Culture

Astronomy Beginners Guide 7 – Choosing a Telescope.

By Duncan Lunan

Everyone interested in astronomy will consider buying a telescope at some point, and will ask ‘What type of telescope should I buy?’   Unfortunately there is no easy answer.   It is really a question of what can be afforded and whether it represents value for money.  When I began writing these notes for my astronomy column ‘The Sky Above You’ in 1983, I was told that Japanese telescopes were to be avoided:  they passed through so many hands before reaching the customer that prices could be severely inflated.  In addition, very few shops which sold Japanese telescopes had workshop facilities or ready access to spares.

That last point is still one to bear in mind and ask about before spending serious money;  but for the rest, Japanese telescopes are now at or near the top of the range, and the prices reflect that.   Back then, the best of the British manufacturers produced and sold telescopes which represented excellent value and out-performed their Japanese equivalents.  Japanese optics were usually excellent, but the stands supporting the telescopes were often too light to hold an instrument which magnified an object several hundred times.  ‘Shake’ in an astronomical telescope will make it impossible to get a satisfactory view of anything – if you can’t keep it still you can’t examine it.   Again that point remains true, and is one to watch out for, but Japanese telescopes and the best US ones are pretty solid nowadays.   Another major change was the entry of Russian telescopes to the market in the 1980s:  at well below western prices they were often excellent value for money.

Refractor or Reflector

The choice of telescope is between a refractor (using lenses) or a reflector (using mirrors).  All telescope sizes are quoted in terms of the diameter of the lens or mirror.  This determines how much light the telescope can collect and how great its ‘resolving power’ is.  Resolving power is the ability to distinguish small detail.  Magnification is not the yardstick of performance – you can only magnify the detail which the objective  (lens or mirror) can see.  The general rule is – the more the advert emphasises the amazing magnification, the more suspect the quality of the instrument.  Magnification can be increased by changing the eyepiece; nothing short of a larger diameter will improve the telescope.   When I was a curator of Airdrie Public Observatory, we were seldom able to use powers much greater than 100x or 120x.   We had more powerful eyepieces, up to 420x, but they could only be used in exceptionally clear and steady conditions.   The late Prof. Archie Roy’s experience at the old Glasgow University Observatory was similar:  the time he showed me the Moon at that magnification, it was the first time he’d ever been able to use it. 

The late John Braithwaite of Dalserf Optics (who provided much of the material for this article), for many years Scotland’s only telescope manufacturer, advised first-time buyers to avoid small refractors – although good second-hand ones, especially from reputable dealers, can be worthwhile.  That said, no instrument should be discounted in starting astronomy: anything which enables you to see more than you can with the naked eye should be pressed into service.   Remember how many great discoveries were made with tiny instruments, by today’s standards! 

Binoculars

Many beginners’ books on astronomy will advise purchasing a pair of binoculars rather than a telescope, in the first instance.  By definition a telescope shows only a small area of sky and one has to learn how to use it:  young people, in particular, are often put off by cheap optics and the difficulty of obtaining a clear focus, or even finding the target.   Furthermore, a telescope is generally only as good as its mount – it doesn’t matter how good the image is, if it’s whisked away by the first puff of wind or touch on the focussing mount!

The advantage of binoculars is that they combine a large field of view, very useful for learning the constellations and the fainter objects within them, with wide aperture and consequently good light grasp – approximating to the type of telescope called an RFT (Richest Field Telescope)  often used in comet searches, variable star work etc, where high magnification is not what’s required.

The disadvantage of binoculars is the problem, again, of keeping the image steady.  Binoculars are heavy, especially for young users.  Patrick Moore recommended a frame, hung round the neck, which took the strain off your arms.  Another approach is a beginner’s telescope which combines the cheapness and light grasp of binoculars with a table-top tripod or a camera tripod fitting.   Until his untimely death in February 2012, John Braithwaite produce a table-top telescope of this type which sold for well under £200.

In general terms the difference between a refracting telescope and a reflector, is that a refractor gathers light by means of a large lens at the front of the telescope (the object glass);  the reflector gathers light by means of a large mirror at the rear.  In both cases the aperture is crucial to the resolving power of the instrument and sets an upper limit on the magnifying power which can be used at the eyepiece.

Both have their disadvantages.  In a refractor the image of a star will be surrounded by diffraction rings and the poorer the quality of the object glass, the more these will be subject to false colour.  In a reflector the corresponding problem is diffraction spikes with the same pattern as the arms supporting the secondary mirror.  (Many artists, having seen photographs of stars taken through reflectors, show all stars and suns with this effect – often with the spikes at a variety of angles!)   As John Braithwaite was wont to say, a telescope, no matter how complex, is just two pieces of glass held in a fixed relationship.   However that’s achieved, every telescope design is a compromise.

One very interesting combination is the catadioptric telescope, which combines lenses and mirrors to produce an effective focal length much longer than the tube of the telescope.   In general long focal lengths are better for lunar and planetary observing, but the catadioptric design is another compromise which also gives excellent results with the stars.   Back in the 1960s Questar was the top of the range, costing roughly as much as a top quality sports car  (or you could go for the professional model which gave you an entire portable observatory in two suitcases, for the price of a small house.)   But the American Celestrons and Meades, and Russian Maksutovs, have put catadioptric telescopes within the reach of average buyers,    

In general, a good refractor of a given aperture will out-perform a similar reflector.  But then again, a good quality object-glass costs much more than a good-quality mirror;  that’s because the object-glass has to be perfect right through while the mirror has only to be perfect at the surface.  This is why Braithwaite’s advice to the first-time telescope buyer was to avoid small refractors.  It’s sometimes said that for worthwhile amateur astronomer, a 3-inch refractor or a 4-inch reflector are the minimum instruments.  (Remember, however, how much was done in history with less.)   Some manufacturers will part-exchange instruments, and this can be very useful if you plan to buy a larger telescope when funds or experience permit.

These days, apparently, some say that nothing under 6-inch diameter is worth buying.  On that, Braithwaite commented that one might say that a Hasselblad camera is essential for photography.  For some tasks, maybe so – but not for beginners.   Without him, these days when I’m asked for advice, I generally consult a friend at the Mills Observatory in Dundee.   They sell telescopes, and he’s able to tell me what’s currently available and what’s best value. 

blue universe
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