5-inch Shootout: 5″ f/12 Refractor vs a 5.1″ f/5 Reflector

Battle o’ the 5-inchers. Tiberius (laevo); a 127mm f/12 achromatic refractor versus Plotina; a 130mm f/5 Newtonian reflector.

 

Introduction: Many telescope reviews conducted on forums or in magazines only assess a single instrument, namely the one under consideration, entirely on its own terms. But while such informaton can be useful, particularly if a fault is discovered, it can be somewhat misleading if no other instruments are compared with it. Take for example, a top drawer 60mm refractor, which produces excellent images within the remit of its aperture, but when it’s compared to a slightly larger telescope of average quality, it begins to show its limitations and the tester gains a much more balanced view of its strengths and weaknesses.

I find myself thinking this way when evlauating all the telescopes that pass through these parts. Such tests are very important and completely warranted. For example, I was once very much enamoured by an expensive 4-inch F/5 Televue Genesis fluorite refractor but quickly fell out of love with it once I compared it to an even more expensive Televue 102 apochromat. In turn, the latter telescope was found to be slightly inferior to a SkyWatcher ED 100 f/9 refractor costing far less than either of the Televue refractors, which left a very bad taste in my mouth, making me deeply suspicious of the claims proferred by those who market so-called ‘premium’ telescopes, as well as the forum fanboys who apparently cannot see beyond them.

But sometimes it pays dividends to compare good telescopes from different genres too, such as my discovery that an 8-inch f/6 Dobsonian proved superior to a 7-inch f/15 Maksutov Cassegrain, even though the former was less expensive. Clearly, you don’t always get what you pay for! You need to find the truth for yourself.

In this capacity, I decided to compare and contrast the capabilities of two very different telescopes of similar aperture; a 127mm f/12 achromatic refractor and a 130mm f/5 Newtonian reflector costing many times less.  I have described the capabilities of this refractor in many previous blogs (now archived by the author). I have retained it as an excellent example of a historically important class of telescope that allowed amateur and professional astronomers to make great strides in understanding the Universe around them and which continues to provide excellent insights into their considerable capabilities. Interested readers will find a veritable treasure trove of classical achromat ‘culture’ in the author’s up-and-coming book, Chronicling the Golden Age of Astronomy, due out in late 2018.

But having said all of this, the Newtonian reflector has been terribly neglected by a generation of amateurs that seem to know the price of everything and the value of nothing. Unwilling to take their cue from the professional community, who have long left the refracting telescope behind in favour of the enormous advantages offered by modern reflective optics, they continue to disseminate misleading or downright false information to unsuspecting newcomers to the hobby, who are subsequently led astray in such a way that their progress as observers becomes severely stunted. That’s why it’s important to continue to question received opinion. If we stop questioning, we quickly become part of the herd culture that so typifies contemperary amateur astronomy.

The instruments compared: The refracting telescope is a high-quality neo-classical instrument (doublet objective) with an aperture of 127mm and focal length of 1524mm (so f/12). The optical tube assembly is about 1.8m long and weighs in at 40 pounds. It sports a fully multi-coated object glass which passes virtually all the light that passes through it. It has a state-of-the art,  dual speed Moonlite focuser, which is fully rotatable and extremely robust.

The beautiful objective lens on the Istar Asteria 127mm f/12 refractor..

 

The wonderful two-speed Moonlite focuser on the Istar refractor.

Such a bulky instrument requires a substantial mount and even when provision is made for its mounting (with its various counterweights)  it can prove very awkward to use in the field, particularly when the instrument is pointed high in the sky.

The Newtonian, in contrast, even with its dovetail plate and finder attached, is featherweight in comparison. Both primary and secondary mirrors possess quality, high-reflectvity coatings, reflecting 97 of the light incident upon them and with a small 27 per cent central obstruction with its upgraded optical flat, it loses very little light to deliver tack-sharp, colour free images with high contrast. It’s focuser, however, is of the simple, single speed, rack & pinion variety. It needs to be accurately collimated for such testing but this requires just a minute of one’s time to attain perfect results.

The innards of the 130mm F/5 Newtonian reflector.

The simple rack & pinion focuser on the 130mm Newtonian.

 

Test 1: Comparison of the high magnification images in daylight; conducted August 21 2018.

Both telescopes were set up on their mounts (the reflector was mounted on a simple Vixen Porta II alt-azimuth) during a warm, overcast day and the instruments charged with a high magnification (~ 50x per inch). The refractor delivered a power of 277x, while the reflector yielded a magnification of 283x. Both telescopes were aimed at the topmost bough of a horsechestnut tree some 80 yards distant and the instruments carefully focused.

