As you may know, I’ve written quite a bit (perhaps more than any other telescopist) about the venerable Shorttube 80 achromat. It’s a well–travelled ‘peashooter’ ‘scope that has punched well above its diminutive aperture and with thorough testing in the field, has exceeded all of my expectations. My particular model was purchased for about £150 and that extra cash bought me an instrument with a fully rotatable focuser that can accommodate 2 inch and 1.25 inch oculars, a well baffled internal tube and a retractable dew shield. The original crown & flint elements (branded as Opticstar) were replaced with a SkyWatcher objective and were professionally spaced and centred for optimal performance.
My 80mm F/5 achromat continues to serve as a highly capable birding ‘scope for crisp daylight images rich in contrast using my equally versatile Mark III 8–24mm Baader zoom, which delivers a very useful range of magnifications from 17x to 50x and with a generous field of view. Indeed, I consider this telescope to be far more versatile than a variety of 90mm Maksutovs that have come and gone over the years. The 8mm setting of the Baader zoom delivers a near ideal 1.5mm exit pupil which is especially good for enjoying larger deep sky objects (DSOs) in a generous 1.4 degree true field. Extensive field experience with this particular instrument has demonstrated that it can be used profitably at double this power to get close up views of smaller DSOs like small planetary nebulae and open clusters. For the widest ultra–rich sweeps, I adopt a simple but good 32mm multi–coated SkyWatcher Plossl eyepiece delivering a power of 13x in a near 4 degree field.
Because it’s a fast achromat, many observers have dismissed it as a high–resolution telescope incapable of delivering adequate high magnification views of the Moon, planets and double stars and this author would be the first to concede that it needs a bit of help when extending magnifications over 100 diameters. That said, because I have grown very fond of this modest little ultra–portable telescope, I have sought long and hard to try to improve its optical performance at higher powers to make the instrument as versatile as possible. And that will be the subject matter of this blog.
As I have related before, I was rather taken aback by the ability of this telescope to resolve double stars within the remit of its aperture. Indeed, many hours of field work has allowed me to resolve an impressive suite of binary and multiple star systems very much at odds with received opinion. But like everything else in life, you’ll never know unless you try. Systems like Delta Cygni, Epsilon Bootis, Iota Cassiopeiae, Xi Ursae Majoris etc have all been resolved with this telescope under good seeing conditions and with nothing in the way of additional help. In recent months, I have been evaluating the performance of a 90mm f /5.5 ED apochromat in regard to the resolution of double stars and found that, like the venerable 80mm f/5 achromat, that this very fast telescope was equally capable of resolving such systems (indeed a little better owing to its greater aperture). The former instrument is fitted with a very fine 11:1 dual speed micro–focuser which has greatly aided precise focusing at high magnifications (up to about 250x) in the field. The shorttube 80, in contrast, is far more difficult to focus under the same high magnification regime owing to its single speed Crayford–style focuser, faster relative aperture (f/5) but also because at ultra–high powers the chromatic error of the telescope makes precise focussing far more challenging. And it was this issue that I decided to re–investigate.
My experiences with the 90mm f/5.5 ED refractor as well as a 130mm f/5 Newtonian reflector showed that I could push these fast telescopes to powers as high as 250x and above profitably to tease apart tricky binary stars such as Mu Cygni A & B under good atmospheric conditions. I very much wanted to be able to do the same with the 80mm f/5 shorttube. Because of the latters’ smaller aperture, 250x would be about the upper limit that I could expect. My aim was to obtain the sharpest possible stellar images freed from (as much as possible) the chromatic ‘fog’ that attends pushing such a fast achromat to such high powers. Without changing the prescription of the lens (using an ED element), or stopping down the aperture (which I was unwilling to do), the only possible way was to remove the offending wavelengths from the image by blocking the principal offending wavelengths. And that led me, once again, to explore the use of filters.
