A Work Dedicated to Brian Carter
The cure for unobtanium is a good dose of speculum!
Here I shall provide an in-depth evaluation of the SkyWatcher Skyliner 200P; an 8 inch ( 203mm) f/6 Newtonian reflector.
Friday, January 30 2015: The telescope was ordered from Rother Valley Optics, UK. The complete telescope was purchased for £288, inclusive of delivery to my door. I chose this telescope owing to its almost universal acclaim by amateur astronomers of all levels of experience, from novices through to seasoned veterans. I was interested in getting something larger and more powerful than the most excellent 6″ (152mm) f/8 model but not something that would be too large and cumbersome. Many literary sources discussing Newtonian optics recommended as slow a mirror as one could get away with. As a purely visual observer, I was pleased to hear that SkyWatcher were offering their 8-inch Dob at a respectable f/6 relative aperture; a good compromise between portability, functionality and performance with a modest eyepiece collection.
An inexpensive Skywatcher laser collimator was purchased at the same time.
Monday, February 2: The telescope arrives in two boxes; one containing the telescope and various accessories and the other containing the flat-packed Dobsonian base. The telescope was presented in excellent condition, as were the accessories, but it was quickly discovered that the lazy Suzan rocker box supplied with the telescope was actually for the smaller 6 inch model. Oh dear; never mind!
A quick phone call to the retailer was enough to arrange for the delivery of the correct size base and the collection of the 6 inch base.
23:00h: A set of tube rings and a dovetail plate were found for the telescope and I was able to attach the instrument to my heavy duty SkyTee Alt-Azimuth mount. Turning to Jupiter, it was easy to see the optics were quite a bit out of alignment but I was taken at the sheer brightness of the planetary image. Star testing confirmed that the optics were mis-collimated.
The simple but sturdy focuser on the telescope can accommodate both 1.25 and 2″ oculars. The package came supplied with an 8x50mm straight through finder- always an impressive addition in my opinion.
Good but sturdy; the focusing unit on the Skyliner 200P.
The innards of the optical tube were painted matt black and the parabolic primary mirror looked immaculate with no sleeks or flecks of dust/paint. Very nice indeed! Two eyepieces were supplied with the instrument; a ‘wide field’ 25mm focal length ocular delivering 48x and a higher power unit giving enlargements of 120 diameters.
The inner sanctum of the SkyWatcher Newtonian. All is well presented.
Tuesday, February 3: The laser collimator arrived this morning and I quickly got to work aligning the optics. This device makes very light work out of precisely aligning the optical components. After placing the device firmly in the focuser, the secondary mirror was precisely aligned with the central spot of the primary and then the primary was adjusted slightly to complete the collimation. This only took a few minutes to do but should make a huge difference to the images garnered by the telescope.
An inexpensive laser collimator makes aligning the optical train child’s play.
23:30h: Seeing fair to good(Ant II-III).
With the telescope fully acclimated in -6C temperatures, I conducted a quick star test using my trusty 24-8mm Baader Hyperion zoom to confirm that the optics were properly aligned. The full Moon was easily framed with lots of room to spare at the 24mm (50x) setting of the zoom, the image being razor sharp and blindingly bright at the same time. Aiming at Polaris, I was very pleased indeed with the beautiful concentric diffraction rings either side of best focus at 150x. No astigmatism was noted with no signs of significant zones. This is a good, smooth mirror by most anyone’s standards. In focus Polaris A was pinpoint sharp with delicate diffraction spikes consonant with a good Newtonian image. Its faint companion was easily picked off wide away.
I know I know; it’s a Dob….. but I couldn’t wait to test it.
Comparing the views of Jupiter with my 17cm f/16 Maksutov Cassegrain at~ 150x, I was very impressed how well the 8 inch Newtonian was performing. At a glance I could see finer detail than in the smaller aperture Maksutov, although the sky background in the latter was considerably darker.This could be attributed to stray light from off axis moonlight though and will require further investigation. I could also see the faint impression of the slender spider vanes superimposed on the in focus image, though after a few seconds, one could effectively ignore them and get on with enjoying the images.The brightness of the planet in the Newtonian at 150x will call for higher optimal magnifications to be pressed into service in due course.
Turning the telescope on Iota Cassiopeiae, the instrument was able to cleanly resolve the three components fairly easily. Almach, now fairly low in the northwestern sky was beautifully rendered, the golden and blue colours of the components coming through easily. The faint diffraction spikes around the stars were quite fetching to my eye. I don’t know what to make of them as yet.
All in all, an impressive first light for the econo-Dob.
Thursday, February 5: The replacement base arrived safely today and the 6 inch base collected. Assembly took about a half an hour. All tools were supplied with the package.
The Skyliner 200P Dobsonian telescope.
18:30h: Seeing II, temperature -0.5C, clear, tranquil sky after dusk. Telescope kept outside all afternoon with optics covered. Little chance of dew as the air is still very cold and dry.
Theta Aurigae: companion easily picked off from the bright primary at 150x.
speculum vs speculum
22:30h: Some low altitude cloud rolled into the valley for much of the evening while I was teaching. The telescope remained on duty throughout. Also fielded my 17cm Maksutov (acclimated) for comparison. Only a few targets visible at any one time. Seeing improved and now excellent, as judged on a very near perfect rendition of Castor A and B through the Maksutov at 343x. I turned the instruments to Jupiter and studied the images delivered.
The winner: without a shadow of a doubt- the 8 inch f/6 Newtonian.
Both served up very fine views but the extra resolution and light gathering power of the Newtonian in these braw conditions put it comfortably ahead. An incredible amount of ultra-fine fine detail could be made out all across the planet but my eye was drawn to the high latitudes of the southern hemisphere, where a few curious white spots and a riot of linear structures quite literally jumped out at me! Where the smaller Maksutov suggested, the larger Newtonian plainly revealed.
The above was based on unfiltered observations, but I felt the icing on the cake was delivered with the addition of Al Nagler’s latest technical innovation: the Televue Bandmate BPL. It diminishes the glare around the planet, darkens the background sky and brings out all kinds of reds, oranges, yellows and whites within its enormous atmosphere. To my eye, the image at 170x with the filter was truly magnificent! I didn’t try any higher powers as there was no need to do so.
I would invite others to pull out their GSO/Synta 8 inch Dobs, spend a few minutes lining up its optics, letting it fully acclimate (I didn’t use a fan) with its cap on and comparing its views with a more ‘revered ‘scope of similar aperture.
To the rich man I speak: the man who has ‘seen’ it all in the cold light of day.
Maybe you could buy one in for a while at least? Nothing to lose, right??
But be prepared to be shocked!
SkyWatcher Skyliner 200P: planet killer extraordinaire!
Sic transit gloria mundi!
Friday, February 6: I am still somewhat in awe of what I saw last night and no one can take that away from me. It only takes one good night to reveal the truth about any telescope, whatever preconceptions one may have had about it. Would I have been better off with a larger mirror; a 10- or 12-inch maybe? I believe that the difficulty in producing a fine mirror scales with the square of aperture. A 12 inch reflector takes four times more effort to make well compared with a 6 inch. Larger apertures may have some advantage over the 8 inch but my previous experiences with these larger Dobs suggest that I would not get as many nights where everything comes together like it did last night. Mileage counts for me. No, the right choice was the 8 incher since I now know that it will play ball with the environment in which I reside.
I spent some time this morning mulling over old books on planetary astronomy. One choice example is Bertrand M. Peek’s tome on Jove; The Planet Jupiter: The Observers Handbook (1981 edition) in which he states categorically that “An 8 inch [reflector] is probably adequate for all purposes.”
Fred W. Price and a host of other authorities I respect, say much the same thing.
Fortune continues to smile my way, as the settled conditions look to last at least another night.
Thank goodness for small mercies!
18:50h temperature -1C, seeing II
Had the ‘scope out for a couple of hours while I was teaching. Before the Moon rose above the horizon, I managed a very competent split of Eta Orionis (1,6″) at 225x. The Great Nebula in Orion (M42) was jaw-droppingly fine; a great heaving mass of ‘chlorescent’ gas at 120x. The Trapezium stars, including the more elusive E and F components, were easily sighted.
