Review Of: the Shorttube 80 Telescope: a User’S Guide by Neil T
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Review of: The ShortTube 80 Telescope: A User’s Guide by Neil T. English This book was kindly donated to SAS by Mike Geisel in November 2019, and is another volume of the prolific Patrick Moore Practical Astronomy Series. This is a first edition, published by Springer in 2019, so it should be quite up-to-date. A nice point is the small number of typos; many other first editions have much more. After the usual front-matter (Preface, Acknowledgements etc) are the thirteen chapters of the text: 1 Under a Dark Sky 2 Anatomy of a ShortTube 80 3 Mounting a ShortTube 80 4 Exploring the Deep Sky 5 Improving the High-Power Performance of the ShortTube 80 6 Watching the Sun 7 Exploring the Moon and the Planets 8 Exploring the Realm of the Double Stars 9 The Meade EXT 80 10 Imaging with the ShortTube 80 11 Souping Up the ShortTube 80 12 Founding Days 13 Afterword: The Little ‘Scope that Could Index The first chapter (Under a Dark Sky) is a tour (perhaps a real single night, maybe a composite spread over months or years) of celestial objects which are suited to a small refractor, viewing in mid- October. The Author includes open star clusters, emission nebulae and larger galaxies, as well as the waning crescent Moon rising in the east in the wee-small hours, but no planets. The author mentions less-familiar objects such as Collider 70, an open star cluster, better known as the sprinkle of ~140 mostly 6th - 9th magnitude stars centered on Orion’s Belt, and including the three 2nd magnitude Belt stars. The Collinder (1931) and Melotte (1915) Catalogues each contain a few hundred bright open clusters, typically a few tenths of a degree wide. Many appear on both Catalogues. Here is a new resource for wide-angle telescopes. There is a category of telescope called the Richest-Field Telescope (RFT) which is ideal for such wide-angle objects. The ShortTube 80 approaches RFT parameters... more later. The location given for the tour is Scotland, and the Author lives in Glasgow, which has a latitude a little south of 56° N, so many of the objects viewed are unfamiliar to us SE Queenslanders (27°-28° S); the difference is about 84°. With this reservation, the tour is a good flavour of the ShortTube 80’s forté. Chapter 2 (Anatomy of a ShortTube 80) as its title suggests, walks the reader through the “anatomy” (or architecture?) of the ShortTube 80, sometimes nicknamed “Shorty”. A brief list of specifications is: 80 mm aperture air-spaced conventional achromatic doublet, and 400 mm focal length, for f/5 focal ratio. This combination of features gives wide-angle performance that is difficult to achieve in other inexpensive telescopes. This chapter reveals a slight misnomer in the book’s title... The ShortTube 80 is a specific model (no longer in production) sold by Orion. Several retailers sell telescopes with similar optical parameters, and the Author includes these variants under the “ShortTube 80” name. Many versions are packaged with inexpensive entry-level accessories such as a 90° star diagonal, a 25 mm eyepiece (16X), and a 10 mm eyepiece (40X). Some come in a hinged aluminium-look hard case with sculpted foam to protect the contents. A 1.25-inch rack-and-pinion focussing tube is the usual type. Many are packaged for retail sale with a cheap alt-az mount. (The most common version is probably SkyWatcher’s SW804AZ3S model, packaged with a findersope, 45° diagonal, 10-mm and 25-mm eyepieces, and mounted on their AZ3 alt-az mount.) Some versions of the ShortTube 80 are of reasonably high quality; the basic optical system or Optical Tube Assembly (OTA) is often the strongest link in the package. To make them compact, many versions have a slide- back retractable (metal) dew shield, and a long focusser travel. Many have a metal front end-cap that screws into the dew shield; probably all include a plastic dust plug for the focusser. Some have a built-in or attached dovetail for mounting; this might be too short if you change the telescope’s balance by using heavy after-market eyepieces, especially 2-inch ones, in combination with a matching star diagonal. It may be necessary to buy a longer dovetail bar and tube rings to suit. I have experience with a refractor of this class... the Prostar 80 was sold (but is no longer stocked) by MyAstroShop for various prices in the $AUD 350-450 range. Mine was an OTA (no diagonal, eyepieces, finder-scope or mount, but it included a zippered black canvas-style padded carry-bag with handles). It has a 2-inch