Introduction: Over the last decade or so, efforts have been made by a small subsection of the amateur community to promote large and very expensive apochromatic refractors. Their (few) owners cultivate the idea that there is something magical about these instruments, that they (somehow) disobey the laws of physics and produce images that are ‘transcendent’ and worthy of ‘homage.’ But as a scientist and a Christian, I don’t believe in magic.
Being openly sceptical of this movement, I recently acquired a large (for an apochromat) 160mm f/8 oil-spaced triplet apochromatic refractor manufactured by CFF, Poland, in order to assess its optical performance in the field, and in particular to compare and contrast it to a much more modest instrument; a 204mm f/6 Newtonian reflector that had underwent a few inexpensive modifications which have significantly improved its performance. In this blog, I aim to provide the reader with a realistic overview of what these instruments are like to use under field conditions and whether the relatively enormous cost of the refractor is really justified.
Brief overview of the instruments: The CFF refractor is mechanically well endowed, having a retractable dew shield, a state-of-the-art Feather Touch micro-focuser. It requires a 3-inch extension tube to reach focus visually using a 99% reflectivity, 1/10 wave PV dielectric diagonal and a straight-through 50mm finder telescope (none of which are supplied with the instrument). The instrument came with a dovetail plate and well machined tube rings with a carry handle attached. It was supplied in an attractive, foam-padded carry case, with small wheels attached to facilitate field transport. The optical tube assembly is quite heavy; close to 14 kilograms (~31 pounds) when fully kitted out for field use. More details here.
The comparison instrument is a SkyWatcher 204mm f/6 Newtonian, described in much greater detail here. As previously described, it has a 204mm primary mirror with enhanced coatings (HiLux from Orion Optics, UK). The secondary mirror (also HiLux coated) has a diameter of 44mm, giving a central obstruction of 22 per cent. The optical tube weighs in at a significantly lighter 8.6 kilos (19 pounds), fitted with its right-angled 8 x 50mm finder telescope.
The focal lengths of both instruments are very comparable; 1200mm for the Newtonian and 1280mm for the refractor. This was considered to be very fortunate, as the same eyepiece delivers effectively the same magnification in both instruments and so a fair comparison could be made without the complicating factor of introducing more variables into the mix.
Comparative ease of set up: The 160mm, being significantly heavier than the Newtonian, is considerably more difficult to carry around and mount. Lifting a 31 pound instrument onto a tall alt-azimuth mount is not for the faint hearted, even for the author ( who enjoys good health at 47 years of age). The Newtonian is much easier to carry about and mount on its Lazy-Suzan. Like walking on eggshells, one is constantly aware that the slightest bump could mis-align the much more complex triplet lens system of the refractor, with no prospect of fixing the problem in the field (indeed it would have to go back to the factory for professional realignment). In comparison, the Newtonian induces far less anxiety on the user. Even if the optics are misaligned accidently during transport, a simple laser collimator can be used to realign the optical train in a matter of a few minutes.
Daylight tests: No sooner had the CFF 160mm refractor arrived on Wednesday October 26, that work was begun evaluating its high power prowess in comparison to the freshly collimated 204mm reflector. Over three days leading up to the end of October, the instruments were set up in a temperature stable outdoor environment, away from direct sunlight. Both instruments were charged with a quality 6mm orthoscopic eyepiece and a 2.25x Barlow lens yielding a very high power of ~450x. The instruments were aimed at the topmost boughs of a horse chestnut tree located ~ 200 yards distant. The instruments were carefully focused on the intense colours of autumnal leaves and the images compared.
Result: The 204mm Newtonian delivered a significantly brighter and sharper image of the leaves. The 160mm was clearly running out of light in comparison, and a trace of chromatic aberration was detected in the refractor image.
Over the next few days, I canvassed the opinion of several other individuals, with the instruments set up in exactly the same way. These included my wife, sister-in-law and my next-door neighbour. Their assessment was in perfect agreement with my own; namely that the 204mm Newtonian delivered significantly better images at 450x. They reported how dim the image was in the 160mm refractor in comparison with the reflector and that the image was a tad sharper in the latter.