19:30h In a race against time, I assessed its wide field capability with two of my eyepieces; a Mark III 24-8mm Baader Hyperion zoom and 40mm Erfle. Both eyepieces performed extremely well in the very forgiving f/6 Newtonian. Cruising through central Orion at 30x in a 2+ field is a joyous experience, with stars staying acceptably sharp over the majority of the field with only the extreme edges showing coma and field curvature. I noted identical behaviour with the zoom eyepiece at the 24mm ( x50) setting.
You’ll need to use the supplied adapter to get 2 inch oculars like this 40mm Erfle to work.
Caldwell 14, the Great Spiral in Andromeda (M31), the Auriga open clusters and M35 were simply spellbinding in the telescope. I have a 8 inch f/6 doublet achromat awaiting return to its manufacturer but I can tell you this Skyliner 200P package – costing 11 times less and weighing half as much – is a much superior telescope in almost every conceivable way!
Is a 8″ f/6 rich field refractor really worth it?
Oh vanity of vanities!
Later tonight it’s Jupiter watching time. Away in to watch the rugger the noo.
23:30h:Smashing opening performance by England against Wales in the Six Nations championship.
Low waning gibbous Moon in the southeast. No clouds.
Seeing deteriorated a little this evening (Ant III), as judged by the appearance of slightly swollen seeing disks in Castor A and B in the acclimated Maksutov. I decided to test how fast it would take the 8 incher to come to temperature by taking it directly out from a warm indoor room to the cold of pre-midnight air at -3C ( so ~ 25 degree C swing). The optics were left covered throughout. When I returned to the telescope an hour later, the images had stabilised. That’s no surprise though, at 8.6 kilos (19 pounds) fitted with its finder, the Newtonian actually weighs less than the 20 pound Mak and enjoys a much greater area over which to rapidly radiate heat.
I again compared the views of Jupiter served up by the Maksutov (17cm) with the 20.3cm Newtonian and the result was the same; the Newtonian shows finer details than the compound, confirming that aperture wins.
I’m not really known for having great affection for Newts, but this dapper telescope has got my pulse racing.
Saturday, February 7: This afternoon I leafed through my old observing notebooks from the years 1996 through 1997. Back then I was living in the city of Aberdeen, Northeast Scotland, and enjoying my first salaried job as a lecturer at the local university. In the summer of 1996, I purchased a Celestron 8″ f/6 Star Hopper Dob and at a much higher relative cost than today. “Wolfgang,” was big and awkward, with a cheap, oversized sonotube OTA. I observed Jupiter throughout the summer and autumn of 1996 with this telescope and my drawings reveal a good amount of detail; rather similar to the details I have recorded in more recent years with my 5″ f/12 achromatic. I was very pleased with the images Wolfgang served up – it was the base that failed first – but the planetary detail revealed by my new Skyliner 200P is easily better (on all but the worst nights) than any five incher can deliver. I was puzzled by this, and it got me digging a little deeper into my notes. Turns out Jupiter was fairly low in my sky during those years, sprinting through Capricorn (1996) and then Aquarius(1997). Now, Jupiter is considerably higher in the sky and the differences between these instruments are much more apparent. That’s one theory. Here’s another one: Maybe the SkyWatcher mirror is truly better than the one I got with Wolfgang? Can technology improve in 20 years? It is not unreasonable to think that it could.
Man and his technology!
22:15h: Sky clearing of clouds, -2C, good seeing (Ant II): One final comparison between the 17cm Maksutov and the Skyliner 200P. Target: Jove: Both telescopes serving up super fine images. The 8 inch image is more detailed though. GRS approaching the planetary limb…..cool.
23:00h: Since the conditions were quite good, I decided to push the telescope by examining the diffraction images of stars at very high powers. Using the 2.25x Barlow with the zoom, I dialed in a maximum power of 338x and examined the brilliant white pair Castor A & B. In this cold weather, the heat from your body can affect the image, especially when you place your warm hand in front of the optical train to reach for the handle. This quickly dissipates though and when you are wrapped up warm and well insulated the effects are minimised.
I was really quite impressed at how clean and together the morphology of the Airy disks presented at these high powers. The diffraction spikes are quite charming and have an aesthetic grace all of their own. I have a very good mirror; a mirror that will surely see a lot of use over the months ahead.
Monday, February 9: With the moon now out of the evening sky, I spent a few minutes yesterday evening enjoying some dark, transparent skies with the Skyliner 200P. Using the low power (50x) setting on my zoom, I turned the telescope towards Cassiopeia now nearly overhead, and spent a few minutes drinking up the views of a few favourite open clusters including NGC 457 (with its ET eyes staring back at you), M103, M52 and a real corker: NGC 7789 which, at higher powers (75x), resolves into a plethora of faint stars. The traction on the mount is quite good and it’s easy to find the ‘sweet spot’ for most any application, from low power scanning to tracking planets and smaller deep sky objects at higher powers.
The SkyWatcher model in my possession probably has a similar mirror to the US branded Orion XT8 Dobsonian. US-based telescope reviewer, Ed Ting, has very interesting things to say about it here:
“It’s not a Zambuto,” he says, but it’s “shockingly good!”
These telescopes have been used to great effect by very well established observers on both sides of the pond. Dr Paul G. Abel, a professional astronomer and BBC Sky at Night presenter, uses the same model as my own – albeit on a motorised equatorial platform – to make all his detailed observations of the planets. I contacted Paul to enquire about whether he had any modifications done to the optics. He told me he had not and saw absolutely no need to do so. A true draftsman, his portfolio of work is of the highest quality.
Arizona based astronomical artist, Jeremy Perez, needs no introduction in amateur astronomy circles. Using only an XT6 and XT8, Perez provides first rate illustrations to all the major astronomy magazines including Sky & Telescope, Astronomy and Astronomy Now,and has contributed to several major books on the subject of astronomical sketching. In addition, Jeremy maintains an excellent website where his versatile work with the XT8 is clearly demonstrated. Worth checking out I’d say.
What inspirational blokes we have here!
21:45h After enjoying some gorgeous views of M36, M37 and M38 at low and moderate powers, I turned the telescope to Jupiter, not yet at an optimal altitude, and was delighted to see that a satellite transit is currently underway. Will report back again later.
Seeing wasn’t great tonight (Ant III mostly) and it was a bit on the breezy side, which made the image shake a wee bit. Fairly mild (+5C) though. I decided to keep magnifications down at 150 diameters. I followed the eclipse until it reached the middle of the disk and then noticed the GRS had reappeared. A fantastic experience!
Geein’ it laldy!
Thursday, February 12: On family vacation until next week.
Monday, February 16: My short time away from this telescope got me thinking about its enormous potential. As it is, the Skyliner 200P is an excellent general purpose instrument; a veritable ‘diamond in the rough’. Yet, it can be improved still more and with only a modest additional investment. The reflectivity of the mirrors(worst case scenario) is ~87 per cent, so combined produce an overall light gathering power of 0.87^2 or 76 per cent. Light loss due to the area of the secondary mirror reduces this by just a few more per cent. That’s not bad at all:- still significantly more light gathering power than a 6 inch refractor but with greater resolving power.
Curiously, there are services available to increase the overall reflectivity of both mirrors to 97 per cent. For example, Orion Optics UK offer their Hilux coating which would bring its light gathering power into line with a similar sized refractor and maintain this ultra-high reflectivity for two or three decades!
The secondary mirror in the Skyliner 200P has a diameter of about 50mm, giving a 25 per cent central obstruction. This provides full illumination over ~ 25mm of the central region of the 46mm diameter field of view; a good compromise for all round use. Yet, Orion Optics UK also offer the amateur the opportunity to buy a smaller secondary with a diameter of 36mm, thus reducing the central obstruction to just 18 per cent, in order to optimise the instrument for planetary and double star work. Heck, one could keep both flats and inter change them as and when required!
Man and his technology!