The usual minus violet filters, which include the fringe killer, semiapo filter and Wratten no. 8 light yellow filter were a step in the right direction but not really good enough to do the job, as all of these do not remove all of the offending unfocused blue–violet wavelengths. What I sought was a filter that would remove all of the unfocused light but still allow as much light as possible to be transmitted to the eye. High efficiency light transmission would be a very important parameter, as at very high powers the images become especially dim in such a small telescope. My search was narrowed down to two such filters; an inexpensive light green Wratten no 56 and the (somewhat more expensive) Baader 495 longpass filter. The latter was brought to my attemtion by Dave Russell, an amateur astronomer based in Upstate New York, who carried out extensive tests using his 140mm f/5.7 Vixen Neoachromat. Below are the published transmission curves for these filters.
I conducted a series of tests during cool, bright autumnal afternoons, where the temperatures did not fluctuate up or down. My targets included. for the most part, leaves from the topmost boughs of trees located between 80 and 150 yards away. The 1.25″ diagonal on the shorttube achromat was threaded with either the Wratten no. 56 or the Baader 495 longpass filter. The 80mm f/5 was charged with a power of 250x using a 1.6mm Vixen HR eyepiece, and as a control, I stopped down the 90mm ED ‘scope to 80mm using a makeshift cardboard aperture mask and used a 2mm Vixen HR ocular to derive the same magnification (250x).
The results for both filters were remarkable! The light green filter did very well indeed, removing the vast majority of the offending short wavelengths, but the 495 long pass filter removed all of it. The images derived were very sharp and punchy using both filters, as if a foggy veil had been lifted from both images that greatly increased the contrast and sharpness at these uber high magnifications. This type of transformation was not noted with the aforementioned minus violet filters which only remove about half of the offending secondary spectrum. Both images were really quite excellent and almost as sharp as that derived by the ED ‘scope on the same target using the same magnification. The only significant differences were the colour casts generated by the filtered achromatic images; the Wratten gave a green cast, the Baader longpass, a yellow hue. In addition, I carefully noted the brightness of the images. This time there was a clear winner; the Baader 495 longpass filter, which was clearly transmitting more light to the eye.
Consulting the data on the Wratten no. 56 filter revealed that its light transmission is about 53 per cent. But as you can clearly see in the hand–held images of both filters below, the Baader 495 longpass transmits significantly more light.
These daytime tests showed that the 80mm f/5 achromat could indeed generate very sharply focused images when the blue–violet secondary spectrum was completely removed. It also showed that the Baader 495 longpass filter was generating a significantly brighter image at 250x since its passes practically all visible wavelengths beyond about 495nm. In contrast, the Wratten absorbed most of the offending blue–violet and a significant fraction of red wavelengths (as indicated by the transmission curves shown above).
The author is aware that the Vixen HR series of oculars are very expensive and may be beyond the budget of some amateurs. Thankfully, there are other ways of deriving the same high magnifications using less expensive eyepieces. For example, I could achieve a power of 240x with the shorttube 80 by mating two shorty Barlow lenses to a 6mm orthoscopic eyepiece with broadly similar results.
Under the Night Sky
Keeping the longpass filter in the diagonal and turning the telescope to the bright stars Vega and Deneb, the filter imparted a beautiful golden tint to them which I didn’t find distracting. In most fields where the brightest stars were absent, one would be hardpressed to notice that any filter was in place. Turning next to the Pleiades and Perseus Double Cluster, the filtered images appeared every bit as bright as the unfiltered view with little or no light loss that I could discern; and which came as quite a relief to me. The lattter clusters, in particular, appeared very striking in the generous field of view served up by the Baader zoom at 50x. Red stars were especially easy to see and indeed were slightly enhanced using the filter. More generally, stellar images appeared tinier and more intense with the filter in place. The Andromeda Galaxy, now very high in the sky, stood out beautifully against a sable hinterland at 13x. Then I swung the telescope over to Lyra, now sinking into the western sky and located the Ring Nebula. Comparing the filtered and unfiltered views at 100x, I was very pleased to see that the images were quite comparable with only slight dimming in the latter. This showed that the filter was passing most if not all of the doubly ionised oxygen (500.7nm) which sets these planetary nebulae aglow. This will come as good news for those amateurs who like using larger aperture short focus achromats in pursuit of emission nebulae.
