Broadly speaking, I subscribe to the argument that as an amateur, you should care primarily about using a telescope you look into rather than as a way of capturing images on a computer. That doesn’t mean I’m not interested in astrophotography, but nothing quite beats the visceral impact of seeing into space through a telescope eyepiece. And before you can hope to take the kind of photos you see online or in books, you need to know a lot of stuff which you can best learn through visual observation first.
I’ve had a number of telescopes over the years, and in 2015 decided to treat myself to another. During the course of my investigations into a market which had changed significantly since I last bought a scope in 2001, I found a lot of valuable information on the web, and a lot of contradictory opinion too. So I thought it would be worthwhile capturing what I think I know, and if some of this turns out to be wrong, I can update it.
Does aperture really rule?
The usual mantra is that ‘aperture rules’, i.e. a telescope which can gather lots of light will outperform one which can’t. However, in the real world things aren’t always that simple. Most amateur astronomers:
- are on a limited budget, measured in hundreds or perhaps a few thousands of pounds.
- require a degree of portability as they don’t have a suitable permanent site where they can build an observatory.
- spend their time in places where seeing conditions are seldom excellent and often poor.
- want to be able to use their telescopes quickly rather than wait for them to cool down after setting them up.
Of course it depends what you want to do, but
- A small telescope on a good mount is much better in practice than a larger telescope on a poor mount, unless all you want to do is get brief glimpses of faint deep-sky objects in wide-angle eyepieces.
- A smaller telescope will stabilise faster than a larger one, and a refractor ( which will be smaller in aperture at any given price point ) will be less sensitive to some atmospheric disturbances than a reflector or SCT.
- While a larger telescope will always capture more photons than a smaller one, it won’t necessarily show finer detail.
- Larger telescopes used at relatively low magnifications may produce a large exit pupil, and visual observers may not be able to see all the light. And if the telescope has a central obstruction, the image may be unusable.
Aperture and detail.
One of the first things they tell you is that because light behaves as a wave, small apertures are limited by diffraction in the detail they can resolve. This is of course true, but it’s also somewhat misleading. The bottom line is that under the kind of conditions an amateur is likely to encounter, it is rare for the atmospheric conditions to permit resolution much better than 1 arcsecond, and this is the same as the diffraction limit of a telescope of around 130mm, or say 150mm if there’s a central obstruction such as in a reflector or SCT. So, whereas a 70mm refractor will be much less effective than a 100mm refractor at resolving small details, there will be very little, if any difference between a 200mm SCT and a 300mm SCT.
Now, the typical eye has a realistic comfortable resolving capacity of about 3 to 4 arcminutes, so in order to allow a visual observer to see that one arcsecond resolution, you need a magnification of somewhere around 200. Anything higher than that won’t increase the visible detail.
What this means in practice is that if you are only interesting in viewing relatively bright objects such as the moon and large planets, bright double / variable stars etc, you will probably get as good a view from a 120mm refractor of highest quality as from anything else.
So why a bigger telescope?
- There are a lot of faint objects out there, and if you want to see them, or see detail in them, then you need the light-gathering power of a larger aperture. That 120mm instrument may resolve Jupiter in good detail, but it won’t show you Pluto, ever.
- If you do get really good seeing, then a larger telescope will take advantage of it in a way a smaller one can’t, and for serious observation that may be important to you.
- Even if you can see faint objects, a brighter image will potentially allow the cones in the eye to work, providing finer detail and colour.
What about the exit pupil?
If we take a typical dark-adapted eye pupil as being somewhere between 4mm and 7mm, we won’t gain any light gathering power from a telescope with a larger exit pupil than that. At our best typical atmosphere-limited magification of 200, that would equate to an objective of around 1m as being the largest size of value for the visual observer. And if you can afford a 1m telescope, you don’t need to be reading this! But a lot of interesting deep sky objects are quite spread out. Something a degree in size in a top-end eyepiece with a 60 degree FOV will only be visible in its entirety if the magnification is 60 or less, And the visual observer will start to lose light in that case if the objective is any larger than about 30cm – a decent size, sure, but quite a bit smaller than the larger amateur Dobsonians out there.
A final gotcha
So, for bright objects with lots of detail like planets, 120mm refractors and a magnification of 200x ought to be pretty good. For faint fuzzies, a 300mm scope ( probably a mirror-based one at that size ) and a magnification between 60 and 200 should serve as well as anything. But…if what you want to do is observe very faint individual point objects, e.g. small asteroids, then you might find that an even bigger scope is justified. Although you may also find that a smaller scope and a decent camera is a better option. But photography…that’s a whole other discussion.