Small modifications to the existing secondary, such as thoroughly blackening the sides of the mirror and the supporting structure would cut down on glare from bright objects. Flocking the tube with low reflection material, especially opposite the focuser, would also improve the overall contrast of the image reaching the eye.
Black paint skillfully applied to the secondary flat mirror and supporting structure would be a worthwhile project to improve overall performance.
What exciting telescopes these 8-inch Dobs are; lucis in caelo!
23:00h: With all the moving about the ‘scope has thus far endured, I decided to check collimation of the Newtonian, discovering that it was a wee bit off perfection. That’s how accurate the laser collimator is! Set it upon my kitchen table and tweaked collimation until it was spot on again and then set it out to cool.
Tuesday, February 17:
Ambient: +1C, blustery northwesterly sleet showers coming and going all evening.
Seeing quite poor (Ant III-IV). En passant, neither the fully acclimated 17cm Maksutov or the Skyliner 200P were giving good stellar images this evening. Only the easier doubles were resolved; Gamma Leonis was pretty at 150x and Mizar & Alcor lovely at 120x, their seeing discs large and ‘boiling’ in both instruments, but sightly more so in the bigger Newtonian. I attribute the latter to its greater sensitivity to atmospheric turbulence
Jupiter looked rough even at 150x but I was still able to watch the GRS whirl across its disk in both telescopes, the slightly larger Newtonian showing it that little bit more easily throughout.
Thursday, February 19: Having discussed the tremendous value of the Skyliner 200P with my editor, I decided to embark on a project to make the aforementioned modifications to the instrument in order to maximise its potential. The project will be featured in an up-and-coming issue of Astronomy Now and, at some later stage, on this website too.
Thanks for reading my story so far.
I leave you now with a short video on Sir Isaac Newton’s original reflecting telescope, its speculum mirror and the ingenious design of the tube and mount.
Man and his technology!
Wednesday, April 1, 2015:
I got both mirrors re-coated but in the end decided on just one secondary of 44mm diameter, and not two as I considered previously. This secondary provides a very decent 22 per cent obstruction – less than my Maksutov and a good compromise between a smaller secondary and a larger one. This still provides excellent illumination even towards the edges of the field with a low-power 2-inch eyepiece. I bought a water butt that perfectly fits the mushroom knobs under the base of the lazy suzan mount and provides both a stable and more elevated platform for more comfortable, seated visual use. I have undertaken all the modifications to the tube mentioned previously.
After putting the whole thing back together this afternoon, I set the telescope outside with its caps on about an hour before my teaching finished. As luck would have it; the sky was both clear and tranquil (seeing I-II) and I trained the instrument on Jupiter. Wow! Incredible detail such as I’ve never seen in the smaller Maksutov! I watched a beautiful shadow transit of one of the Galilean satellites. The GRS was near the centre of the disk and the satellite shadow was directly north of it.
I made a sketch to record those moments.
Time: 20:40-50h UT
Seeing: Ant I-II
CM II: 236 degrees
Jupiter as seen in an 8″ f/6 Newtonian reflector on the evening of April 1, 2015. North is at the bottom.
By 22:00 BST, I had successfully split Theta Aurigae and the magnificent triple system, Iota Cassiopeiae.
This is my best telescope, no doubt about it, but it’s also considerably less (even with the mods) expensive than the 180mm Mak and my 5 inch refractor.
In time, it will become my most used telescope.
Aperture wins. Period!
Don’t let any wooly thinking cloud your judgement!
And so, it only remains for me to bestow a name on my powerful speculum; eight inches of Newtonian bliss.
Only one appellation trips off the tongue: Octavius!
Sailing away with young Octavius.
Saturday, April 4, 2015
An Evening with Octavius & Cornelia
Time: 22:00 BST
After a beautiful bright and reasonably warm day, I set up both my 17cm Maksutov and Octavius side by side and allowed them to fully acclimate over a couple of hours with the optics capped. I watched the Moon rising in the East and brilliant Venus in early gibbous phase setting in the West. When dusk arrived, I discovered that the seeing was near perfect! Hardly a quiver on Theta Aurigae at 340x in the Maksutov. Comparing the Jovian images one more time, I got the same result; two lovely instruments capable of rendering excellent images but one clear winner; the 8″ f/6!
The images are more stable in the Maksutov, owing to its greater insensitivity to atmospheric turbulence but that said, when the images settle down in the Newtonian, it easily shows its superiority to an attentive telescopist.
I would welcome people to test this claim and report it to as many astro folk as possible.
The biggest surprise though was the efficacy of the reflector in resolving the tricky binary system Iota Leonis. At magnifications of 225x, the 8 inch speculum showed me the clearest view of its elusive secondary that I have yet seen in any telescope. It’s raw resolving power is a law unto itself!
This is very encouraging as it means that there will be nights where I can push the 8 inch on pairs that are deemed very tough for my smaller instruments.
There is something very sweet and pure about this telescope; it’s understated power and charm shine through when just a little care and attention is afforded it.
My exchange with the public.
Battle o’ the Specula.
A Triumph for Octavius!
For the defeat of badness with goodness!
For reason over darkness.
For diligence over laziness.
For vanquishing bluff with truth!
For heart filled humility!
And the ostrasization of hubris.
A Triumph for Octavius!
Go in Peace Octavius!
Tuesday, April 7, 2015
Continuing to learn from Octavius in good seeing. Started a blog on my adventures on CN to save some space on my site. Thank you CN!
I am eternally grateful to Cornelia for teaching me the ways of Octavius.
On vacation until next week.
Wednesday, April 15 2015
Work on CN aborted owing to account suspension.
After a rather grotty few days compared with what our neighbours south of the wall have enjoyed, the clouds cleared in the late afternoon here and it stayed that way for the rest of the evening. After work, I set Octavius outside to cool for about 40 minutes from my heated office. That was enough time to get it serving up excellent images of Jupiter in very good to excellent seeing (I-II). My favourite magnifications are about 170x with the Televue BPL filter in place. Though less than 50 degrees altitude at this time, the 8 inch speculum rendered some really fine details within the planet’s vast belt system. The NEB was a heaving mess and the SEB distinctly bifurcated. When I began my observations at dusk at 20:30UT, the GRS was just coming round the eastern limb of the planet and over the next half hour I watched it grow more and more distinct as it edged its way into the disk.
I made a sketch of my impressions at 20:45-55 UT CM II 182 degrees.
It is hard to describe the image of the planet in the Newtonian. It ‘flits’ back and forth between slightly blurry and magnificently sharp over the seconds and minutes. I have learned to completely ignore the spider vein shadows superimposed on the planet’s disk. They are largely cosmetic in nature and have little or no impact on the resolution of fine detail.
I have not encountered much in the way of the ‘tube currents’ that other observers have reported. Indeed, I have had the instrument long enough now to judge that I do not need a cooling fan. The mirror is relatively small and of low mass and cools very quickly, even if taken out of a warm room. I am mindful also of many distinguished observers from yesteryear (Philips, Webb, Browning etc), who produced some very fine work over many years using nothing more than passive cooling to the night air. Fans did not exist during their lifetimes.
Friday, April 17, 2015
Time: 21: 20h UT
Seeing: II-III, fully clear sky.
I set up my fine 5″ f/12 achromatic refractor (Tiberius) as well as Octavius to again compare the views in each telescope when I pointed them toward Jupiter. I also compared the views of Theta Aurigae in both telescopes.
Will report back; a shadow transit has now commenced on the eastern limb of the planet.
Saturday, April 18, 2015.
Time 23:25 UT
Both telescopes easily split Theta Aurigae at about 150x. The refractor view had a darker sky background and the pair more stably presented. The Newtonian showed the pair more easily but less stably. The explanation is probably complex; the smaller aperture of the refractor and its greater thermal stability were probably at play here. But if I were not measuring the system, the Newtonian would be the better choice; simply because everything is brighter and better resolved.
I enjoyed a wonderful shadow transit and recorded my impressions of the event at 22:45h CM II 192 degrees.
Jupiter as it appeared in fair seeing in the 8″ f/6 Newtonian , 170x w/TV BPL on the evening of April 17.