At higher powers, the stellar images snapped to a very tight focus with zero chromatic fogging much more easily than in the unfiltered view. This is a real bonus as there was now far less ambiguity to the position of best focus than when attempting to focus without the filter. Some may dislike the colour shift in the stellar Airy disks but, truth be told, this was of little concern to me as all I wished to do is to get very tight and tidy stellar images at the highest powers. Indeed, the ED ‘scope (a very good FPL 51 doublet) also imparted a slight yellowing to the Airy disks at high powers (above 150x) which departed somewhat from their true colour. Were I looking to get accurate colour information though, I would unhesitatingly recommend neither instrument over a 130mm f/5 Newtonian reflector which will always reveal the true colour of the subject stars as well as being able to resolve tighter pairs than either of these refractors (and retails for about 1/7th of the price of the ED telescope to boot). In short, the goal was never to turn the achromat into an apochromat but only to allow me to achieve the brightest and tightest Airy disks at the highest powers.
High Power Night Time Tests
Shortly after 7pm local time on the evening of October 27 2017, I set the telescope on its Vixen Porta II mount and directed it at a first quarter Moon that was now culminating low in the south. A brisk westerly wind was blowing, carrying cloud patches over the landscape but there was enough clear spells for me to assess the image using the longpass filter. I charged the instrument with my Baader zoom and dedicated 2.25x Barlow and dialled in the 8mm setting yielding 113x. This particular configuration provides a wonderful panoramic view of our companion in space, its generous field allowing the entire lunar hemisphere to be examined at once.
Even without the filter, the image was quite good but it was plain to see that the crater rims were tinged in unfocused blue–violet and as Dave Russell previously reported with his 140mm Neoachromat, the entire surface was bathed with a faint ‘lavender fog’ which softens the image ever so slightly and which cannot be focused out. With the filter in place, the Moon took on a striking yellow–green countenance but the sharpness of the image was noticeably improved. Gone was any trace of unfocused light and the lavender fog completely removed. The prominent northern craters, Aristotle and Eudoxus were beautifully sharp, and the many ridges within Mare Tranquilitatis really stood out. The image just snapped to focus (and at f/5 you’re either there or you’re not!). Moving further south, the craters Hipparcus and Albategnius were stunningly presented with perfect delineation between their sunlit and shaded floors. And then my eye met with the rugged southern highlands presenting Theophilus, Cyrillus and Catharina perfectly as if etched out with a laser. To be honest, I found it hard to take my eye away from this visual banquet but I eventually switched to higher power, inserting the 2mm HR ocular delivering 200x. Again, the image snapped to a sharp focus and yielded wonderful details. The lunar Apennines were simply astounding to study at this high power with excelllent contrast and no image breakdown whatsoever! Mount Hadley stood out boldly near the location of the Apollo 15 landing site.
My findings agree very well with the reports made by Russell. This is one excellent filter for lunar studies, transforming an otherwise mediocre 80mm lunar ‘scope into a very good performer. Because of its small aperture, a telescope such as this is relatively insensitive to the vagaries of the Earth’s atmosphere and I went away from the field fully confident that I could push the instrument to still higher magnifications on this wonderful target under better observing conditions.
Impressed? Chalk it down!
On the evening of October 28 2017 at 7:30pm local time, I set up both the 80mm f/5 achromat (with the 495 longpass in place) and the 90mm ED refractor stopped down to 80mm to get another look at the Moon in the south. Conditions were blustery all day but a clear spell came between 7pm and 8pm. Hoping for the best, I quickly discovered that the seeing was horrendous lol (Antoniadi IV). What a difference a day maketh! Views were just passable at 113x in the 80mm f/5 and lousy at 200x. But precisely the same was true with the ED ‘scope. At powers of the order of 100x the images were just useable, but at double that magnification, it was well nigh impossible to get a sharp focus.
