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by Neil English 07/08/10 | Email Author

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The Story of a Journey: the Skylight f/15

By Neil English

The refractor is the prince of telescopes. It has earned that honour not only by virtue of its pedigree, which dates back four centuries, but also because its unobstructed optics produces the sharpest, highest contrast images of any telescopic design yet contrived by the artful mind. Like countless other ‘forty somethings’, I made my debut in the late 1970s with one of the smallest incarnations of this tried and trusted refractor tradition; a 60mm f/13 achromat. That little spyglass changed my life. Back then, I couldn’t pronounce ‘chromatic aberration’ let alone explain it, but I didn’t need to either. My trusty Tasco served up fantastic views of the Moon, the satellites of Jupiter and even the rings of Saturn. It brought myriad faint clusters of starlight into sight that I never even knew existed. A seminal moment came when I chanced upon a curious line in my astronomy book. It said that near the bright star Vega, lying high overhead during warm summer nights, a small telescope like mine could unveil a marvellous sight – a pair of stars, epsilon1 and epsilon 2 Lyrae – and their amazing secret; that each star itself had a close companion. My first view of the system made a deep impression on me and stoked my life-long love of double star observing, not for any scientific advancement (I’m much too clumsy for that) but for the sheer fun of it. You see, looking at double stars is rather like stroking my cat; it calms me.

A plethora of binary and multiple stars are within reach of a small telescope, even from an urban setting. I quickly grew fond of seeking out their subtle (and not so subtle) colour contrasts and the sheer fun of being able to split pairs close to the Dawes limit of the scope I was using. As new products flooded the market in the early part of this decade, I began to experiment with various telescope designs to see which one would serve my needs best. And my circumstances changed too; the arrival of my two sons meant quality time under the stars was now at a premium.

So, I needed a scope that had enough aperture to deliver nice, high power views down to the resolution limit imposed by my turbulent atmosphere (rarely one and typically two arc seconds), as well as a system that I could set up with minimal hassle, at a moment’s notice. The ideal choice for me was a good 4-inch refractor. Like many double star enthusiasts, I had gained a lot of experience using high quality 4-inch f/10 refractors before ever contemplating buying an Apo of the same aperture. Eventually though, like everyone else, I succumbed to Apo fever and took the plunge into the brave new world of colour-pure observing. I’ve owned, used and reviewed a number of doublet, triplet and four-element Apos (from f/5 to f/9), over the last decade and can attest to their great all-round performance. But in my specialist pursuit – splitting double stars – I gradually noticed a trend. The faster the focal ratio, the harder it was to get consistent results in the field. Close binaries like mu and Delta Cygni and Eta Orionis were more comfortably discerned at high magnifications (> 200x) in long focal ratio instruments (f/8 or larger) and, once resolved, required little additional focusing. In contrast, with shorter focal ratio instruments (f/5 or f/6), the frequency with which I managed a successful split was noticeably less. The views were noticeably more stable through the longer scope. Could this effect be related to the higher elevation of the longer scope’s objective off the ground and away from heat sources? Of course, it crossed by mind. But I always observe on grass and usually begin my observations several hours after sunset when ground heat has had time to dissipate. What’s more, where I live, the heat differential between the air and the ground is seldom very high. This is Scotland, not Arizona! Longer focal length instruments performed better during average and less than average seeing conditions. For example, on nights where there was a gentle breeze blowing, the longer scopes almost always did better. Furthermore, that advantage seemed to disappear on nights of good to excellent seeing. This pattern was quite consistent and independent of seasonal temperature variations.  What did all of this mean? There was something here, but I couldn’t quite put my finger on it. My ‘Eureka’ moment arrived when, by chance, and out of sheer curiosity, I reviewed a 4” (102mm) f/15 achromat of classical design for Britain’s ‘Astronomy Now’ magazine. In short, my success rate at teasing out tricky double stars with this telescope literally changed almost ‘over night’. Where I could manage difficult pairs like Iota Leonis only ‘every now and then’ with my f/9 scopes (Apo or achromat), I could do so ‘frequently’ with the long focus refractor. Now, I had looked through many short focal length Apos that were well corrected for the Seidel aberrations before. The best explanation I could proffer was that the enormous depth of focus of the long achromat was stabilising the image. Simply put, moments of good seeing came more frequently in the long scope than in the short scope. It also put another puzzle that simmered in my head for years into perfect focus. It explained why so many of the views I enjoyed through classical refractors housed in observatories the length and breadth of the country offered up such steady images.

