.

If you are talking about a ~5″ Apo like a 120ED, then yes they do give very nice views, but the 8″ f/6 Dob will still show you more detail and at 1/4 or less the price.

I think the views in the 5″ f/9.4 achro refractor I have are good, and on some nights they really are very good, but I have done quite a few side-by-side comparisons with my 10″ Dob over the past few months and as a result…the frac is back in it’s case in the shed and the Dob is by the back door ready for action whenever there is a break in the clouds.

RikM, (Gloucester, England), from an online thread entitled: 1st Planetary scope: Refractor vs Dobsonian.

Quote: “f-ratio is important. A 6″ f/11 might very well best an 8″ f/5 on most typical nights observing planets,” End quote.

Focal ratio is not relevant it’s the size of the obstruction that matters. So long as the secondary obstruction is under 20% of the primary diameter by area, the scope behaves like an unobstructed instrument. More than 20% and you start to see its effects. The effect you see isn’t due to light loss it’s due to increased diffraction caused by the large circumference of the secondary. This decreases contrast. However, there’s no reason a 25% or even 30% obstructed scope can’t perform very well. Why? If the scope is already a large aperture instrument with good optics then even with a hefty central obstruction it can still show superior contrast and detail. Optical quality and aperture matter more than focal ratio. I see this every time I observe Jupiter at f/4.

Umadog, (Basel, Switzerland), from an online thread entitled, 1st Planetary scope: Refractor vs Dobsonian.

My Skywatcher 8″ F/6 Dob beats my 5″ F/8 Apo [Takahashi FS 128] on the planets ……


Dweller, (Lancashire, UK), from an online thread entitled: 1st Planetary scope: Refractor vs Dobsonian.


Indeed, this is the point. The reason that beginners should be steered toward f/6 Newts is because this is a cheap way of getting good images. The mirrors are easier to make well and the eyepieces need not be expensive. Coma correctors aren’t necessary. Large (and fast) can be better, but quality is a lottery if you’re buying Synta or GSO, although those manufactures can produce some nice stuff. If you go home-grown you have a much better chance of a good mirror but you’re paying a lot more for it. In the end a lot of this comes down to economics not optics. Finally, there’s the hassle factor. To get the best out of a refractor you don’t have to do anything very special. To get a good view out of a Newt (particularly a big one) you have to plan ahead with cooling and collimation.

Umadog, (Basel, Switzerland), from an online thread entitled, 1st Planetary scope: Refractor vs Dobsonian.

If you have been dreaming of the day when the Chinese are able to make a portable 8 inch APO for under a thousand quid, keep dreaming! Seriously, this is not going to happen any time soon because APO-quality 8” ED blanks from the major suppliers (Ohara and Schott) don’t exist and if they did would be hideously expensive. Even if the glass was available, big APO lenses will always need a lot of highly skilled hand finishing along with very careful assembly in a sophisticated cell. The only major suppliers of big APOs today – TEC and APM/LZOS – charge the price of a new BMW for an 8” and if that situation changes it will be because they start charging even more! Then there’s always the Takahashi FCT-200 with a list price of $125,000 (but at least it includes the mount).

On the other hand, if you dream of getting similar performance to an 8” APO for under a thousand pounds, then dream no longer. Long focus Newtonians have always been simple to make well and with the advent of interferometer testing they can be made to an exceptional level of optical quality. Add in the possibility of a very small central obstruction, easy collimation and just two light scattering surfaces and a long-focus Newtonian has the potential to perform closer to a big APO than almost any other design.

Roger P. Vine (Welwyn, England), from an online article entitled;Orion Optics 8 inch Planetary Dobsonian Review.

In a Newtonian 8″ to 12″ with small central obstruction under 20%, very thin spider, longish F ratio above F6, excellent tube construction with well ventilated mirror and a decent flotation mirror cell (no mirror glued to plywood). Shorter F ratios require that the object be exactly centered in the field to avoid comatic aberrations. Also, the shorter the mirror, the more you will have to fiddle with the collimation. The mirror should have the best coating you can afford, avoid cheap coatings that lose contrast over time. Get a coating that you can clean without introducing pinholes. Add to that a smooth functioning focuser and you will have a very effective planetary instrument.

Roland Christen (founder of AstroPhysics, IL, USA), from his online article entitled: What is the Best Planetary Telescope?