Both the 5 inch glass and the Newtonian revealed the event well but the superior resolution of the Newtonian made it far more compelling. ‘Perfect’ linear features in the 5-inch come alive in the 8 inch speculum and were transformed into a sea of ‘imperfections,’ as a feast of micro-contrast details – entirely invisible in the refractor – popped in and out of view. And all the while, the image always flits in the Newtonian and I did experience a minor tube current that lasted for about ten minutes before it vanished.
A 8-inch f/6 Newtonian has not received much acclaim as a double star telescope but I wonder if this is yet another urban myth. I am very encouraged by the ability of the instrument to split some tricky double stars – I added Epsilon Lyrae (observed just after local midnight) and Izar to my tally this evening. I would like to flesh this interesting topic out further after I have performed more tests, but in the meantime I encourage you to look at Jeremy Perez’s website to see how well an 8-inch Newtonian can perform on binary star systems. I am also mindful of the work of the English amateur astronomer,Thomas Teague, who has measured many double stars with a quality 8″ f/5 Newtonian and a Celestron Astrometric Eyepiece.
I would now like to quantify the overall quality of the 8″ f/6 reflector in light of the modifications I have made to the instrument.
Here are some pictures of the modifications made.
Upgrading of the reflective coatings on both mirrors to 97 per cent. These should remain durable for at least 25 years, even in our rather damp climate.
Replacement of the secondary by a smaller one of semi-major diameter 44mm. Mirror edges blackened.
A new 44mm flat with edges blackened with matt black paint.
The interior of the tube opposite the focuser was flocked as well as the drawtube of the focuser itself.
Flocking the tube opposite the focuser is a good move.
These modifications give the instrument the contrast transfer of a 204* – 44 = 160mm unobstructed aperture – so a little better than a 6 inch refractor. Light gathering power will be significantly better than a 6 inch refractor though. ( *The diameter of the primary mirror as measured by Orion Optics UK)
The central obstruction does reduce the system Strehl in a quantifiable way (see below).
Graph showing the effect of a central obstruction on the Strehl ratio and Airy pattern.
Taking a 22 per cent central obstruction and reading off the graph we get a Strehl of ~0.9.
All in all, I have a very fine telescope in my possession, as experience at the eyepiece testifies.
The Skyliner 200P: £288 delivered.
Water Butt: £20.00
Flocking paper: £9.00 (including post and packaging)
Secondary mirror replacement with enhanced Hilux coatings ( including optical work to check surface flatness, post and packaging): £81.40
Primary mirror re-coating with Hilux (including post and packaging): £147.20
Total Cost: £550.60
Is it any wonder why I like to wax lyrical about Octavius!
My sister in law came to visit yesterday and my wife and I treated her to a view of Jupiter at sunset. At this precise time, the Great Red Spot was prominently on display and had just moved into the eastern hemisphere of the planet but the image was beautifully sharp. “Wow!” she explained, ” I have never seen the planet so well!
The superior coatings on the mirrors seem to have reduced light scattering in the image, improving the overall view.
When I ventured out later the same evening, I obtained a great and easy split of Iota Leonis at 339x. My first attempt at Delta Cygni was unsuccessful at 22:00 UT but it was rather low in the east at this time. When I tried again at 22:30 UT, the companion as well resolved at 225x.
Sunday, April 19, 2015
Yet another clear night tonight (remember you can always check my weather!) and time to explore some of the glories of the deep sky with Octavius.
I will report back on those activities later but for now I wish to share other user testimonies of the unmodified SkyWatcher Newtonian. I do not trust the judgement of those who express unbridled enthusiasm for apochromats. I believe their judgement in these matters is ‘coloured’.
Here are 12 independent assessments of the 8 inch Dobsonian for your perusal.
In addition, I would like readers to compare and contrast the Jupiter drawings of Dr. Paul Abel (mentioned earlier) – a highly experienced planetary observer – who regularly uses the same telescope (albeit entirely unmodified!) as my own.
You can see a drawing of Jupiter he made with his 8″ f/6 in good seeing here.
Now, compare that drawing to one he made with the venerable 6″ f/15 Cooke refractor at Hampstead Observatory, London, just a few days afterwards (also in good British seeing).
Would you concede that the drawings reveal similar amounts of detail?
Tuesday, April 21, 2015
21: 40 UT
The Lyriads have been dandy this year! Hoping for the best of them tomorrow evening!
I have been very busy preparing my students for their science and mathematics exams, which are starting in just a few short weeks. But Octavius is so easy to set up and get working that I can put him to good use more or less immediately after I finish with them. The key is preparation; leave the telescope out with its caps on for about an hour and you’re cooking with gas.
Sunday evening presented near ideal conditions and I set the telescope to work showing me many deep sky objects. As dark skies will only remain for a few more weeks, time is against me and so I will refrain from providing that report until I have visited all the targets on my list.
Last night was interesting. At 21:10 UT, I had both the five inch refractor and the 8″ speculum out in the open air and fully acclimated. The seeing was unexpectedly grotty though (Ant IV), as I discovered while looking first through the refractor. I had to look very hard to see the GRS in the long glass; the details having been smeared out by the turbulent air. Remarkably though, while the image in the Newtonian was roiling in the bad seeing, I could much more easily pick it out on the Jovian disk. A modest aperture advantage is a boon on planets like Jupiter even in rough seeing. A good lesson learned!
Beginning at 20:10UT this very evening, I uncapped Octavius, turned him toward Jove and was delighted to see that the seeing had once again improved. The telescope was placed on its Lazy Susan mount an hour earlier and I gave Ciaran, my final student this evening, his first ever telescopic gander at a beautiful crescent moon dominating the lower western sky and Venus to its upper right. But first, I turned the telescope on Jupiter, dialled up 170x in the Baader Hyperion zoom (with TV filter attached) and focused the planet in the centre of the field. Ciaran peered in and after just a moment of eye-brain coordination shouted out, ” That’s so amazing! It looks like a spacecraft image!”
And it was! No GRS this time but plenty of palpable activity across its vast globe. The planet literally comes alive in the large aperture Newtonian.
He asked to look at Venus and I obliged. “This will look different,” I explained, as I unscrewed the filter. Choosing a power of 150x or so, I centred the planet and invited him to take a look.
” Wow!, he exclaimed, ” its dazzlingly bright! What’s that cross spread over the disk?
“Ah, that’ll be the shadow of the telescope’s spider veins on the planet,” I explained, turning the telescope to show him the secondary assembly.
” Oh I see!”, he said.
We ended by taking a quick look at the Crescent Moon and though quite low down, was wonderfully sharp and filled with astonishingly fine details (magnification about 80 diameters).
“Oh my God!” Ciaran said, ” that’s so beautiful!”
They say that what goes into a person cannot defile them, but what comes out, can. Ciaran spoke from the heart and revealed the truth about this telescope. And he did not defile himself.
Wednesday, April 22, 2015
The spell of fine weather continues and the telescope performed flawlessly all night. Jupiter was magnificent. The Crescent moon was eye-wateringly beautiful! I was able to effortlessly push the magnification to 340x without any degradation of the image.
Collimation has held remarkably well thus far; f/6 is clearly a good place to be in this regard.
I visited a suite of double stars to assess the efficacy of the Newtonian. Eta Geminorum Iota Cassopeiae, Alula Borealis & Australis, Iota Leonis and later Izar. All of these are repeat observations – routine but important work in the true assessment of any instrument. All were easily split with the 8 inch speculum. I was most impressed at the calm and well resolved images of Iota Leonis at powers up to 500x. Out of curiosity, I threaded a neutral density filter onto the eyepiece and studied the images of Iota Leonis at 340x. The filter cuts off quite a bit of the glare and attenuates the bleeding of light from the brighter primary. The background sky was also darkened producing images that were incredibly refractor like!
The attenuation of light was modest though, as judged by my ability to easily detect (with the same filter in place) the light from the exceedingly faint (magnitude 10.1) companion to Alula Borealis. I tried this trick because of what I learned from reading the works of skilled observers from a by gone age – vis-a-vis on the large American refractors – and I believe it will be an exceedingly good and inexpensive way to help ferret out faint and tight companions to a legion of double stars within reach of my modern 8 inch reflecting telescope.