Analysis: the 204mm Newtonian with a 44mm secondary mirror has a contrast transfer of 204 – 44 = 160mm. With the 97 per cent reflectivity of both mirrors, the Newtonian collects ~ 40 per cent more light than the refractor, thereby explaining the findings of this author and the other witnesses called to assess the images.
Date: Sunday, October 30
Time: 23:30-00:00 UT
Seeing: fair, (III), mostly overcast with some sucker holes in the clouds.
Instruments fielded: 160mm f/8 triplet apochromat, 204mm f/6 Newtonian reflector. Both telescopes were fully acclimated.
Star testing on first magnitude, Capella, at 250x showed essentially perfect results for the refractor, with nice concentric Fraunhofer diffraction rings inside and outside focus. No evidence of any significant optical aberrations were noted. Both the intra-focal and extra-focal patterns were symmetrical. Both ring systems presented with the same white colour. In focus, contrast was excellent – as good as I have ever witnessed in a quality telescope – and better than seen in the Newtonian at the same power.
Turning to the fairly tricky double star, theta Aurigae, now high in the eastern sky, the faint companion was resolved well in both telescopes at 250x, despite the less-than-ideal conditions. The image was a little tighter and more contrasted in the 160mm refractor, but once the images settled down in the Newtonian, they were very comparable. I was very pleased indeed with how well the Newtonian was coping with this fairly stringent optical test.
A note on comfort: The refractor, having a heavy, long and quite unwieldy tube, was found to be frustrating to use when trained at stars of differing altitudes. On the alt-azimuth mount used, it needed constant tweaking of the centre of gravity in order to keep the stellar images fixed in the field (the locks on both axes being left loose). The instrument would clearly benefit from a sliding weight system to make slight adjustments in balance on the fly. Indeed, this was found to be the case with my 127mm f/12 classical refractor, where a sliding weight system was custom made to accommodate the telescope.
The Newtonian on its simple mount was a joy to use in comparison. No balance issues were encountered.
Date: November 1, 2016
Time: 21:15 UT-
Seeing: turbulent (Antionadi III-IV), very clear, great transparency, cold.
Temperature: 0 C
Both instruments were trained on a more difficult pair; delta Cygni, now high in the western sky. Power employed ~200x. Neither instrument could resolve the faint, close-in pair. The stellar seeing disk remained turbulent in both instruments. This would have to be a low power night, so the next target visited was the Double Cluster, now ideally placed high overhead in Perseus. Power employed ~30x (40mm, 68 degree AFOV ocular).
Results: Both telescopes delivered very fine images of the Double Cluster. The celebrated open cluster was considerably easier to observe with the Newtonian owing to the comfortable position of the eyepiece at the top of the tube. The heavy, 2-inch eyepiece was not an issue with the Newtonian, but caused considerable balancing issues with the refractor, which forced this author to push the tube about half a foot forward on the mount. And then, there was the issue of having to crouch down near ground level to get to the eyepiece, which is not exactly practical or glamorous on a cold, autumnal night like this.
Comparing the images, both instruments produced pinpoint stars across the field with the refractor (at f/8) offering better edge correction compared with the 204mm Newtonian (f/6). And while contrast was better in the unobstructed refractor, it was not hard to convince myself that I could discern more stars in the larger aperture Newtonian. So the situation could be summed up in this way: the refractor offered less light gathering power with greater contrast, whilst the Newtonian offered up better light gathering power with slightly less contrast. Which was more majestic to my eye? I would have to say the reflector, as the gain in contrast in the refractor was simply not enough of an incentive to tip the balance in its favour.
Date: November 4, 2016
Time: 14:00 UT
Further tests conducted during daylight this afternoon, comparing the 204mm Newtonian to the 160mm refractor at the same ultra high magnification (450x) on the same tree top some 200 yards distant.
Result: The Newtonian produced the superior image. It was brighter and sharper.
A Curious Aside: Newtonian reflectors, if well baffled, make very good daytime telescopes, albeit the image is inverted. As a keen telescopist, I have enjoyed many hours over the last 19 months soaking up the very fine, high power views of the Creation with my 8-inch reflector.
Further night time testing to be conducted later this evening.
Date: November 5, 2016.
Seeing: Improved (II-III), good transparency, mostly clear.