I’ve compared my ED120 refractor (4.7″) with a number of Newtonians and have concluded that it will match a good 6″ Newtonian in planetary and lunar detail but the additional aperture will show deep sky objects just a little better.

John Huntley, from an online thread entitled: 150mm Refractor versus 200mm Reflector?

I am a confirmed dobaholic so the only choice is an 8″ dob between these two. The issue really is that unless spending 10x your budget or more, a 150mm refractor will provide less impressive views of low power faint objects and more faults with high power bright objects than an 8″ Newtonian. I’d also consider it more unwieldy and heavy/difficult to use than an 8″ dob. The one exception to the above is for wide open clusters where the frac will provide slightly more attractive views I think.

For me at that sort of budget an 8″ newt/dob is about as good as it gets.

Moonshane, from an online thread entitled: 150mm Refractor versus 200mm Reflector?


I’m a refractoholic but I have to admit that Shane is perfectly right on all counts.

Olly Penrice, from an online thread entitled: 150mm Refractor versus 200mm Reflector?

My STS and his have been under the dark skies of Landis a few times since. On one particularly memorable occasion, in May of 2015, our Teeters were out there alongside two world-class apochromatic refractors, the FS-128 and the comically coveted AP130. Given all the high-end gear, it felt like some star party of the aristocracy. My friend, who knows next to nothing about telescopes, was on hand. How could he possibly fathom the privilege of looking through such fine gear. Not until several years in the hobby, did I get my first eyepiece time with an Astro-Physics refractor or a Newtonian with Zambuto optics. There was, in fact, a 5th telescope on hand: a 4.5” Tasco — hey, every aristocratic star party needs a peasant scope. The FS-128 owner kept trying to pawn off the Tasco to my friend but he refused. (My friend later regretted not taking it). The modest scope did show Saturn nicely. In fact, on this night the seeing could not have been better. We were all treated to awe-inspiring views of the ringed gas giant, arguably the best I’ve ever had. On this night, Saturn would have looked great in about anything, but the two best views were to be had by the Teeters. My friend, who had no skin in the game (so to speak) or reason to be biased, attested to the “cannon ones” affording the best views of Saturn.

Daniel Quirin, from his online review of the Teeter STS18 (8″ f/6) Newtonian.

Once again, these Newts just floor me! Refractors will always be beautiful, but it’s hard to justify a $250,000 refractor to an $8,000 Newtonian that’s beating it. The fact is, is that if you dial the Newt right, your gonna win. Pons has been observing planets for 50 years. He’s earned the right to decide what he thinks is best and he’s got the best of both worlds to prove it.

When I asked him which scope he liked better on planets, he said the Newt was king, hands down and it’s as simple as that. He has no shame in saying so, d[e]spite the fact that he’s spent a good part of his life and a lot of money building the refractor.

People always try to challenge me in a debate. Then they look through one of these Newts and they’re quickly silenced. Pons always told me that they’ll always try to argue, but they’ll quietly go back to their garage and start trying to build a perfect Newtonian on their own.

Daniel Mounsey from an online thread entitled: Refractor vs. Reflector?


Yes, a 4-5″ APO will be very good, but the smallest detail it will resolve on a planet is around the 1 arc-second diameter mark. Resolution is tied intrinsically to aperture. It doesn’t matter how good the optics are or whether you’ve got 99.8% Strehl ratio optics etc, it won’t (indeed can’t) do better than this.

A clean, well-cooled and collimated, say 10″ f/6 or f/7 Newtonian with quality optics around 1/25th wave RMS mark and a secondary obstruction <20% in near perfect conditions will do twice as well as the 5″ APO in the same conditions — ie in the smallest detail to be potentially visible. It will probably cost little more than half as much. If the seeing is mediocre or poor there will be little difference in detail visible except in those occasional moments when clarity prevails for a moment or two — and the Newtonian wins again. The larger ‘scope will produce a significantly brighter image that will take much more magnification before it becomes unacceptably dim and uncomfortable to view.

Don’t get me wrong, refractors are beautiful telescopes inch for inch, but they are practically limited in aperture. (well they are aperture-limited by the depth of your pockets I guess). Aperture of the primary mirror/lens is the prime determiner of how much detail is potentially visible in a planetary image.

