Stargazing with a Telescope by Robin Scagell
Help in choosing a telescope
Points to consider when choosing a telescope:
What do you want to use
it for and where?
If you live in a heavily light-polluted area, you can more or less forget most visual deep-sky observing. While there are many objects that you can observe, the views from the city are not usually very rewarding. There is little point in getting a telescope ideally suited to deep-sky work if all you can see are planets. Light pollution is no bar to observing the Moon and planets. Out in the country, this is less of a problem and you may choose what you want – though if your garden has a poor southern horizon because of trees, the planets may be hard to observe unless you have a portable instrument. You may be unlucky enough to live in a place where the seeing is always bad, for reasons of local geography, in which case you are more likely to want to observe deep-sky objects than the planets.
For planetary work that demands high magnifications, that is over about 150, it is important to have a telescope with a long focal length and good quality optics. Usually this means a comparatively long focal ratio – f/8 or longer. Refractors are often cited as being ideal for planetary work, but this is not necessarily true these days as many refractors are designed to be portable and have shorter focal ratios, around f/5 or f/6, which makes them hard to use at high power.
In my view, the main advantages of the cheaper short-focus refractors (meaning anything that does not use a special glass such as fluorite), are that they are fairly maintenance-free and do not need realigning and recoating from time to time. They are OK for occasional stargazing, or for wide-field photography, but not for high magnifications. You do get a brighter and more contrasty image from a 4-inch (100 mm) refractor than from a 4-inch reflector, but the optical performance from a good reflector should be similar if not better because it will be free from false colour.
The deep-sky observer prefers large aperture and, usually, fairly short focal length. These conditions are met by short-focus reflectors, and the bigger the better. But if you must have portability, a short-focus refractor will do the job but with a smaller aperture.
For photography and CCD imaging, a good mounting is essential. If you have a small, portable instrument do not expect it to be good for imaging. It may be possible, but it won’t be easy and the results will be mediocre at best.
How portable must it be?
In my view, the only reason for having a portable instrument is if you don't have room in your house for anything larger. If you live in a town and need to travel to get a dark sky, take something worthwhile. A 150 mm reflector, for example, doesn’t take up much room in the house or your car, and is many times more useful than a small refractor.
There is a lack of an instrument from a major manufacturer that offers largish aperture and light weight so that it will fit on a photographic tripod. Such instruments are possible – telescope wizard Horace Dall used to travel the world with a six-inch reflector in his coat pocket, for example – but not readily available.
If, on the other hand, you need something that you can store indoors or in a shed because you don’t have a permanent observatory, that’s another matter. You can put up with a bit of effort lugging a mounting outside in return for better results. But if you are no longer as young as you were, take into account the fact that many instruments require you to lift fairly heavy weights. The tubes of the popular 8-inch (200 mm) Schmidt-Cassegrain Telescopes (SCTs), for example, have to be lifted up onto their wedges before use. You must lift not just the tube but the fork mounting as well. You would be as well off with a conventional Newtonian in a PVC tube, maybe even of larger aperture, that doesn’t need to be lifted so far onto its mount.
I have taken an SCT abroad to Kenya, and it is a major undertaking. The tube and wedge fit in one large trunk, while the tripod has to go separately. It is transportable, but definitely not portable. But if you want an aperture that large when you are on holiday, the SCT is probably the best way to do it.
What about the range of SCTs and Maksutovs, such as the larger ETX and NexStar? These are designed to be portable and to be set up anywhere, but when you go to the larger and more useful sizes they are still not so portable that you can pack them in with the rest of your luggage. Their main advantage is not so much light weight as ease of use – they are simple to set up. This counts for a lot – it’s better to have a telescope that you like using and have fun with, than one that is so tricky to set up that you can’t be bothered, no matter how good its performance.
So in summary, consider what you mean by portable and how important it is to you. If you need real portability, be prepared to pay for it and put up with a fairly small aperture.
How much can you afford?
Often, the matter of cost is more important to parents buying telescopes for their children than by the more dedicated amateur. In this case there is one important guideline – don’t buy something small and cheap to get their interest whetted. Invariably, the small telescopes you see in toy departments, superstores, mail-order ads and even photo shops are not worth having and may even be so bad that they will put your offspring off astronomy for life. This sounds a bit damning, and there are a few good, cheap instruments to be had but they are few and far between, and not generally available from toy stores! Instead, aim for something that you can sell on if necessary, and buy with caution.
The terminally hard-up should consider kit telescopes, usually Dobsonians. If necessary, buy the optics only and make your own Dobsonian mounting. It is probably not worth trying to make the optics themselves, unless you would enjoy doing it, to save money. The savings, if any, are minimal these days. Or buy secondhand – there are many older telescopes around that still work well and are often given away because they don’t have motor drives and Go To.
Terrestrial telescopes are generally speaking not suitable for astronomy, though a good-quality instrument should show a bit of detail on the planets and give nice views of some deep-sky objects. They have apertures between 20 mm and 50 mm, and give upright views of the landscape, unlike astronomical telescopes, which invariably give upside-down views. You can pay a lot for some terrestrial telescopes, such as those for bird watchers and shooting enthusiasts, who use them to determine the accuracy of their hits. They are often called spotting scopes, and the more expensive models are generally of good optical quality and will give you some nice astronomical views, though inevitably with a rather small aperture.
Many people have been disappointed by small brass collapsible telescopes sold by mail order. These may not even meet their quoted specifications, and may contain very cheap optics in a brass tube that bumps up the cost.