I decided to wait until after midnight to field the two large telescopes, spending some time earlier under the sky with my 130mm f/5 Heritage Newtonian, between the frequent showers. This telescope steadily resolved the very faint companion to theta Aurigae, indicating that conditions had improved a little over the previous evenings. As the night progressed, it was clear that these showers were dying out and the clear spells were increasing in longevity. After midnight, they had all but ceased. Both the 204mm Newtonian and the 160mm refractor were stored in a cool, dry outhouse for several hours so as to ensure that they would be fully acclimated upon commencement of the next phase in my testing. My primary target, the Great Nebula in Orion (M42), would not transit the merdian until well into the wee small hours. However, by about 00:30UT it had risen to a decent height above the treetops.
Using my trusty 24-8mm Baader Hyperion zoom, I dialled in the best magnification that would frame this celebrated emission nebula and its interesting hinterland. A power of ~60x was thereby chosen (20mm setting on the zoom).
Both instruments delivered beautiful views of the Orion Nebula, showing its brightest regions in a fluorescent green colour. It is on such occasions that one feels so very privileged to have the opportunity of seeing this celestial spectacle in two fine and capable instruments. The contrast in the CFF 160 refractor was unquestionably superior. The sky was noticeably darker which added somewhat to the aesthetic of the image, and yet, all the while, the larger Newtonian showed more. Specifically, I was clearly able to follow those delicate, incandescent filaments trailing away from the main structure for longer before they faded into the background sky proper. In addition, where the 160mm refractor image showed rather abrupt changes from light to dark within the nebula, especially in the regions immediately east and west of the Trapezium, the Newtonian revealed that the situation was more complex and that these same regions had additional streamers, difficult or well-nigh invisible in the smaller refractor, which encroached on the otherwise dark cavities of the nebula.
To sum up, the refractor produced a less detailed image with a darker sky background, whilst the Newtonian showed more features but with a slightly lighter sky background. Which is better? That’s a highly personal choice, I suspect, but for me I felt compelled to declare the Newtonian image that little bit better, simply because, as a telescopist, I value detail over what I perceive as more of a ‘cosmetic’ effect of sky darkening.
Next, I trained the instruments a few degrees to the north of the M42 complex, to the double star, eta Orionis. Charging both instruments with a 6mm (Baader classic) orthoscopic, but was unable to resolve this system in either telescope. Its low altitude (a few hours before culmination), together with the slightly lower than average seeing conditions conspired to make this a no-goer on this occasion. Telescopes of this aperture class are noticeably more sensitive to the vagaries of the atmosphere than smaller backyard instruments; it just comes with the territory.
I had better success with theta Aurigae, which, by now, was very high in the eastern sky. Both instruments resolved the faint companion very well indeed at 200x. Again, the image of the pair was slightly tighter and better contrasted than in the Newtonian, but it nonetheless did an excellent job as well. I was able to somewhat ‘replicate’ the aesthetic of the refractor by observing the pair with a Baader single polarising filter attached the eyepiece, which reduces glare and darkens the sky without causing a colour shift.
That pretty much wraps up my observing report for this night.
Another Curious Aside: Contrary to what has been cultivated by ‘arm chair’ amateur astronomers over the last few decades, Newtonian reflectors make excellent double star instruments (particularly at f/5 and slower relative apertures). This author’s 8-inch f/6 Newtonian completely outperformed a first rate 5” f/12 classical achromat, as well as a 180mm f/15 Maksutov Cassegrain in extensive field tests carried out over the last few years.
Date: November 7, 2016
More daylight testing conducted today, this time from another independent witness: Elaine, my chemistry student.
Both instruments were fully acclimated in the cool, afternoon conditions. The treetop target and magnification was the same as before (i.e 450x).
When asked, which image was brighter, Elaine replied, ” the Newtonian.”
When asked, which image was sharper, Elaine replied, ” the Newtonian.”
Last, but not least, I invited David, my physics student, to make the same comparison. I posed the same questions to him while he spent a few minutes studying the images served up by both telescopes. I then asked him to point to the telescope which gave the brighter and sharper image. He obliged, as shown below:
Significance of the witness testimonies: Some apochromat owners are, irrationally or otherwise, bias, when comparing views through other types of telescopes. They either make ill-informed assessments or deliberately lie (or at least exaggerate) so as to justify the purchase of such instruments. In still other cases, it is beneath their owners to perform such testing. To help redress this issue, all the aforementioned individuals exhibited no such bias (for they had no reason to lie), and thereby provided completely impartial answers to my questions.