In poor, mediocre or average conditions a SCT of similar size to our Newtonian will perform about as well on planetary detail as the Newtonian. In very good conditions or excellent (rare) the Newtonian will produce a somewhat crisper image due to the much smaller secondary mirror used. It is a simple matter of physics due to the size of the secondary obstruction and the wave-nature of light. Increase the secondary mirror size and you push more light out of the Airy-disc and into the surrounding diffraction pattern. As Foghorn Leghorn said to the young chicken-hawk “Son yer can argue with me, but yer can’t argue with figures” — and that’s a fact, not an opinion!

Also, it is a simple fact of life that in a typical commercial SCT used with a diagonal, you need to get 5 optical surfaces right for it to work well. In a Newtonain there are only two.

If you are looking for a quality, visual-use, portable “APO killer” for an Eq6 mount, get yourself an 8″ f/7.5 Newtonian with a 25mm secondary. Longer focal length Newtonians are easier to collimate and much more forgiving of slight errors. Additionally, they are much easier to fabricate! Your eyes (and bank-balance) will thank you for a long time — it will flog the pants off any 4″ APO on any subject in the night-sky save perhaps ultra-wide field viewing. The image will be 4x brighter at a given magnification and will show twice as much detail in the right conditions.

Les D (Australia), from an online thread entitled; PLANETARY VIEWING ?? -aperature rules?or telescope type?

Unless your mirror is absolute and total trash, the reason is cool down, collimation, or seeing. Sounds like you took care of cool down and collimation so… Seeing.

Big scopes are able to resolve more, so they are subjectively more affected by poor seeing. I noticed this last night. I set up my 15″ next to so[me] nice Apo refractors, and stars looked better in their scopes. Seeing was exceptionally bad for me last night, stars were bloated little orbs over 150x, but they looked fine in the refractor. Peering at the moon, I didn’t see that shimmering you describe, but defocusing a star I could see very very fast upper air movement.

Now I know my mirror is not trash, it’s actually quite good and I’ve seen pinpoint stars at 300x in it and views of Jupiter that look like photographs. Last night, I couldn’t even see the GRS. So on nights of bad seeing, a small scope will be subjectively better because it isn’t big enough to resolve the poor seeing, at least not as well as a larger scope. That’s normal.

I[t] beat the pants of[f] the refractors on M13 though, and the Leo triplet, and M104, and M51… Shall I continue the list?

Brian Carter (Atlanta, Georgia), from an online thread entitled:Jupiter in my Dob vs Refractor?

Are those little apochromatic refractors really better than reflectors? They certainly have been advertised as such. In fact, refractor manufacturers have always alleged that reflectors are, well… just a little less than the ultimate – workable, useable, but really not first rate – images just a little sour. And in fact, many a view through a reflector confirms the sour image reputation. Views through refractors are invariably sharp and crisp, neat and gratifying to the eye. But are reflectors really a poor man’s telescope, a less than optimum instrument? As you might imagine, I don’t think so. And here is why I think not and why the “super little refractor” thing is just another load of advertising hype……….

While a Newtonian reflector of aperture and design proportions sufficient to function as a serious instrument for lunar and planetary observing is not going to be as readily portable as a small refractor or Schmidt Cassegrain or Maksutov instrument, such an instrument will optically match or out-perform all other forms of astronomical telescopes inch for inch of aperture in larger sizes. The problem is that such a Newtonian reflector requires slightly more care and consideration in use, but will be considerably less expensive to construct than any of the other telescope types. The point to emphasize here is that the Newtonian reflector is in no way a substandard instrument when compared to other compound reflecting optical systems or refractors. It is every inch the equal of these instruments, and, I believe, in many ways superior. Design the instrument well, construct it out of quality materials and with care, and fit it with quality optics. Give the instrument chance and it will absolutely amaze you.

Robert F. Royce (professional optician), from his article, Reflector vs. Refractor.

As early as 1972, the renowned British telescope maker E.J. Hysom conducted a careful series of experiments with mirrors of various diameters and thickness using a sensitive thermocouple. Hysom determined that a 30mm (1.2 inch) thick mirror cools at a rate of 3.3°C per hour, while a 76mm (1.8-inch) thick mirror cools at a rate of only 0.9°C per hour.
With the aid of a fan these rates could be increased by a factor of three.
Thomas Dobbins, from an online article entitled: The Recent Evolution of the Planetary Telescope: Part 2.