This was also confirmed on a double star situated much higher up in the western sky. Turning to Epsilon Lyrae 1 & 2, neither telescope could convincingly split the stars at a power of 200x. These observations provide an important lesson in their own right. Even small telescopes are not immune to seeing. And though the ED refractor is of unquestionably higher optical quality than the shorttube 80, the seeing conditions on this occasion completely overwhelmed both instruments.
On two evenings of good to excellent seeing (October 25 and 29, 2017) I was finally able to test the high resolution performance of the longpass filter with the 80mm f/5 achromatic telescope, using the aforementioned stopped down ED ‘scope as a control. Examining the images of Epsilon 1 & 2 Lyrae at 250x, the filter enabled me to achieve precise focus easily and consistently. The Airy disks appeared clean, greenish yellow in hue and round as buttons! Light transmission was excellent. The same was true when I tested the telescope on Delta Cygni; the companion being readily seen in the very good seeing conditions presented on these two evenings. But the proof of the pudding, as it were, lay with Mu Cygni A and B (very near the Dawes limit for such an aperture), which I have found to be exceedingly difficult to image well using the native 80mm f/5 achromatic optics, with the best views delivering a vaguely dumbbell like morphology. For reference, the 130mm f/5 Newtonian makes light work of this system and the pair is just resolved at 250x using the full aperture of the 90mm f/5.5 ED refractor. But stopped down to 80mm in the latter ‘scope, the pair was consistently seen to be touching. Using the longpass yellow filter on the ST80 finally allowed me to see these stars distinctly at 250x, the two Airy disks indeed touching each other, like a tiny snowman in the sky, the fainter companion (4.8/6.2) orientated roughly northwest of the primary; and just as the stopped down ED ‘scope had delivered. This is good evidence that the 80mm f/5 telescope can resolve to its theoretical limit, presenting images that are accurate representations of reality.
I am confident that the filter can indeed improve the performance of the telescope at the highest powers. It further dispels the myth that the ST 80 cannot do this kind of work. As usual, I attribute this to an admixture of laziness, the presumption of inadequacy, and sheer lack of interest. As ever, I would warmly encourage others to follow my work and to confirm these or related findings in due course.
The reader will remember that such a telescope has no need of such a filter (or any other filter) for low and medium power work (up to 100x or so). Indeed, this author is of the firm conviction that the very acceptable levels of secondary spectrum produced at these modest powers are desirable, educational and comely, in equal measure.
The longpass filter will also be utilised as a tool to enable me to obtain the sharpest possible images of the brighter planets when they come around. And while, in this capacity, I have obtained very encouraging results last season using the No. 56 Wratten filter, the longpass should help it perform that little bit better.
What other telescopes might benefit from using the yellow longpass filter? Certainly, a variety of fast achromats come to mind but I would caution that as the aperture gets larger and the focal ratio is kept low, sphero–chromatism will quickly overwhelm the ability of such a filter to perform as it has done in this 80mm ‘scope. The reader will recall Russell’s comments that the secondary spectrum on the ST80 is not overwhelming to begin with (indeed it is similar to that seen in a 6 inch f/8 instrument) and so the filter need only be used where the highest resolution is sought (planets and double stars). The author is also aware that Russell’s instrument is a Petzval design, so has a longer focal length doublet to start with. Still, I would encourage others to experiment using the more recently introduced 6 inch f/5.9 achromats, as well as the ubiquitous 100mm, 120mm, and 150mm f/5 achromats on the market.
That’s where this blog winds up. So ends the tale of my quirky little achromatic.
Thanks for reading!
Postscriptum: For more on what can be done with the Shorttube 80 achromat in regard to splitting close binary systems see pages 39 and 43 of the the book, Astronomy with Small Telescopes (2001), where a number of suitable targets and magnification regimes are discussed. Happy hunting!
Neil English’s soon to be published work; Chronicling the Golden Age of Astronomy, calls upon the vast resources of history to debunk many other telescopic myths.