Tonight promises to be clear and very cold; perfect for testing the colour fidelity of both instruments.
Date: November 8, 2016
Seeing: II, quite good, partially cloudy, chilly.
A Comparative study of Gamma Andromedae (Almach)
After enjoying some spell-binding views of the comely triple star system, iota Cassiopiae, in both telescopes (now situated nearly overhead), I trained the telescopes on the famous colour contrast double star, gamma Andromedae (Almach) and examined the images at 200 diameters. Then, retiring to the comfort of a warm fire, I gathered my thoughts.
Both telescopes served up beautiful, calm images of this famous double star. The refractor gave a darker background owing to its superior contrast but I could see that that size of the Airy disk was ever so slightly smaller in the Newtonian under these better conditions, fully in keeping with its greater aperture. The colours of the stars were subtly different in both telescopes; the refractor seemed to give deeper colours – marmalade orange and blue – a consequence I suppose of the darker sky background it throws up. The colours thrown up by the reflector were just a little softer in comparison. The Newtonian stellar images also produced beautiful diffraction spikes which I have grown very fond of.
Which telescope produces the ‘purer’ stellar images? Well, the apochromat, by necessity, approximates the true colour image, bringing three wavelengths of light to a sharp focus – with wavelengths at the extremities of the visual range being less well corrected – a necessary compromise when using lenses. The CFF 160 is optimised at 532nm, but as the eye adapts to night time conditions, peak sensitivity shifts to shorter wavelengths as mesopic vision kicks in. In contrast, the reflector focuses all wavelengths of light equally in comparison. Thus it must, by necessity, have produced the true colour of the primary and secondary stars.
As a telescopist who appreciates true colour images, I deemed the Newtonian image more compelling and worthy of study over the refractor.
Date: November 10, 2016
Time: 15:30 UT
More Citizen Science: Two further eye witnesses were brought forward to judge the images during daylight, as described previously. This time it was my friend, Kevin(left), and my brother-in-law, Bill (right). I asked them to study the images and deliver their verdict by pointing at the telescope offering up the superior images in these low light conditions.
The Gibbous Moon looks like a promising target later this evening. I hope to view our natural satellite with both these instruments and report back.
Cheerio the noo.
Time: 21:15 UT
Seeing: III, a notch down on previous evening, gibbous Moon transiting fairly low in the south.
The CFF refractor copes well in the sub-zero conditions upon us just now. After taken outside from a warm indoor environment, the high power images are acceptably good after about an hour. The Newtonian cools faster though. 40 minutes tops, but does suffer more from thermals.
Both instruments were charged with a 32mm Plossl, power ~38x to assess the overall appearance of the whole lunar image. Both instruments served up razor sharp images of the waxing gibbous Moon. Contrast was noticeably better in the refractor, with a darker sky background. High power views at 200x were also excellent, with fine definition despite the less than perfect seeing conditions.
The Newtonian offered up a brighter background sky at low power but still more than acceptable. High power views at 200x were very comparable. Examining the floor of the crater Plato, only one craterlet was visible in both telescopes (the central A crater).
All in all, I’d give the nod to the refractor on this target.
Turning the telescope mid-way up the western sky in Cygnus, I aimed both instruments at delta Cygni, to see how they would cope under these conditions. Though not ideally placed, the companion was faintly visible in both telescopes. Over a period of about two minutes, I perceived that the companion was more stably held in the refractor. The reflector through up more thermal issues over the same time period, but there was always plenty of opportunities to watch the image settle down, when it produced images that were very comparable to the refractor.
Vigil ended at 22:00 UT
Date: November 12, 2016
Time: 17:45-18:45 UT
Seeing: II, slight westerly breeze, partially cloudy, bright Moon rising in the east.
Further star testing of the CFF 160 refractor shows very nice, concentric Fraunhofer diffraction rings inside and outside focus. Spherical correction appears to be very good (~ 1/6 wave P-V). I could detect no coma or astigmatism in the stellar images at powers up to 300x. I am 100 per cent satisfied that the unit is working in perfect accordance with its design specification.