An 8″ mirror doesn’t have these thermal stability issues that are fundamental to larger apertures. Cool-down is relatively quick as long as you have a pyrex mirror. An 8″ plate glass mirror with a fan will also cool down quickly.

8″ f/8 newts (provided they have a solid/split tube design) can be staggeringly good planetary scopes. Wholly apart from the materials costs of refractor glass, you’re more likely to get a perfectly figured optic than you are with an apo. Refractors have more optical surfaces that have to be accurately figured (4 in a doublet, six in a triplet) whereas a newtonian only has one. At f/8, if using a low-profile focuser, the central obstruction is miniscule and the increase in contrast over a shorter focal length newt can be dramatic. Also, using a single-arc two-vane curved spider like a protostar can go a long way towards minimizing overall diffraction. Also at 8″ or less, the flexure inherent in that design is negligible enough not to affect performance.

Zamboni, from an online thread entitled: OPT 8″ f/9 Planet Pro Dobsonian.


The Astronomers Without Borders’ One Sky Newtonian; an affordable but good quality, ultraportable 130mm f/5 tabletop telescope.

We have 4 scopes that always see some use. A 8″ F8 Newt (Dob mount), a 111mm APO (actually, a lot of 4″ refractors), a 16″ SCT and a 17.5″ F4 Newt (dob). The 8″ has some of the best optics I have ever had the pleasure to use, a true one of a kind scope. Planets are fantastic through it. Actually, pretty much everything is. But, when I want to look at galaxy clusters or similar, the 17.5″ is the scope. When I want to study the details in planetary nebula or small single galaxies, I like the 16″ SCT. On exceptional nights, the 16 is great on the planets too but those nights are far and few between here in the Great Lakes State (only 1 really comes to mind in the last 10 years…).

Jason B, (Michigan, USA), from an online thread entitled: 1 inch Apo vs 12 inch SCT.

Under the stars, this telescope really shines now. It really is nearly the equal of my 10 inch f/6, a ‘scope I have been told by many who look through it, has Zambuto like quality. Planetary detail is excellent. Deep sky is just great. I find myself surprised over and over again by this telescope. The figure on that primary is just excellent. We did not touch what the original guys at Cave Optical did with the figure, we just recoated it. I reviewed this ‘scope on the Todd Gross astro equipment ratings site, and I’ll tell you now what I said then. If you like vintage ‘scopes and you don’t have one of these, try to find one. You won’t be disappointed! 

Edward Conley, (North Branch, MI, USA), from an online review of a Classic 8″ f/7 Cave Astrola Newtonian.

FWIW Rolando [i.e. Roland Christen] said the best view he ever had of Saturn was through a 12.5″ Cave – 800x was no problem.

deSitter, from an online thread entitled: Cave Telescope Estimate of Worth.

Jupiter on the morning of October 8 2010 by Jason H ( Central Florida, USA); afocal footage from a Criterion RV 8 f/7 Newtonian reflector.

When I rece[i]ved my 6″ F/8 Criterion RV-6 I was amazed at the detail I could see on Jupiter. Since then I have heard many others say how well their RV-6 scopes performed. Why did these scopes seem to perform so well? How do they compare to “modern”: Newtonians like Zambuto mirrored scopes?

Jim Philips (South Carolina, USA), on an online thread entitled: Criterion RV-6 Dynascope.

Well I always like to have an excuse to repost a picture of my restored 1960 or 61 original RV6. Yes the optics are as good as everyone reports. I agree with what others have written that the 6 inch at f/8 is relatively forgiving and if well made performs excellently even with a spherical curvature. After seeing a neighbors RV6 outperform my Astro Physics 6 inch f/8 triplet,(early model), I sold the refractor and restored my RV6 to almost like new condition.

Bill Nielson (Florida, USA), on an online thread entitled: Criterion RV-6 Dynascope.