The air was so dry here that even though the telescope was left out for several hours, it did not dew up and even after I brought the instrument in shortly after local midnight, only the merest veneer of moisture settled on the outside of the tube; the mirrors being entirely dew free! That doesn’t happen very often round these parts.
My growing scepticism of many of those who have dismissed the Newtonian as a capable double star ‘scope impelled me to, once again, seek the knowledge of my ancestors. Though regrettably, their work has been summarily ignored by many of my contemporaries – I have ‘discovered’ for myself that truly dedicated observers achieved some mighty impressive results with instruments not too dissimilar to my own.
I speak especially of the work of the great British Victorian astronomer, Thomas William Webb (Anno Domini 1806-85), who resolved many sub arc second pairs with a Newtonian reflector – and had no reason to mislead others. Earlier this afternoon, I received a copy of a book chronicling his life and work.
Webb owned many smaller refractors in his early days – a 3.7 inch Tulley refractor, a similar sized Dollond and even a fine 5.5 inch Clark refractor – but eventually he settled on a 9.3 inch With-Berthon silver-on-glass reflector, which he used with great enthusiam for the last 20 years of his life. It is especially noteworthy that despite having the privilege of observing through similar sized refractors on magnificent equatorial mounts (owned by fellow astronomers in England, including those of Dawes and Huggins), he expressed no desire to acquire one of his own.
In the words of Robert A. Marriot, author of Chapter 8 of the book, The Stargazer of Hardwicke: the Life and Work of Thomas William Webb;
Having observed with these large telescopes, with which he was obviously impressed, Webb must have had at least some desire to own such an instrument himself, and yet for the last two decades of his life he was perfectly content to use his 9.3 inch With-Berthon reflector, which, although beyond his definition of a ‘common telescope,’ always served his needs ’till the dappled dawn doth rise’.
Believe me when I tell you that I know exactly how the Reverend Webb felt!
Saturday, April 25, 2015
Seeing III, excellent transparency, cool northerly airflow.
Octavius enjoying some late evening sunshine.
I am thoroughly enjoying the views of Jupiter with Octavius but the planet is now well past the meridian when full darkness falls upon the landscape. I can locate the planet just before sunset and with the TV filter in place can still make out some very fine details. The GRS is now in full view on the eastern hemisphere and can be very clearly made out with the generous aperture. Soon, I will show my boys the wonders of the first quarter Moon.
The boys were blown away by the clean, crisp images delivered by the Newtonian. They had great fun zooming from 50x to 150x with the Baader zoom.
I returned to Venus and enjoyed looking at its marble white, slightly gibbous form. 150x is about as high as I’d like to go on this planet. It’s hard to imagine that a world that looks so comely from our clement vantage is the most hellish place that one can conceive of.
I find the diffraction spikes on Venus to be a source of endless fascination. Like the presence of modest levels of secondary spectrum in achromatic telescopes, they have a singular beauty all of their own.
A Cruciform Venus
Sunday, April 26, 2015
Time 22:25h UT
Ambient: Seeing poor (IV), transparency excellent, temperature -2C, light north-northwesterly winds bringing in frigid polar air.
We had all four seasons in one day today. A freak Arctic blast of cold air brought hail and a light dusting of snow which quickly dissipated in the bright April sunshine.
Northerly air flows generally bring poor seeing to this valley and I could only manage 150x at the most on a handful of objects visited. Jupiter was poor this evening, the Moon reasonable at the lower powers. Caught a brief glimpse of Theta Aurigae’s faint companion though at 150x, which gave me a lift. All four components of Epsilon Lyrae also reasonably well (though still low down in the north east) resolved but the swollen seeing disks made the sight far less compelling. Ditto with Castor A & B and Gamma Leonis.
Called it an early night; glad to embrace the simple comfort of an open fire.
Thursday, April 30 2015
With every passing hour, my admiration for the Newtonian grows. Myths cultivated in refractor forums have served to steer people away from them too long and my contempt for those who advocate apochromatic refractors in the larger sizes grows stronger day by day.
Disillusioned by contemporary telescopic culture, I am finding myself spending more and more time reading the literature of older, accomplished observers. Just this morning, I spent some time leafing through Fred W. Price’s book, The Planet’s Observing Handbook. Like every other author of his generation (and many before him), he recommends – for sound, practical reasons – the Newtonian as the instrument of choice in the long term study of planets. The book is filled with beautiful drawings made with his own 8-inch reflector. There is something very pure and uncorrupted about his work that very much appeals to me.
In recent years, there has been a movement in the direction of ultra fast Newtonians, mainly to maximise aperture and portability. But not so long ago, opticians were making Newtonians with slower f ratios – in the f/7-12 range. With their very small central obstructions (CO) and amazing resolution and light gathering power, they would have beaten the pants off those pretentious large aperture Apos being offered to the unwary today.
Those who want that little bit more from a Newtonian would do well to consider models produced by smaller firms such as Mag1 Instruments, Orion Optics UK, and R.F Royce, to name but a few. Though more expensive than mass produced products, they are still far less pricey than their refractor equivalents and deliver excellent results that will serve you for a lifetime.
Personally, I am entirely sated by the performance of my 8 f/6 speculum, but I would love to see Synta or some such come out with a 8 inch f/7 or 10″ f/6 Newtonian models with enhanced coatings and a ~20% CO at a reasonable price. A big optics house like that should be able to pull this off without much alteration to its existing manufacturing ethos.
Check out this thread for encouraging signs of cultural change within the hobby.
Octavius: instrument of change.
Another very cold and clear night, 2C, bright gibbous Moon in the south, light northerly airflow. Seeing lower than average (Ant III-IV).
Conducted some more observations of Jupiter and a drawing made at dusk as it appeared at 20:45h.
CM II 271 degrees. Altitude ~45 degrees.
Giant Jove. 170 diameters w/TV BPL
I hope milder weather will come soon and with it the return of better seeing. Then I can begin investigating some sub-arc pairs.
Friday, May 1, 2015
Cornelia, my large Maksutov, has been put up for sale to raise funds to donate to the Nepal emergency appeal.
Saturday, May 2, 2015
Playing with numbers: Last night, the seeing improved quite a bit and I was treated to a beautiful image of Jupiter with the Newtonian at 170x despite its lower altitude. Later, I visited some of the show piece doubles in Bootes and Lyra. The Newtonian has thus far proven to be very powerful in this regard. All in all, I’m very impressed with its efficacy; quick cool down, excellent performance on the full panoply of celestial targets and relative insensitivity (compared with larger specula I’ve owned) to the vagaries of the atmosphere.
That got me thinking about an old chestnut oft pondered upon by amateur astronomers; is there, in any objective sense, an ‘optimum’ sized telescope for the pursuit of my double star adventures? Then I re-visited knowledge garnered by my diligent forebears.
According to the work of John Sidgwick in his Amateur Astronomer’s Handbook, the maximum useful magnifcation (M) is given by M = 28D^0.5 where D is expressed in millimetres. Setting D= 200mm gives M = 396x, so about 50 per inch of aperture for my 8″ speculum. This seems like a reasonable upper limit for most applications conducted by amateurs and over quite a wide range of fielded apertures.
The late professional double star astronomer, Paul Couteau, considered the minimum magnification needed to best see and measure doubles to be M~ > D, where D is again expressed in millimetres.
Since magnification M is F/f, where F is the focal length of the telescope and f is the focal length of the eyepiece we have:
F/f > D, so F/D > f. F/D is the focal ratio (relative aperture) of the telescope and the formula predicts that this minimal magnification will be achieved with an eyepiece of focal length approximately equal to the the f ratio of the telescope you use; an observation born out well in my field experiences over the years with many kinds of telescopes.
But over what range of apertures does this hold true? It stands to reason that Couteau’s minimum magnification ought to be less than Sidgwick’s maximum magnification. This can be expressed mathematically as an inequation:
D < 28D^0.5
Squaring both sides yields;
D^2 < 748 D
Thus, one solution yields D < 748mm.