There are some good Russian terrestrial telescopes that are worth having. The 20 x 50, for example, gives nice views and is excellent value, but the popular Turist-3 is no longer widely available in the UK, though as far as I can tell they are still in production. There is an alternative 20 x 50 named Yukon which I hoped would be a good replacement, but I am sorry to say that the sample I tried had a strongly curved focal plane. As a result, only objects within a small central area were in focus at any one time. This is bad enough for terrestrial use, but for astro use it would be intolerable. The smaller 10 x 30 telescope is still available, though it is too small for satisfactory astro use.
Binoculars are always a good starting point for amateur astronomers. They are specified as, for example, 7 x 50, where 7 is the magnification and 50 is the size of the main lenses in mm. The bigger the main lenses, the more light they collect – but the heavier they will be. The brightness of the image also depends on the magnification – the higher the magnification, the dimmer the view. So high magnification binoculars need really big lenses. See my reply to a question for more information on this, and of course I have written a book, Stargazing with Binoculars, in conjunction with David Frydman.
The smallest refractors commonly available these are 60 mm aperture. The cheapest are sold by toy stores and are misleadingly described because they have stopped-down lenses so they are incapable of giving good views at the excessive magnifications they boast. Always be wary of small refracting telescopes, even if they are intended for children. Some of the worst offenders even bear the name of a major and well-respected US organization and are sold by a major UK catalogue company.
This is the area where you have to be wary of what you buy. A reasonable 60 mm refractor will show plenty of craters on the Moon, will reveal the belts of Jupiter and the rings of Saturn with some clarity. The key thing to look for is an achromatic lens – that is, one which corrects (to some extent) for the false colour inherent in a simple lens. Many of the telescopes sold in toy stores and catalogue stores have simple lenses that give a very poor view, despite the detailed pictures shown on the box. They also claim that they have, say, 50 mm or 60 mm lenses when in fact there is a stop behind the lens that cuts the working aperture down to 20 mm or so. This would be like advertising a TV as having a 25-inch screen when in practice only the centre 10 inches gives a picture.
Even a cheap telescope with an achromatic lens is not guaranteed to give you a good view, but at least you will be in with a chance. Lenses (and mirrors) are not made on a production line all identical – each one is individual, so you can’t tell how good it will be without trying it. A good 60 mm refractor will show many deep-sky objects, but you need good dark skies to be able to see anything worthwhile.
A Sky-Watcher 90 mm refractor on altazimuth mount
Fluorite and ED lenses
A 100 or 114 mm reflector gives a significant improvement in image over a 60 or even a 75 mm refractor, and to my mind is the smallest telescope worth having for many purposes. You can begin to see enough detail on a planet such as Jupiter that you can readily detect changes, and the number of deep-sky objects visible is also increased noticeably over small instruments. However, don't be tempted by cheap 114 mm reflectors that claim to deliver 1000 mm focal length with a short tube, such as the Sky-Watcher Skyhawk 114 and the Celestron Astromaster 114 Short. The extra lens that is inserted to give the long focal length degrades the optical quality so as to make them very little use, which spoils two otherwise excellent ranges of instruments.
These days, 130 mm reflectors are becoming very popular as a good compromise between cheapness and aperture. The Sky-Watcher Explorer 130 as it is known in the UK costs just £169 (as of 2012) though I prefer the motorised 130M at £199. The motor makes a world of difference to the usability of the mount.
Larger reflectors The bigger, the better, by and large. A 6-inch (150 mm) Newtonian will show a huge range of objects and will generally show masses of planetary detail, while remaining portable enough to move around easily. As you get to larger sizes of Newtonian, the light grasp and potential planetary performance improves at the expense of portability. But even a 10 inch (25 cm) Newtonian of, say, f/6 is not a particularly cumbersome instrument, though it will need a fairly substantial mounting.
These instruments in the larger sizes such as 11 inch and 14 inch aperture are very widely used by planetary photographers. Image processing makes up for the slight loss of detail caused by the large secondary mirror.
Many telescopes these days are sold with motor drives and/or computer control. A telescope with basic motor drives will follow an object through the sky at the correct rate. This is not as easy as it might sound, because the rate varies depending on where the object is in the sky. The Pole Star, for example, hardly moves at all over an hour or so, because it is near the pole – the sky’s ‘pivot’ – but a planet will drift out of the field of view in a minute or less. There are two ways of making a telescope follow objects wherever they are in the sky. Either you can set it up as an equatorial mount, with one axis pointing at the pole, or you can give it a computerised controller that knows how different objects in the sky move. In this case you need to tell it where it is on Earth, the date, and how it is aligned, before it can follow objects correctly. The controllers that do this usually also have a database of objects as well, so that they can then take you to them. This is called Go To.
Basic Go To instruments are available for under £250 (the Sky-Watcher Explorer 130P SupaTrak Auto) and while the cheaper instruments will not have the Go To precision of more expensive models, they can be excellent beginners' instruments. The model quoted does require you to know one or two stars in the sky when setting up the Go To mount, however.
One other drawback with the TAL-1 is that it is less adaptable than Far Eastern instruments. So don’t expect to be able to buy the full range of add-on goodies, such as computer control.
My own view of the Go To craze is that it does not actually help the beginner as much as it should. The ETX Autostar software in particular is really badly designed and I wouldn’t recommend it to any beginner. And even when you do get it working, it is much better to learn for yourself where the objects are in the sky.
One thing which neither the ETX nor the NexStar manual gives due prominence to is that both telescopes should be accurately levelled before use, using a spirit level. It doesn't matter how accurately you point them north and with the tube dead level, if the base is not properly level they will not find their two reference stars accurately.
|Click here to read some (genuine) questions answered (and also some more comments about ETX telescopes).|
Site updated 18 April 2012