Went to my club Saturday nite and happened to set up right next to a gentlemen who was using a 5 inch Takahashi refractor. I was using my 8 inch Orion Intelliscope. We struck up a conversation and soon began swapping scopes on different targets. Now as some of you know, i got the Dob to tide me over while saving fro a premium APO. Well, to make a long story short, my lowly, mas produced mirror beat the state of the art fluorite lens on every single target, planets included. Interestingly, it wasn”t i who first acknowledged this, it was the guy who owned the Tak. He kept bringing his own ortho eyepieces over to my scope, and shaking his head. As a recovering CRF, this was very validating for me. I am really no longer seeing any advantage at all to ultra expensive refractors. Not to mention that, while stunningly beautiful, and well made to say the very least, his scope and mount combo is a boat. Mine was out and ready in under 5 minutes. In conclusion, I am no longer aspiring to get the 4 to 5 inch APO, rather my next upgrade will be a 12 inch newt, which, because of cost, can happen a lot sooner. Personally, at this point, I see refractors as excellent, rugged, grab and go travel scopes. I am quite happy to be in the reflector camp at the moment.
Jonnyastro, from an online thread entitled 8 inch Newt vs 5 inch Apo.
Aperture rules and a lot always depends on the seeing which is the great equalizer, but a well made Newt with a reasonably small secondary mirror can be a great planetary scope. The secondary mirror will always lower contrast compared to the unobstructed Apo, but the higher resolution of an 8” brings something to the view that the 5” telescope can’t.

Snart, from an online thread entitled 8 inch Newt vs 5 inch Apo.

I just got back from a weekend star party and pretty much had observed the same thing. My well collimated DOB showed more and better than anything that had less aperature. Since I cant afford anything in the APO category, it left me pretty pleased with my equipment…….. VERY encouraging. I guess my homework and the help supplied from CN has led me to the right stuff!


Steve k, from an online thread entitled 8 inch Newt vs 5 inch Apo.
The hang up over reflector verses refractor seems to originate in the 1950’s-1960’s. Even Patrick Moore will say buy a three inch refractor or a six inch reflector.
However times and tech have changed. It just takes time for the astro community to accept this. I own a 14 inch reflector and I also own an 8 inch mak. I have also owned a five inch apo. The most used scope is the 14 inch reflector. I have been into astronomy for 30 years, I have been very active and I know my stuff.
Don’t forget if someone buys an apo for $2000 they want it to out do any other type of scope that costs a third less,their opinion will rely on the cost. It is human nature.
Gordon, from an online thread entitled 8 inch Newt vs 5 inch Apo.
It just boggles my mind that a piece of equipment costing in the three hundred dollar range can outdo one costing in excess of 5000 with mount.
Jonnyastro, from an online thread entitled 8 inch Newt vs 5 inch Apo.

Being a newt guy, I’d agree that an 8″ reflector can beat a 5″ apo refractor. However, I would point out a few things: The newt may require more cooldown time, and it may be more affected by seeing conditions, tube currents, etc. The newt will show diffraction spikes around bright objects unless a curved spider is used, while the refractor will obviously not. The refractor may show a “cleaner” image, but not necessarily more detail. This is especially true if the newtonian has a large central obstruction, isn’t flocked, etc….The great thing about newtonians is that they’re easy to modify. A flocked, collimated, cooled down newt with a curved spider, nice focuser (being perfectly in focus is important on planets!), and good optics will be right on par with an apo refractor of the same aperture minus the secondary obstruction, IMO.

Erik, from an online thread entitled 8 inch Newt vs 5 inch Apo.


like i always show my students; a 6″ unobstructed, perfect optic is creamed in resolution by a 10″ 20% obstruction 1/10 wave newt:

http://www.astromart…?article_id=473 (thanks darren!)

this is why i always wonder when people say refractors are best on planets…..


dave b, from an online thread entitled 8 inch vs. 5 inch Apo.

KWB said

like i always show my students; a 6″ unobstructed, perfect optic is creamed in resolution by a 10″ 20% obstruction 1/10 wave newt:
That’s fine, Dave but your skirting the issue. I’ve now been painted into the corner. Can you give the nod to a 6 inch
F/8 reflector against a 150mm Tak,AP,TMB,etc,?

if they both cost the same, i would take the 155mm AP.

if the 155mm AP and an 1/8 wave 8″ newt both cost the same, i would of course take the 8″ newt.

dave b, from an online thread entitled 8 inch vs. 5 inch Apo.