That is a most interesting result, as 748mm translates into an aperture of the order of 30 inches! Bigger sizes are (presumably) too perturbed by the atmosphere to be used( without modern technology).
Can it be a coincidence that virtually all double star work has been conducted with instruments of this size and smaller?
I think not!
But, we may go still further. Is there in any sense an optimum sized telescope between zero and 748mm? Interestingly, we may also express this mathematically.
Specifically, what aperture, D, provides the biggest difference between the maximum and minimum magnifications employed, that is, what is the optimum value of D for the function arbitrarily defined as f(D)= 28D^0.5 – D?
Differentiating the function with respect to D and setting the result equal to zero yields:
f'(D) = 14D^-0.5 -1 = 0
Solving for D yields D = 14^2 = 196mm.
Thus, in this simple analysis, the optimum aperture turns out to be 8 inches!
Bigger is better, of course, but not nearly as often!
References: Argyle, R (ed.) Observing and Measuring Visual Double Stars (2012).
Tuesday, May 5, 2015
A spell of unsettled weather has befallen us. All day long, sullen rainclouds drenched the landscape with life sustaining moisture. It was good, proper rain.
I’ve become much more curious about the potential of the Newtonian to explore hitherto uncharted territories in my observing experience. One of the posters on this thread was kind enough to share his experience with me regarding his success with the sub-arc second pairs; Eta Coronae Borealis and Zeta Bootis, using a 8″ f/6 speculum.
Literature describing those kinds of activities conducted with Newtonian reflectors is well nigh scarce in comparison with the documented achievements of astronomers who have used the classical refractor. But I did find one interesting contemporary reference: Observing and Measuring Visual Double Stars (2nd Edition), 2012. In Chapter 11, written by Christopher Taylor, he crystallizes his thoughts on that very subject – The Newtonian Reflector in Double Star Astronomy.
Good with numbers, Taylor presents a proper (and I mean proper!) theoretical consideration of the Newtonian telescope in the pursuit of double star astronomy. Though he rightly acknowledges the superior stability of the images in long focus refractors, he nonetheless concludes that, given f/6 or slower relative apertures, “All the supposed optical defects of the reflector are removable or fictitious“, and further reminds us that, “a good 0.3m reflecting telescope is a far less expensive item than an equally good 0.3m refractor!“
But there’s more. Taylor, rather admirably, backs up his conclusions with the results of his own observational experience using a ‘disembodied’ (my choice of word) old 12.5 inch f/7 Calver reflector with a 16.3 per cent central obstruction, erected in a “good spot” in his garden in rural Oxfordshire. There is no tube; the mirror is completely exposed to the night air and, perched seven or so feet above it, sits the secondary assembly. The mount, massive though it is, turns out to be an altazimuth! LOL!
The magnificent apparatus weighs in at an incredible 1500 pounds (680 kilos)!
On page 140, he describes his achievements with this mirror with which he employs a power of 825x on nights of excellent seeing. Though he does not mention the frequency with which he enjoys such nights, Taylor presents tables and sketches of drawings he made of exceedingly close pairs like Delta Equulei and Beta Delphini (and many others); pairs that have orbital periods of just a few years or decades! Taylor describes the essence of why these investigations are so invigorating:
An unforgettable demonstration of this was provided by the 2005 periastron passage of the famous 169-year pair gamma Virgini…. In May of that year, gamma Vir swept through 0.3 arc seconds approach of the two stars with an apparent rate of revolution which briefly peaked at 2 degrees per week! This superb phenomena was closely followed by the author’s 12.5 inch right through closest approach, at which it was still found possible to make formal measures of p.a., yet it appears that sadly, few observers equally well equipped saw much of this spectacular double star event of the century.
His enthusiasm has rubbed off on me. I understand why he felt so passionate about seeing these things; for like ships passing in the night, it is only in the sub-arcsecond realm that amateurs can truly hope to enjoy a ‘ringside seat’ on the dynamics of distant suns sailing far beyond the Solar System.
We cannot perceive what we cannot conceive.
Friday, May 8, 2015
Even though I have quite an extensive library on astronomical topics, of which I am very proud of, I came to realise that it was heavily biased towards the classical refractor. But I have become much more interested in the work of historical figures who used the power of the speculum mirror to advance the cause of astronomy. Accordingly, I have began to redress that imbalance by buying up several books – old and new – chronicling the lives of the Herschels who brought the reflecting telescope to the fore.
The Herschel Collection.
Saturday, May 16, 2015
I have not had much time to use Octavius of late owing to my teaching commitments. But I hope to redress that soon. I have examined quite a few deep sky objects on and off over the last month and will report back on my findings in due course. The length of true darkness at this latitude is now very short, heralding the onset of summer twilight which will persist until late July.
I have been reading some of the articles of Sky & Telescope associate editor, Gary Seronik, who is a long-time Newtonian enthusiast. In particular, I found Gary’s article on his 6 inch optimised Newtonian to be a good read. Seronik shows us how to get top quality views from a Newtonian at a fraction the cost of a refractor of the same aperture. And he derives even greater pleasure from the fact that the telescope is entirely home built.
Perhaps most importantly though,Seronik reminds us of the importance of aperture, noting at the end of that article that despite building and using a super duper 6 inch f/9 Newtonian, a 8 inch f/6 is, in his own words, “crushingly superior” on all targets.
No matter how much you mollycoddle a telescope, increasing its aperture is a better way to go.
Monday, May 18, 2015
CN account un-suspended but I will not be contributing there any more for personal reasons.
Friday, May 22, 2015
More musings on numbers: This thread has alerted me to the joy of establishing a theoretical basis for my experiences with Octavius. ‘Aperture’, you have heard, ‘wins’. Many know this to be true, as I do, within certain limits. But is there a physical basis for believing this? Why does resolving power scale inversely with aperture?
This leads us to Airy’s result for a circular aperture where he showed that the resolving angle, theta = 1.22 lambda/D, where lambda is the wavelength and D is the diameter of the aperture.
The derivation of the Airy formula is really quite involved but an email from a gentleman in the States (thanks Jim!) prompted me to look at a simpler approximation based on one rendition of Heisenberg’s Uncertainty Principle. Airy himself did not have knowledge of such physics, as this was only developed in the early 20th century. What follows is an elaboration of the approach of Dick Suiter, in his book, Star Testing Astronomical Telescopes.
Starting with the momentum/ position inequality.
delta p x delta y~ h where delta p is the ‘fuzziness’ in the momentum in the y direction and delta y is the fuzziness in position y. h is Planck’s constant.
Rearranging this formula we get;
Delta p/p ~ h/(p x delta y)
The momentum of the photon can be expressed as E/c and since E = hf where f= c/lambda we get E = hc/lambda (wavelength). Delta y can be considered to be the diameter of the circular aperture, D.
So p = E/c = hc/(lambda x c) which simplifies to h/lambda
so delta p/ p ~ h/ (p x delta y) ~ hc/fhD and since c/f = lambda the formula reduces to lambda/D.
Thus, the resolving angle, theta, or resolution of the telescope, can be expressed as
theta ~h/(p x delta y) = h x lambda/ (h x D) or just lambda/D.
It is interesting that this approximation is very close to Airy’s result of 1.22 lambda/D!
So it really is true; larger aperture resolves finer detail!
What is less well known to amateurs is that resolving power scales with wavelength, as shown. The shorter the wavelengths observed under, the finer the detail we ought to see.
Octavius has eight inches of aperture; Tiberius, only five!
Wednesday, May 27, 2015
With the chemistry examinations taking place tomorrow, my academic work has come to an end for another year, so I have more time to finish this particular blog.
So, let us reason again, you and I.
I have already stated that I believe Octavius would give very similar planetary views to a 6- inch refractor and have a light gathering power at least equivalent to, or better than, a 7-inch refractor.
One other way to establish the veracity of these claims is by comparing CCD images of a tough target like Jupiter.