Just one reasonably searching target visited this evening: delta Cygni – which is better situated in the early evening as it is higher in the sky. Both telescopes were charged with a power of 200x and the system critically examined.
I can report no significant differences between the instruments this evening. Both telescopes clearly showed the faint companion equally well. The stellar images were calm, round and beautifully characterized in both instruments. As mentioned previously though, observing with the reflector is so much more comfortable than with the refractor, owing to the differences in position of the eyepiece (pictured above).
After a light supper I uncapped the telescopes, which were kept outside throughout, and examined the images of the bright gibbous Moon, now higher in southeastern sky. Again, I examined the images at ~ 38X and 200x.
As previously reported, both telescopes delivered very crisp images of the lunar regolith. The Newtonian image was noticeably brighter though, with a brighter sky background. The 160mm refractor showed the images in higher contrast, as expected. But it was then that I reached for my Baader single polarising filter to see how it would alter the lunar images in the Newtonian. It would dim the image a little but the reader will remember that the reflector gathers more light than the refractor.
Well, I have the great pleasure to report that the addition of the filter (which does not impart a colour shift), produced images of the lunar regolith which were every bit (if not slightly better) as good as the refractor in terms of overall contrast. I am finding that this inexpensive little optical accoutrement is a very effective ‘replicator’ of the refractor image.
How does that make me feel?
Elated, of course!
Date: November 14, 2016
Time: 13:20 UT
I finally took the plunge and acquired a second set of Bob’s knobs for my 8-inch Newtonian. They make fine tuning the collimation much easier than using a hex wrench.
I set the telescope outside on this incredibly mild afternoon (the thermometer recorded 14C!) and with a power of 200x, I enjoyed the clean, crisp views of the intricate contours of the pine cones and golden needles of a European larch 120 yards distant.
The Newtonian is a wonderful telescope! Such extraordinary power at such modest cost! In truth, I’ve not really discovered anything magical about the 160mm refractor in my testing so far. Sure, it’s a beautifully made instrument but it takes so much effort to set up in comparison with the Newtonian and is just not as comfortable to use. You see, these things are important issues for a dedicated telescopist. The easier and more comfortable it is to operate, the more you will use it.
My other objection to the refractor pertains to its very high cost. It’s just a silly design, in my opinion, for the returns you get at the eyepiece. The Newtonian offers far more promise in comparison. There was once a time where slower Newtonians were all the rage. Even moving to f/7 would all but eliminate the need for collimation on a regular basis, making it just as hassle free as a refractor, and without increasing the mass appreciably. I once felt that a folded refractor design (not the Zerochromat though) would have provided the ideal way forward, but having immersed myself in the operation of Newtonian telescopes, I have firmly convinced myself that they offer greater promise. A 8- or 10-inch f/7 mass-produced Dob would be a wonderful addition to the telescope market. The primary reason is the reduction in maintenance, which seems to be an important issue for many amateurs, as well as producing a larger, coma-free field of view. Big companies like Synta has the in-house technology to make such a product a reality. They are clearly capable of producing very high quality mirrors (the primary mirror on mine is unchanged) and I would bet that people would be willing to pay that little bit more for a telescope with a stronger, more rigid tube to keep the optical train firmly in place, night after night.
Just what has the telescope market to lose in introducing such an instrument to the masses? Surely, it would grow the hobby like nothing seen in recent decades. The images delivered by such a telescope on a good night would sate even the most nit-picky of observers and the money saved could be put to better use.
Super Moon tonight, but heehaw chance of seeing it. Mild weather at this time of year almost invariably means cloudy and wet weather.
Date: November 15, 2016
Time: 13:22 UT
Geez ma Dob!
There is little else I can report visually at this stage, although I still have to compare and contrast the images of Jupiter in the Newtonian and refractor at some point in the up-and-coming months. But, yet again, I’m not expecting any magic. The planet will barely reach 30 degrees altitude from the far south of England, which means that it will never ascend over 24 degrees in altitude from my location here in Scotland. That doesn’t bode well for seeing fine detail though. Still, the comparison needs to be made and I shall report back in due course to complete this blog.
Needless to say, I’m blissfully happy with how Octavius has performed in the scheme of things. I have truly made it my own.
I bid you adieu the noo.