When I owned an 8″ Mag1 Portaball with a Zambuto mirror, I used to compare the views of the planets through my telescope with refractors. Over a two year period, there were a few refractors that came close to providing better views on a few exceptional nights, but I didn’t find a refractor that could compete head-to-head with my reflector. (The best refractor, the one that came the closest, was an AP 155, if I recall correctly.)

I now own a 12.5″ Mag1 Portaball (also with Zambuto mirror.) I’m still waiting for the night where the refractors demonstrate their clear superiority. I’m not holding my breath. Under crappy seeing conditions, I’ve seen the phenomena of a refractor providing what its owner called ‘a more aesthetically pleasing view.’ This is another way of saying when the seeing is bad, smaller aperture scopes don’t see the bad seeing as well as [a] large aperture scope. (In this type of condition, one can ‘stop down’ the larger scope and see the same sort of views that are seen by the refractor.)

When the seeing is good to excellent and when optical quality is excellent, aperture wins every single time. And dollar for dollar, high quality reflectors rule.

But don’t take my word for it. Check out Gary Seronik’s article “Four Infamous Telescope Myths” in the February 2002 issue of Sky and Telescope. You can also go to star parties and try a few experiments. Under good seeing conditions, take a look at the planetary views through a correctly collimated reflector equipped with a Zambuto, Royce, Swayze, Hall, etc. mirror. Then take a look at the views through a 6″ refractor that’s many times more expensive. I think that the results might surprise you.

Finally, consider this Mars image, made by Wes Higgins with a 14.5″ Starmaster. In the past, when the optics in most large reflectors were mediocre at best, I believe that high quality refractors provided the best views. Now, with high quality optics readily available in large reflectors, I believe the situation has changed.

Barry Fernelius, from an online thread entitled, Reflector versus refractor.

What more and more people are doing right now in France, is to buy those chinese 8″ or 10″ f/5 or f/6 Newtonians, play with them for some time, then have the primary mirror refigured to an exceptional quality for around €1,000 (US$ 1,250) with enhanced reflective coatings. They perhaps add a better focuser and tweak the spider a little bit. After that treatment on a 8″ reflector with 20% central obstruction, a 6″ APO can no longer match it for visual work.

Rhadamantys, from an online thread entitled, Reflector versus refractor.

At the risk of beating a dead horse, my experience is that an APO refractor delivers consistently a[e]sthetically pleasing results every time, with detail limited only by atmospheric conditions and aperture. A high-quality, well designed newt can also deliver [a]esthetically pleasing views, with detail limited only by atmospheric conditions and aperture. Everything else being equal, quality aperture wins, every time. Not surprising that (last time I checked anyway) Thomas Back’s personal scope for planetary viewing is….wait for it…. a 20″ Starmaster. Nuff said?

Gary in Ontario, from an online thread entitled, Reflector versus refractor.


A 130 mm F/5 with a decent mirror and a 2 inch focuser. No CA, much faster than the Mak or the refractor for EAA and very rugged.

I’ve owned several.. It’s scary sometimes how good they can be. I remember one night under dark skies.. I was doing the low power, wide field thing with my NP-101 and swapped it out for a 130 mm, F/5 Newtonian with the 31 mm Nagler and a paracorr. I wasn’t giving up much with the $200 scope.

Jon Isaac(San Diego, California), from an online thread entitled: 4″ refractor vs. 4″ Mak.

My world changed forever when I first took a modern SkyWatcher 8″ f/6 Newtonian for a serious spin under a dark sky. It was far less expensive than any of the other telescopes I had personally owned, including some fine refractors and Maksutovs, and it outclassed them all on every type of celestial target. It was the sweetest of revelations! Vainglorious eejits continue to bicker over and covet this new breed of ridiculously overpriced refractors with low dispersion glass for visual use; and with little or nothing to show for it. Life’s too short. Modest and marvellous in equal measure, Newtonians are my instruments of choice, based solely on visual performance, when I want to pursue either serious or casual observing. And I’ve earned the right to do so!

Mr. Hardglass.


Neil English is author of Choosing & Using a Dobsonian Telescope.


De Fideli.


2 thoughts on “The War on Truth: The Triumph of Newtonianism.

  1. Good Morning Sas,

    Merry Christmas to you and yours.

    Thanks for the thumbs up!

    Best wishes,


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