Crayford-style two-speed focusser; and the bag is roomy enough to hold a few accessories such as a star-diagonal and an eyepiece or two. I wanted the 2-inch focusser, because some of my eyepieces are 2-inch only (others are dual-diameter), purchased specifically for my other telescope, a 1970’s orange Celestron-8; and I didn’t want to buy a set of quality 1.25-inch eyepieces too. There is sufficient travel in the focusser to accommodate the longer length of a 2-inch diagonal, as well as the shorter mechanical length of a DSLR camera adapter. Its characteristics using my eyepieces are as follows: Eyepieces parameters: Focal length Apparent FOV Power True FOV Exit pupil Baader Hyperion aspheric 36mm 36 72 11 6.48 7.2 Masuyama 26mm 26 85 15 5.53 5.2 Meade MWA 15mm 15 100 27 3.75 3.0 Meade MWA 10mm 10 100 40 2.50 2.0 William Optics XWA 5mm 5 110 80 1.38 1.0 The Southern Cross is 6.00° tall, the Pointers are 4.38° apart; and targets such as the Hyades (face of Taurus the Bull), Orion’s Belt plus Sword, and each of the Magellanic Clouds (Small Cloud plus globular cluster 47 Tucanae) fit inside the view circle if I use my Baader Hyperion aspheric 36mm eyepiece. My eye-pupil diameter of 5.5 mm in effect “stops down” the 80mm lens to 63mm with this eyepiece; this costs ~0.5 magnitudes in the faintest star visible. My Masuyama 26mm eyepiece is a good match for my fully-open pupil; and captures all of the above deep sky objects, except the Southern Cross (just missing the foot), and it misses either one star of Orion’s Belt (δ Orionis), or the tip of his Sword – but captures M42, with all of the Belt. The author also mentions that this style of telescope is very good as a spotting scope for observing targets like birds. For nearby objects, the focus position will be outwards from the position needed for astronomical subjects. Different models may have different outermost focus positions; something that should be assessed according to your requirements. A further use is as an autoguider for astrophotography through a second imaging telescope. Modern CCD guiding cameras are so sensitive that you might not need the light-gathering power of an 80 mm telescope, and a smaller dedicated autoguider would work well or better; but if you don’t need one, why spend the extra money? Author Neil English says that the ShortTube 80 is also good for solar observation using a front-end solar filter. I’m confident that this would be so, but I haven’t tried it; solar viewing generally doesn’t interest me, except for rare events like transits of Mercury (and Venus). As English points out, most telescopes in this class can be upgraded with after-market accessories... Crayford-style dual-speed 2-inch focusers, premium eyepieces, high-reflectance diagonals, German equatorial mounts etc. But if you can afford a large portfolio of improvements, then a higher-quality OTA such as an ED doublet, or a triplet might be a better investment; these have special expensive glass in the objective. An 80 mm ED doublet or triplet OTA sells for ~$AUD 1200 -2500 but a suitable package might be negotiated at a good price. Beware of “putting lipstick on a pig”, by trying too hard with a ShortTube 80. English takes the reader through the suitability of several aftermarket high quality eyepieces, and warns against using eyepieces with very long focal lengths. The issue is the wide-open size of the user’s eye-pupil. If the telescope’s exit pupil exceeds the observer’s eye pupil, the effect is as though the 80 mm diameter is “stopped down” in the ratio of eye pupil ÷ exit pupil. The corresponding light ratio is in the ratio of the squares of the two diameters, and that amounts to a specific loss in magnitude of faintest star visible. If the diameter drops to 72 mm, the loss is ~0.2 magnitudes; for 64 mm the loss is ~0.5 magnitudes. The compromise is minimal, unless you need to see the very faintest stars accessible, or the very finest detail. I think English exaggerates the issue. The Author next discusses the chromatic aberration found in such a telescope. (ED doublets and triplets are much better colour-corrected, so chromatic aberration is less obvious in these more expensive telescopes.) There are various ways to reduce the effects of chromatic aberration... a light yellow Wratten #8 filter, or minus-violet filter; otherwise, use a more expensive interference filter specially designed for the purpose, variously named Semi-Apo, Fringe Killer or Contrast Booster. All of these reduce light transmission by 50% or more, causing a loss of ~0.75 magnitudes (or more) of faintest visible star.