All aberrations fall off rapidly as focal ratio increases. Consulting some books on telescope optics, I was able to find out how the various aberrations scale with focal ratio. I’ve put them in a table for your convenience.

Aberration How they scale
Spherical 1/F3
Astigmatism 1/F
Coma 1/F2
Distortion 1/F
Field curvature 1/F
Defocus 1/F2

Sources: Bell, L, The Telescope, Dover (1981), Suiter, H.R, Star Testing Astronomical Telescopes(2009), Willmann-Bell

Now, the best short focal length Apos can be beautifully corrected for all five Seidel aberrations (the first five listed above) and false colour. Have you ever looked through a TEC or an Takahashi refractor? So what’s the secret of the long focus achromat? As you can see from the table above, I’ve deliberately highlighted the feature that longer focal ratio scopes suffer less from; defocus aberration, or, in other words, long scopes enjoy more depth of focus. What benefits does this confer? Well, for one thing, it makes focussing easier – that’s why fast scopes need micro-focusers almost by necessity. The position of best focus is found more quickly and with less guesswork than in an instrument of lower focal ratio. Furthermore, defocus aberration falls off inversely as the square of focal ratio, so an f/15 scope gives you nine times more depth of focus than an f/5 instrument. It certainly explains why I fiddle far less with the focus knob on a long scope compared with its shorter counterpart. But could depth of focus really stabilise an image? Perhaps the atmosphere causes some defocus and a scope with greater depth of focus can better accommodate that tendency to defocus? I asked a professional astronomer, who uses adaptive optics on one of the large observatory class telescopes for his reaction to this idea. And this is what he was willing to tell me;

“As far as I can tell, defocusing aberration from the atmosphere is, at most, a 10% effect on the overall seeing-aberrated wavefront, so that's the maximum amount I can see focal ratio mattering. Of course, there are many other factors leading to greater image stability where the F-ratio might matter more, so my position is that long F-ratios are manifestly good for image stability, but just for reasons that have little to do with seeing.”

I then contacted Dr. Chris Lord of Brayebrook Observatory, England. He’s a highly respected optics guru and authority on double star observing. I asked him if seeing can affect long and short scopes differently?  Here’s what he had to say on the matter:

“Consider an ideal system and no atmosphere. A plane wavefront becomes a converging wavefront after refraction through or reflection off the objective. The focus, because of diffraction, becomes a funnel shape, and the depth of focus is arbitrarily defined by the wavefront error at the Rayleigh limit (I redefined it as that of the objective wavefront error). The part of seeing that causes phase retardation can tilt the wavefront, or warp it. I considered a simple tilt. The wavefront is still plane, but does not arrive at the objective at the same time. To the objective, the wavefront will look as though it is arriving at an angle to the optical axis. It will not be focused on the optical axis but to one side and either in front of or behind the focal plane (it will be focused on the Petzval surface). Consider a bundle of tilted wavefronts, all tilted differently to one another, arriving at the objective in rapid succession. To the eye, located on the optical axis, examining the focus, the star image will appear to move from side to side, and go in and out of focus. The distance from the entrance pupil to the focal plane is an exact number of wavelengths. If the wavefront is tilted more than 1/4 wavelength, it will be focused outside the arbitrary depth-of-focus. If the depth-of-focus is taken to be the objective wavefront error, then its value is shallower than the 1/4 wave Rayleigh limit, assuming it is better figured than 1/4 wave. The depth-of-focus is also shallower with fast f/ratio optics, and so wavefront tilt induced by seeing becomes more apparent. When the Fried length exceeds the aperture, (typically 4 to 8-inches in the UK) tilt dominates phase retardation. Instead of seeing a dancing diffraction pattern you tend to see a swollen seeing disc. When you are observing under these circumstances focus can only be judged by the smallest seeing disc. Within the defined depth-of-focus it does not remain the same size. When the aperture is bigger than the Fried length the spurious disc is visible, and its size remains the same (for a given focal ratio) within the depth-of-focus. Slower f/ratios produce bigger spurious discs, and bigger seeing discs, i.e. a 40-inch f/20 telescope will produce a bigger seeing disc than a 40-inch f/4 telescope. The seeing disc is always bigger than the spurious disc. You have to keep in mind seeing has two aspects, oscillation & scintillation. It’s the oscillation aspect that affects the criticality of focusing. The wavefront tilt, when expressed in wavelengths, remains constant regardless of focal length. A wavefront tilt of x wavelengths for y aperture will have 2x wavelength tilt at 2y aperture.”

Did you get all that? Mind boggling stuff for sure! All said, there may indeed be a sound physical basis to explain what I have seen at the eyepiece. Certainly, more work needs to be done on this fascinating possibility.

To summarise; in short and medium focal ratio formats, you really can’t beat an Apo over an achromat of the same specification. The latter are generally better corrected in almost every way. But I have compared the same double stars in my f/10 achromat and a number of high-quality 4-inch f/8 and f/9 ED doublets (both budget and premium models). Both the Apo scopes and the achromat served up well resolved views of these test stars on average nights but apart from showing their colours more faithfully, the Apos really had no other advantage so far as I could detect.  For double star observing at least, as the focal ratio increases beyond f/9 or so, the advantages conferred to the Apo rapidly diminish. When the focal length to aperture ratio exceeds 3, the achromat has colour correction that is visually comparable with a short focal length ED objective. But a 4-inch f/15 will have minimal Seidel aberrations and much greater depth of focus to boot. And therein, I think, lies the Achilles heel of the modern, compact Apo. There's no way round it either – unless you purposely build a very high focal ratio Apo - and it’s ever present,  irrespective of what compact Apo design you consider. For these reasons, I turned my back on the exciting Apo market and put my faith in a modern long focus achromat of classical design.

Introducing the Skylight

If I were to obtain the ultimate scope for indulging my passion for double star observing, I could dispense with the idea that it was necessarily to be found in some ultra-compact high-end apochromatic motif. Portability was important to me though, as I like to set up and get observing in less time than I can eat a bowl of cheerios. The D&G achromats, while remaining dream scopes, were not an option, as the smallest instrument currently being made by the company was a 5-inch f/12 instrument which would be prohibitively cumbersome given my need to move the scope a few times during a typical observing session. The beautiful, all brass, long focus achromats made by I.R. Poyser (Wales, UK), were also great temptation but they proved prohibitively expensive (and maybe a bit too decorative for my needs) and not nearly long enough for what I really had in mind.  That left the more economical 105mm f/15 Antares Elite series achromat as the only viable option. Yet, after reviewing the Skylight f/15 I had yet another choice. This is a hand crafted 4-inch f/15 classical achromatic refractor inspired by the golden age of English telescope making and the refractors of T. Cooke & Sons.

Figure 1. The 3-inch Cooke (right) that inspired the building of the Skylight f/15(left).