Results: Both instruments served up sharp, detailed images of the well-developed foliage. The Newtonian was much harder to focus accurately owing to its fast f ratio(5), in comparison to the refractor (f/12). They were very comparable in terms of image brightness but the reflector showed a consistently better image. It was a shade sharper and completely devoid of chromatic aberration (CA). The refractor did show some CA in comparison, which manifested a faint chromatic fog, lowering image contrast and sharpness.

I called two other visual testers to the telescope; my wife and my my next-door neighbour, Trevor. Teaching them how to focus the telescopes finely, I let them examine the images in both telescopes for a few minutes, eventually enquiring of them which instrument delivered the better high power daylight views.

Their verdict was the same as my own, namely, that the reflector delivered the better image of the tree-top foliage.

Trevor points to the telescope offering the superior daylight images.

 

Comments: It might have been anticipated that the refractor would offer the brighter image, but CA takes some of the unfocused light and spreads it around the field, slightly lowering the overall brightness of the focused image.

Test 2: Double Star Performance: August 22 2018

Tiberius (laevo) et Plotina; fratrem certamen accendebant.

Wide field performance tests aborted until the Moon was out of the sky. Some double stars were critically examined instead.

Conditions: Brisk southwesterly winds, partially clear, visibility rather poor except near zenith. Temperature + 10C, rather cooler than of late.

Both telescopes were set out to cool from the late evening (19:00 UT) onwards, so completely acclimated to their environments.

4 systems examined at high powers (260x  and 277x on the 130mm f/5 and 127mm f/12, respectively):

Epsilon Lyrae 1 & 2

Delta Cygni

Pi Aquilae

Mu Cygni

Results: Only a brief observing spell possible with by telescopes this evening between 21:00  and 21:25UT as low cloud moved in and made the sky increasingly difficult to navigate. By 22:00 it had all but competely clouded over again.

Both telescopes resolved all four systems well at the powers mentioned above. The breezy conditions and the high altitude of three of the test systems made it very challenging to observe in the refractor owing to its long tube and positioning of the eyepiece very near the ground (a Televue 2-inch EverBrite dielectric diagonal being employed to make observations easier). The same systems proved far more comfortable to observe in the Newtonian, for obvious reasons. The long refractor really needs a massive equatorial mount to do it justice; something I am not interested in pursuing.

The refractor definitely pulled ahead though in terms of ease of focus of the subjects (at f/12 you’d expect that), while using the reflector with its simple rack & pinion focus and f/5 relative aperture was always much more challenging. Indeed, I had forgotten the considerable advantages the classical refractor has over faster systems in this regard. The refractor images showed little in the way of diffraction artifacts, the stellar Airy disks being very tight and round as buttons. Contrast was a tad better in the unobstructed refractor and the images were marginally more stable as judged by their reduced tendency to morph out of perfect focus as they moved across the field. That said, I was very pleased at how well the reflector held its own; the more prominent diffraction rings having no discernible effects on the resolution of these point sources. And while contrast was a shade better in the refractor, I did not judge it superior enough to warrant a discontinuation of my double star adventures with the 130mm Newtonian. Its wonderful comfort is a huge virtue in this regard.

Comments: The CA described in the daylight tests had no effect on the resolution of these test doubles (an observation well borne out by an enormous body of historical literature), although their colours were slightly distorted (yellowed) compared with the Newtonian(which by nature always delivers true colour images). Indeed, the secondary spectrum was only slightly apparent on Delta Cygni A, being quite a bright star. What is more, I felt it added slightly to the aesthetic appeal of the refractor image over the reflector, but this is a completely subjective judgement.

Test 3: Deep Sky Capability: September 6 2018.

The sloth discovers heehaw….ken.

 

Although the last three nights have been excellent for deep sky observing, I decided to leave this test until the evening of September 6 2018, to make sure no moonlight interfered with the observations. Still, the effort was very rewarding and insightful. As you can imagine, these instruments are very different beasts in regard to their demands on eyepieces. At f/12 even cheap wide angle oculars behave like champs from the centre to the edge of the field and this means that one does not need to splash out relatively large sums of money for well corrected deep sky views using heavy 2-inch oculars. The maximum true field that can be achieved with this refractor from my eyepiece arsenal is 1.79 angular degrees, power 38x. In contrast, the much faster f/5 optical system in the Newtonian requires better eyepieces that can correct for the significant off axis aberrations including coma, astigmatism and field curvature etc. But it is able to deliver a considerably larger true field than the refractor (2.3 degrees with a Celestron X-Cel LX 25mm and 2.5 degrees with a standard 32mm Plossl, though with inferior correction towards the edge of the field).

The closest match I could make to the 38x of the refractor was to couple a 1.6x Barlow to the Celestron X-Cel LX 25mm yielding a 1.44 degree true field and a power of 42x.

Low power, wide-field oculars used in the tests; a 40mm ES Maxvision and a 25mm Celestron X-Cel LX coupled to a 1.6x Barlow. The oculars yield 38x and 42x in the 5″ f/12 glass and 130mm f/5 Newtonian, respectively.