In this capacity, I searched tinternet for comparison images taken with a 8″ f/6 Newtonian and a refractor that has garnered quite a bit of prestige in the amateur world; the TEC 140 (a 5.5 inch triplet refractor).
Here is the best sequence of images of mighty Jove I could find taken with the TEC 140.
Now, please compare these images with those taken with an unmodified 8″ f/5 SkyWatcher Newtonian. You can see some good ones here.
I have no reason to suspect any foul play, as both imagers would have striven to show off the best attributes of their instruments. After all, ’tis only human nature to do so!
Would you say that they are broadly equivalent?
I think the TEC images are a bit over processed and were probably (that’s a hunch, I can’t prove it however), captured using a more sophisticated CCD camera than that used by the owner of the Newtonian. The f/5 SkyWatcher Dob was completely unmodified though; so it will have a (larger) central obstruction and standard mirror coatings than my own telescope.
And yet it produces similar or even greater amounts of detail than the prestigious TEC!
But let’s now factor in the cost of these telescopes;
Suppose I were to order up a 8″ f/6 Dob from this retailer.
And then buy the TEC 140 (optical tube assembly only) from the same retailer.
The price differential is 20x!
Is there something I’m missing here?
Mein Gott im Himmel, ich verstehe nicht!
The ‘majesty factor’ perhaps?
A more ‘transcendent’ image maybe?
I can’t quantify either of those things, unfortunately, but I’m sceptical, as I know, having owned my speculum for a few months now and having personally tested many refractors of similar or larger size over several years, that both types of telescopes produce beautiful images of planets.
Octavius, with its 22 per cent central obstruction and light gathering power broadly equivalent to a refractor of the same size, must be a better all-round telescope than the famous TEC 140. Not just because I want it to (I do admit to disliking these telescopes for logical reasons); but because it has to!
Octavius the Meritorius!
Sunday, May 31, 2015
A note on further experiments
The primary mirror of the 8-inch SkyWatcher is very good. Very smooth optics with no astigmatism. I have experimented with removing the mirror from the cell a few times now to explore the effects of tightening the three clips that hold the mirror in its cell. A word of caution:-
Over-tightening the mirror will induce marked pinching in the optics that can ruin high power images.
I have found that they should be tightened just enough so as to hold the mirror rigidly in place but only so tight as the clips can still wiggle a little. The difference between getting it right and over doing it is like night and day!
Is your primary over tightened?
I have not thought much more on improving the thermal properties of Octavius because I could possibly be blissfully happy with its native performance without a fan. But I have learned that in days gone by some reflector enthusiasts have lined their tubes with cork or, more recently, with polystyrene flocking.
Will I give this a go?
Maybees aye, maybees naw!
Some notes on low power eyepieces:
I’m not one to fuss about eyepieces, especially these days when even inexpensive stuff is so good. Of all the astronomical kit discussed by amateurs, it is arguably eyepiece discussions that produce the most heat and the least light. Don’t get sucked down that black hole what ever you do LoL!
Since Octavius is destined to become my most used telescope in the coming years, I have thought some more about getting the best bang-for-buck, low power ocular for Milky Way sweeps and for framing the largest deep sky objects. At f/6, the 65 degree AFOV Erfle gives good performance but my attention was piqued by a new eyepiece marketed by Explore Scientific. As part of their Maxvision range, the company offer a 40mm model with improved coatings and eye relief, an adjustable eye cup and a larger 68 degree AFOV.
The Antares 40mm Erfle (left) and the Explore Scientific 40mm eyepiece.
After testing the unit for a review (you can see that in Astronomy Now magazine) I decided to buy one as it offers enough of a performance boost to justify the switch, eventhough it weighs in at nearly three times that of the Erfle (1.25 kilos LoL)! And at £117 plus shipping, it wasn’t an extravagant splurge. Now I can play with a 2.25 angular degree field with my 8-inch speculum. M31 here I come!
Testin’ an’ that.
Man and his improving technology!
Tuesday, June 2, 2015
As I briefly touched on previously, some amateurs have found that using a fan which blows air over the primary to be a useful tool in bringing the optics to thermal equilibrium with the outside air. Where I live though, there are never great temperature swings at any time of the year and my months of testing suggest that it would not really be necessary.
I’m a very lucky man!
Besides, I am not overly enamoured about the prospect of having some electrical appendage grafted onto Octavius. It just wouldn’t feel right to me.
Wednesday, June 3, 2015
I would like to give you more.
I would like to describe my deep sky experiences with Octavius over the last few months.
Thursday, June 4, 2015
Octavius is a very impressive deep sky telescope. With a good 40mm wide angle eyepiece, I can coax fields of view in excess of two angular degrees out of it. On the evening of Sunday, April 19, I caught sight of the Pleaides (M45) just above the north-western horizon, where it seemed to hover above a distant rooftop. Excitedly, I trained my 8 inch speculum and focused the stars down to tiny, crisp pinpoints. I was delighted to see that the entire cluster fitted neatly into the field of my six element Erfle! I have since replaced this unit with a significantly better Explore Scientific Maxvision ocular offering a larger field of view (~2.25 degree) with slightly better contrast and edge-of-field correction. From a dark and transparent sky, my 8-inch speculum provides lovely, contrast-rich views with only a slight drop off of light at the extreme edges of the field. This edge-of-field vignetting is entirely acceptable though.
The improved coatings on both mirrors came into play when I visited the great globular cluster, M13 in Hercules. I could push the telescope to 200x before the image became too dim. The view was spectacular though, more reminiscent of what you would get with a standard 10-inch Dob. The cluster was beautifully resolved with hundreds of stars cleanly resolved to the core. The widely reported ‘propeller’ streams curving out from its main body were very obvious with a direct gaze. When I swung the telescope over to M92, I wasn’t disappointed either; a veritable swarm of white stars at 200x, more condensed than M13 but still well resolved almost to the centre.
Spring galaxies also benefited from the improved coatings. The celebrated M81 and M82 were easy pickings for this telescope, with both ‘Island Universes’ revealing excellent structural detail at powers of 150x or so. The improvement was obvious compared to a 17cm Maksutov set up along side it.
More exciting still, I could more easily trace out the spiral arms of the Whirlpool Galaxy, M51, in the constellation of the Hunting Dogs. The difference between the views in the smaller Maksutov and the larger Dob was subtle but quite obvious. The same result was observed when I swung the telescopes down to the Sombrero Galaxy (M104) in Virgo. The greater light grasp and resolving power both came into play to deliver a noticeably superior view in the Newtonian than in the Maksutov, where it was easier to see the prominent dust lane bifurcating this classic edge-on spiral at powers of 150x or so.
Open clusters are a pure joy to observe with Octavius. The Double Cluster (C14) in Perseus was beautifully framed at 30x. I spent a good twenty minutes in the wee small hours of Monday, April 20, in sheer awe observing this spectacular communion of stars. The superior light grasp of the reflector over either my 17cm Maksutov (now gone) or 127mm refractor was painfully obvious, my eye being able to differentiate white, blue-white, creamy, orange and downright ruddy constituents at moderate powers (100x or so).
Having become intimately familiar with the appearance of the three bright Messier clusters in Auriga in my 5-inch refractor, I was simply stunned at the difference manifested in the 8 inch speculum. While they are beautiful in the 5 inch glass, the view of M36, M37 and M38 in the larger speculum are quite simply in a completely different league!
The same was true when I compared and contrasted the views of the famous planetary – the Eskimo Nebula (NGC 2392) in Gemini – using the 8-inch speculum to my smaller Maksutov and refractor telescopes. The superior light grasp and resolution of the Newtonian allowed me to push the magnification to significantly higher powers. I found 250x presented the best views, with the bright, 10th magnitude central star being clearly differentiated from not one, but two diffuse, concentric shells of luminous matter. In contrast, the smaller ‘scopes maxed out at powers at or slightly below 200x. Shell detail was also a notch down in these smaller instruments.
All in all, I am positively delighted with the deep sky views served up by my 8-inch light bucket. From my relatively dark country sky, it will provide a lifetime of wonderful sights to visit, some for the first time, but many to be returned to again and again with the passage of the seasons, like faithful old friends.