Having had an opportunity to evaluate the performance of the Skylight f/15 prototype, I asked the builder, Richard Day, of Skylight Telescopes London, if he would deliver a telescope to my specifications (the details of which I’ll get to later). Six weeks after placing my order with Skylight, the instrument arrived. As I pulled away the meticulously wrapped refractor, layer by layer, the first thing that struck me was how utterly enchanting shining brass presents against the long, slender lines of a charcoal black, powder coated tube. And boy did it go on and on! Spanning 1.6 metres in length from tip to toe, you could tightrope the Grand Canyon with this scope! Seriously though, it’s immediately obvious that the maker of this instrument ardently tried to connect the owner with the halcyon days of f/15 refractor building. The finder scope was found in a lovely decorative box and I was also delighted to see a personalised note from Mr. Day.

Let’s work our way round this beauty. Starting with the dew shield; well, what can I say? Fully 11-inches long and made of solid brass! I’d never seen this on a Cooke refractor before but another British firm of repute, William Wray of London, who flourished in the mid 19th century, did produce very fine instruments for the serious amateur astronomer with brass dew shields equally long in comparison.

Figure 2. Check out the size of the brass dew shield (lower right) on this beautiful 3.5-inch f/15 Wray refractor!

The fluted lens cell – which is fully collimatable - is characteristic of the types employed in the heyday of long focus refractor building. Inserting a Cheshire eyepiece into the focuser, I did some minor tweaking with a hex key to achieve essentially perfect collimation. The high specification objective is of older pedigree; a classic Fraunhofer air-spaced doublet which is still widely acknowledged to be the optimal optical design in achromats for the elimination of coma and spherical aberration. The anti-reflection coatings, though meticulously applied to the lens, are very subdued, with a pale lilac tinge. Light transmission appears to be excellent. Indeed, were it not for the presence of these coatings, I felt the objective wouldn’t have looked out of place on a mid-19th century instrument.

Figure 3. The multi-coated Fraunhofer objective lens

Removing the objective, you can really see that Day has done his homework with this scope. The interior is matt black with knife edge baffles carefully positioned, as derived from optical ray tracing. That much was obvious when I was able to detect faint stars right down to the magnitude limit of a four inch aperture. A 3-inch Cooke I examined had similar baffles in place. Moving to the brass finder and its bracket. Again, totally and utterly unique! It is solid brass, has what looks like an MgF2 coated objective with a clear aperture of 40mm, and is fitted with a 20x eyepiece. Its retractable (yes retractable!) dew shield glides smoothly along the body with a satisfying amount of tension. A rack and pinion focuser holds a very charming little prismatic diagonal that can be freely rotated to obtain the best viewing position relative to the eyepiece of the main instrument. Optically it is fairly good and delivers a well corrected field of about two degrees, which proved surprisingly useful as I’ll explain shortly.

Figure 4. The long, slender lines of the Skylight f/15 just goes on and on…

Figure 5. The lovely and intriguing 40mm aperture brass finder

I learned of the interesting background of the finder bracket from conversations I held with Mr. Day by phone while it my scope was being built. “I wanted something special for the finder,” he told me, “but I could find nothing as an off-the-shelf item that was suitable.  As a result, I decided to use this as an opportunity to have something custom made.  The final result is unique, and I'm very pleased with it.  I admit that the style of adjustment screw is an unusual choice… they were suggested by the company who made the brackets.  They had some new/old stock that had been around for many years, and they thought they would look good.  They fit well into the ethos of the instrument.  Those screws are old, and that patina is real (however, I’ve not got an endless supply).  Indeed these types of screw were in vogue when T. Cooke & Sons were still making telescopes!  I liked that thought.” Mounted against the “Darth Vader” black of the main telescope, its brazen caste is similarly proportioned to those found on the classic achromats of yesteryear.

The focuser - a very high quality Baader Steeltrack design, with a nicely colour-matched 1;10 microfocuser - is obviously a big departure from the simpler rack and pinion used by the ancestors of the Skylight f/15.  Its fit and feel is robust and the motions are ultra smooth. When racked out rapidly, the focuser makes a curious ‘whirring’ noise which I have come to like a lot. To be honest, I could have lived with a simpler rack and pinion design on this telescope but having the extra luxury afforded by this focuser was a nice bonus. The solid brass focus plug is another unexpected novelty I have grown fond of.