 

For higher power, deep sky comparisons, I employed a 11mm ES 82 ocular in the f/12 refractor and a 4.8mm T1 Nagler (also 82 degree AFOV) in the 130mm reflector, delivering very comparable powers of 139x and 135x, respectively.

The 4.8mm T1 Televue Nagler ( left) and the 11mm Explore Scientific 82 degree ocular delivering 135x and 139x in the Newtonian and refractor, respectively.

 

Results: Just two targets were examined: M13 in Hercules and The Double Cluster in Perseus. In the low power setting, the refractor offered a slightly punchier image of M13, with slightly greater contrast (darker sky background) than the reflector. The faintest stars in the field were just a tad easier to discern in the big glass than in the Newtonian, but otherwise they were very comparable. At high powers, the results were broadly the same; with the nod going to the refractor, but I was very impressed at how well the little reflector did. If I were to quantify the difference I’d estimate that the 5″ f/12 delivered maybe a 5 to 10% improvement over the reflector on this remote target.

Turning next to the Double Cluster, I returned to lower power. Going back and forth between the images, the views were more comparable than they were different. Contrast was a little better in the refractor, with beautiful pinpoint stars strewn all across the field. The reflector gave almost the same results, with slightly less contrast and colour saturation. The refractor did however pull significantly further ahead at the edge of the field with tighter, better corrected stars, quite in keeping with its f/12 native focal ratio.

Conclusions: This series of tests, comparing two very different instruments of broadly similar aperture is almost never done by amateur astronomers. Doubtless, part of the reason for this is that no one wants to be told that a very expensive refractor could be rivalled by a far less expensive reflector on the same targets. And yet, apart from the clear superiority of the reflector during daylight use, this is very much the conclusion I was forced to draw; the views are very comparable. I believe the results are attributed to the superior coatings on the mirrors which collect very similar amounts of light as well as the relatively small central obstruction in the reflector which tends to keep image contrast high. Had the Newtonian possessed standard coatings, I believe I would have reported a larger difference in their performance on deep sky objects. This, together with very close attention to attaining perfect collimation in the Newtonian readily explains why it performed so well  in comparison with the refractor on high resolution point sources, such as double and multiple stars. At the very high powers employed in test 2, the coma free field is much reduced in the reflector, allowing good images to be maintained from the centre of the field to its periphery.

But in all such comparisons, it pays to also consider the comfort factor; that is, how easy the instruments are to transport, mount and manoeuvre in field use. This is where the Newtonian rocks in comparison to the refractor. It is quite simply a joy to use; no hunching behind an eyepiece very low to the ground, no need to re-balance the telescope when it’s pointed to targets of greatly different altitude etc. The small advantages the refractor has over the reflector pale into insignificance when these considerations are accommodated.

Newtonians have clearly come a long way; with modern high reflectivity coatings, quality primary and secondary mirrors and careful attention to collimation and cooling, they compete very favourably with refractors, at a fraction of the cost. I hope you can appreciate why I almost always reach for the little 130mm reflector in comparsion to the refractor. Granted, the latter may look more majestic in the cold light of day, but all this is quickly forgotten under a clear, dark country sky.

 

 

Neil English explores four centuries of telescopic astronomy in his ambitious new work (772 pages), Chronicling the Golden Age of Astronomy, due out in October 2018.

 

 

De Fideli.

4 thoughts on “5-inch Shootout: 5″ f/12 Refractor vs a 5.1″ f/5 Reflector

  1. I very much enjoyed this comparison by Neil English and it has certainly changed my outlook on Reflector vs Refractor; my being very new to high power telescopes.

    Thank you very much for such an insightful article, Lanny Walker

    • Hello Lanny,

      Thanks for your post. You are most welcome indeed! It’s about time such testing was done!

      Best wishes,

      Neil.

  2. An intriguing comparison. Always find your writings fascinating. My trouble is that I prefer refractors because I think lenses are beautiful, both to look through and to look at. Romantic old fool.
    Thanks Neil, keep up the fine work.

    • Hello Ian,

      Many thanks for your message and thank you for the compliment.

      I find it very insightful to compare apples and oranges like the tests done in this article, as one can never really see the virtues of one type of telescope over the other without doing such comparisons.

      Tests like these are rarely or never done by other reviewers.

      For me, the optical advantages of the 5 inch refractor are too small to justify spending the much larger sums of money incurred in acquiring the refractor over the reflector. But an equally important factor is ease of set-up and comfort in field use, both of which give a decided nod to the lighter and more compact reflector.

      Small refractors are very versatile instruments though. That’s why I still enjoy using my ShortTube 80 f/5 achromatic telescope.

      Kind regards,

      Neil.

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