Haste ye back the dark skies of August!
Friday, June 5, 2015
Having sold off some gear that I no longer need, I raised some more funds to purchase a better finder for Octavius. Nothing fancy; a SkyWatcher 9 x 50 right angled model, which can be bought new in the UK for a very reasonable price (£59). Such an upgrade will enhance my observing experiences by reducing neck strain and should make finding objects that little bit easier.
With June now upon us, the skies never get truly dark here. At such times, I turn my attention to double stars since they are little affected by twilight. Summer can present quite excellent seeing here (as revealed in many previous blogs), especially if its settles down for any length of time. I am very excited about examining progressively tighter pairs to see how much I can push my 8 inch speculum. Once this study is completed I shall bring my public telescope reviewing to an end. I pray that the weather will be kind to me in the coming weeks.
Saturday, June 6, 2015
Time : 00:05h BST
Ambient: 10C, brisk westerly wind, good transparency. Seeing fair to good (Ant II-III).
I got off to a solid start this evening. Octavius was placed outside and left to completely equalize with the night air. No dew as the wind is too strong.
Izar ( Epsilon Bootis) beautifully split at 225x. Faithful colours – yellow and blue.
Epsilon Lyrae 1 & 2 easy at all powers above 100 but best seen at 225x
Alula Australis: lovely near equal magnitude split at 225x
Delta Cygni; easy pickings at 225x
Zeta Herculis – unsuccessful, strongly elongated at 225x and 340x. Will need calmer conditions to bag this puppy.
00:30h: The bonnie triple system, Iota Cassiopeiae, beautifully resolved at 225x, despite its fairly low altitude in the northern sky.
No’ bad ken.
Telescope put away. Time tae hit the hay.
21:00h BST: Heehaw else tae report today.
Dreich weather ken.
Get it roond ye!
Sunday, June 7, 2015
Time: 19:50h BST
The Lord works in mysterious ways. After a day of downright rotten weather, today has been much better and this evening promises more clear skies! I have no idea how good they will be but at least I’ll get a chance to field my best telescope once again.
In addition, it occurred to me that I have another witness to my own weather conditions, someone impartial, who lives near me, and has been a contributor to this blog. He can vouch for my blue skies.
Indeed, maybe the same gentleman can vouch for most, or even all the observing-related weather entries I have made on this blog/review since its inception at the end of January last?
Hey, how lucky am I?
Just a few hours to go before my next adventure.
Fingers crossed eh!!
A White Knight upon a Dark Horse.
Ambient; westerly wind has all but abated at ground level but with some isolated cloud patches moving swiftly across the sky at high altitudes. The cursed wee midgees came out earlier but now the temperature has fallen to 7C, so not likely to pose a problem going forward. Some good images of Arcturus.
More cloud encroaching, seeing a notch or two down on last night (Ant III). But still managed a clean split of Izar at 170x. Seeing disks more turbulent tonight though. Ditto for Epsilon 1 & 2 Lyrae. The quartet were fairly stably held at 340x but with frequent morphing of the image in and out of focus as the stars swept through the field. Don’t see much point in continuing higher resolution targets tonight. Need to wait for better nights.
Monday, June 8, 2015
A Note on Collimation:
As anyone who uses a Newtonian regularly will tell you, alignment of the primary and secondary mirrors is absolutely critical to gaining the best possible images. Some folk get in a right guddle with this procedure though, while others border on being OCD, checking it two or three times per night. LOL!
As briefly touched on earlier, collimating a f/6 Newtonian is fairly straightforward and is rendered almost completely pain free using an inexpensive laser collimator (I use an inexpensive SkyWatcher unit). F/6 or slower systems can be accurately aligned during the day but I tend to always back this up with a star test before beginning serious observations.
You can find a very good demonstration of how to quickly collimate your Newtonian here.
I would say however that it is better to do the collimating on a horizontal bench/table so as to avoid the small risk of one of the Allen keys falling down the tube and hitting that all-important primary mirror – yikes!
Looks like the weather is good again for tonight, so perhaps some better opportunities to split some tougher pairs.
New finder should be with me tomorrow- yeehaw!
At last, a fairly warm and bright day. And the day has given way to what looks to be a clear and tranquil night.
I walked through the garden in the cool of the evening. With a song on my heart and a spring in my step, I retrieved Octavius from my office and set it out to cool.
The warmer temperatures have brought out more insects. The swallows feast on them high in the sky – a good sign that fair weather is here for a while. Nearer ground level, the bats are busy too. No wind. Only the sound of a distant cuckoo breaks the silence.
Will report back later.
Ambient: 8C, twilit sky, clear and calm, seeing appears very good (Ant II).
I have just come in from examining the images of my first two targets.
Epsilon 1&2 Lyrae: Magnificent tonight! Much greater stability to the images, four tiny Airy disks surrounded by well structured diffraction rings at 340x.
Delta Cygni: Companion very easily seen and perfectly formed at 340x!
Just waiting for the sky to get that little bit darker……
Tuesday, June 9, 2015
Lambda Cygni resolved tonight! Couldn’t see anything except the primary at 340x but the image was calm and very well defined. I ran to the office to get my 1.6x Barlow. This yielded a power of 544 diameters. Swung the star to the eastern edge of the field, refocused until it was at its best at the centre of the field – both members (0.9 arc seconds) well resolved! Mirror holding up really well at these very high powers!
8-inch Newt owners please give this system a try, and, if successful, post it where everyone can see it!
Will talk more about it in the morning.
My new finder arrived today. Looks cool!
A nice comfortable finder for Octavius.
I had to get up early and leave for the city this morning, so was not able to make any more observations last night. But that’s beside the point; my modified 8 inch f/6 Newtonian resolved a sub arc second pair – Lambda Cygni – and with relative ease!
Why was I astonished by this result? Well, for one thing, being used to the domination of this arena of observational astronomy by the classical refractor, it was quite a reality check to see that a Newtonian could perform so well in this regard. But there is absolutely nothing in this observation that violates any known principle of physics; I mean, according to the Dawes limit (4.56/D) my Newtonian ought to do considerably better – down to 0.57 arc seconds.
I know from previous experience with a 17cm Maksutov that in some locations I can split pairs as low as 0.7 seconds of arc. And if I can do that with a Maksutov, I can also do it with a Newtonian. Let’s just say I have faith in Octavius; not a blind faith but one that is based on reason and experience.
The fine weather is continuing but this would be a suitable place to finish.
I commend this economical telescope to you and invite you to test everything I have said about it.
It is, by some considerable margin, the best bang for buck in the entire hobby! It is a most excellent, all-round performer, an instrument that will serve up a lifetime of wonderful views and require little in the way of maintenance.
Three Cheers for Octavius: the People’s Telescope!
Update: September 27 2017
It is exceedingly rare that an individual will openly admit the true performance of a 8″ f/6 reflector compared to a ‘premium’ medium aperture apochromatic refractor. But I found one such assessment here. You will note that the gentleman in question compares the performance of his 8″ f/6 Newtonian with a TEC 140. This is what he said (I assume English isn’t his first language) about a comparison between the two instruments on Jupiter;
There is similarity with my 200mm Newton: both instruments encourage using high magnifications. For these average conditions, if my Newton allows more megnification ( about 285x) and makes detection of little contours of faint contrast easier, then there is no major difference between both scopes……..By exceptional conditions (no turbulence) I remember my 200mm going further in magnification (400x) and detection of colours and contrasts….
Then speaking of deep sky objects such as M13, the gentleman admits the 200mm Newtonian offers better resolution than the TEC 140:
M13: Splendid and perfectly contrasted. Periphery resolved but core remains ‘milky’ or granulous. Newton 200mm allows more resolution.
All in all, I feel this is an honest and realistic assessment of both instruments compared to the hyperbolic and emotionally charged statements of the somewhat blinkered (or blinded?) opinions of refractor only fanatics. This author is only too happy to test any similar sized premium refractor against his own superlative 20.4cm f/6 Newtonian.
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