Figure 6. The Silky smooth Baader Steeltrack focuser on the Skylight f/15

Mounting options

As a visual observer, I have developed a strong preference for simple, non-nonsense observing with minimum set up time. And while those wishing to carry out measurements of double stars will obviously want to mount this refractor on a sturdy motorised equatorial mount, I much prefer the elegant simplicity of a stable, yet portable alt-azimuth. I found the Tele Vue Gibraltar to be a good overall match for the size and weight of this telescope (9 kilos) and I opted for the same company’s mounting rings to securely fasten the Skylight f/15 to the mount head, using two large wing knobs. Set up takes just a few minutes and the mount offers just enough tension to allow me to track objects- even at high magnification – across the sky. Of course, those skilled in woodwork could build an even sturdier mount for this scope (something I am currently considering). One of the first things I learned while using the Skylight f/15 in this mounting configuration was the undue heaviness of the brass dew shield. After I mentioned the problem to Mr Day, he had a replacement shipped to me, free of charge, about three inches shorter and about 30 per cent lighter.  Needless to say, it worked much better on my modest Alt-Az mounting. I am now reliably informed that these new, lighter brass dew shields are supplied as standard with the instrument.

Figure 7. The Skylight f/15 astride the Unitron equatorial mount.

Optical tests

The optics on the Skylight f/15 are first rate. Think Tele Vue in a classical accent. In careful star tests conducted at 214x, I could not detect coma, astigmatism, field curvature or distortion. Spherical aberration was exceptionally well controlled. Using a green Wratten filter, I was able to obtain a nearly perfectly symmetrical pattern of diffraction rings round Vega, on either side of focus. The rings looked very smooth and were devoid of any rough zones. I was satisfied that this optic had a figure of at least 1/8th wave. It could be even better, but I don’t think my eyes could discern any meaningful improvements on that figure.  It colour correction was also spot on for a C-F achromatised refractor. Racking inside focus gave a lovely aniline rim to the diffraction rings, while extra-focally it rendered a very plain green.

Being an achromat, it does display false colour around first and second magnitude stars but only at powers above 100x or so. Yet despite this, it served up very high quality views on contrast-hungry objects such as the Moon and brighter planets. Indeed, the achromat’s inability to precisely focus longer (red) wavelengths plays with the eye in a fundamentally different way to the Apo. Personally I love the faint blue halos thrown around otherwise white double stars at high powers and the subtle shades of yellow the Skylight f/15 imparts to Saturn. The Martian deserts have a pale, greenish marinade, while they appear more of an austere fawn through the Apo. You can begin to understand how the planetary observers of yesteryear imagined them to be vast tracts of vegetation! The minimisation of Seidel aberrations in the long focus achromat is responsible for controlling false colour to such an extent that you quickly forget about it.

Inexpensive eyepieces, such as my Meade 56mm Plossl, while throwing up the mild astigmatism in my eyes when used with my f/6.3 scope, performs like champions in the Skylight f/15, with pinpoint stars right to the edge of the field. With that eyepiece, I could achieve a near 2-degree field which is wide enough to frame most deep sky objects comfortably. That said, I didn’t acquire this instrument as a rich field scope. I’ve enjoyed comfortable views of double stars at powers well beyond the oft prescribed 50x per inch of aperture in good conditions and my tests on the Moon convince me that I can push the optic to 100x per inch of aperture on the best nights. One thing you’ll notice is the strikingly large size of stellar Airy disks caused by the telescope’s long focal ratio. Charging the Skylight with a power of 375x on a calm October night, I watched in amazement as the four components of the Lyra double double floated across the field of view, their tiny yellow-green ‘globes’ rippling in the seeing. I was deeply impressed at how steady the image held at such high magnifications, something I just didn’t see frequently with shorter focal ratio scopes, irrespective of their specification. The bottom line for me is that quality long focus achromats provide a real alternative to Apos for the purposes of high resolution work. Furthermore, my experiences with long focus achromats convince me that they are less an ancestral form of the Apo so much as their legitimate siblings.

Where have you been all my life?

Using the Skylight f/15 has been an inspiration for me in more ways than one though. It has helped me resolve some vexing questions I’ve encountered on my journey through the rich milieu of the refracting telescope. Many seasoned observers have reported the alleged greater contrast of longer focal ratio instruments over their shorter counterparts. For many years, I dismissed the idea as an urban myth, a result probably of the greater magnifications reached by a given eyepiece in a longer F ratio scope. But the combined effects of depth of focus and less eyepiece astigmatism have led me to re-evaluate these reports. Perhaps it lies in its ability to ‘hold together,’ as it were, the image of already diffuse deep sky objects, presenting them in a more stark, contrasted way against the backdrop of dark sky.

More significantly, any one exploring the fascinating history of the refractor over the last four centuries is sure to have encountered many tales of astronomers employing impossibly high magnifications with their telescopes. I’ve never quite understood how for example, Wilhelm Struve could ‘routinely’ use 700x on the newly constructed 9.5 inch Dorpat refractor (built by Fraunhofer) while conducting his masterly survey of  double stars. We may conjecture that the air round the observatory was the stuff of legend but we know for sure that he was looking through an f/18 instrument!

Figure 8. Check out the contrast on this Gibbous Moon imaged through the Skylight f/15. Image credit Phil Jaworek.

I would also like to bring to your attention the extraordinary feats of the visual astronomers based at the Lick Observatory which houses the great 36-inch (0.9m) Clark refractor. Now you can find numerous entries published in the Publications of the Astronomical Society of the Pacific from 1900 to circa 1909 of separations of extremely difficult double stars measured with the Lick refractor by Robert Grant Aitken, which have entries ranging from 50-100 milli-arcseconds. What’s more, these data were used to establish the orbital elements of such binary stars and are broadly accepted today. Yet, despite its 10 fold greater theoretical resolving power and even with the assistance of adaptive/active optics, the Keck telescope atop Mauna Kea can only achieve resolving powers that are broadly similar to those achieved by Aitken et al using the refractor. How can this be? Despite the stellar images swimming in a morass of false colour, I suggest that the f/18 focal ratio of the Lick refractor (the Keck is f/1.75) was the decisive factor in stabilising the images enough to allow these early- and extremely difficult - measurements to be made.

Figure 9. The Skylight f/15 readies itself for a rendezvous with the stars of Bootes.

Nature abhors a vacuum, and the Skylight f/15 has helped fill a great void that separates me in space and time from the workshops of the great refractor builders of the 19th and early 20th century. The Skylight f/15 is a beautifully designed and well executed instrument. Far from being a copy, it’s a well studied re-interpretation of the best the past has to offer. And what a past that is! Sure, it’s a little ostentatious and more expensive than similar instruments on the market but I feel the extra expenditure incurred in securing it was worthwhile. It exudes quality, both in terms of its high-specification optics and the exaction of its mechanics. As a dedicated double star instrument, I think it represents the sine qua non in no-nonsense, high-power observing comfort. It will appeal to the heart as well as the mind.  And a scope like that ought to stay in the family…

Figure 10. A scope worthy of Adoption.

Note added in proof: The following link gives some background to the underlying physics relating to depth of focus and the defocus aberration.

You can read more of my adventures with glass in my up-and-coming book: Choosing and Using a Refracting Telescope, published by Springer in September 2010. The author has no affiliation with Skylight Telescopes.

Thanks are extended to Phil Jaworek ( for kindly providing a lunar shot taken through the Skylight f/15.

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