If you are thinking of buying a telescope on ebay for astronomy purposes, there are many things you need to know before you can make an informed buying decision.
I'm writing this short guide because there are a lot of cheap options on ebay, and you need to know what defines the average quality from the true bargain. Can I also draw your attention to some of the other very good guides in this category, particularly those by 1alienx which go into more detail than I will here.
I imagine if you're reading this you've just recently become interested in astronomy and have decided to take the plunge and buy a decent, or at least half-decent telescope. You might have had a look around the specialist websites and dizzying sums of £1000 or more seem to be the going rate for the larger devices. But then you come to ebay and these large telescopes seem to be on offer for £100-150 from names like Seben and Tasco.
These listings seem to be honest - they talk about not giving the newcomer false information about magnification possibilities and make other claims about being anti-disinformation. It's a clever tactic because a lot of what they say does have some truth to it. But, of course, they are not giving you the whole picture.
I will explain. The two basic types of telescope design that comprise all devices in the sub-£300 price range are refractors and reflectors. If you do not know the difference this is where your research must start. Open a new browser window and go to google and type refractor or reflector? or words to that effect. Find an article that discusses the differences and absorb the information. Alternatively, you could use wikipedia. Look specifically for Newtonian Reflector to be mentioned.
I am going to assume then that you now know the merits of each design. OK, so these big telescopes on ebay are all Newtonian reflectors right?
Wrong! The vast majority of the cheap ebay scopes are catadioptric reflectors. Does this really make much difference? You bet. These telescopes seem to have large apertures and focal lengths. You might not exactly understand what all this means but surely it must be great? Bigger is better, right? Not really, because they've cheated, as I will now explain.
The focal ratio in a telescope is what defines the field of view. In layman terms, it's how big a piece of the sky you will see with the telescope. If you have a very long telescope with a relatively narrow aperture (the diameter of the scope at the front) i.e. something that looks like a long pipe, you will see a very small piece of the sky. Now the size of the aperture and the eyepieces will determine at what level of detail you see that small piece of the sky but the key thing to note is that it will be small!
On the other hand let's assume that aperture stays the same but the scope is much much shorter, so that it is like a stubby piece of pipe. This shorter length increases the field of view. Bear in mind that impressive sights in the night sky like the Andomeda galaxy are actually really quite large. If your focal ratio is too high, you will not be able to fit these phenomena into your field of view.
The reason why these apparently quite short ebay scopes have high focal lengths is that they cheat by adding a second focuser inside so that you get effectively double the focal length. This is why they are Catadioptrics and not real Newtonian Reflectors. A stroke of genius? Sadly no. Whenever you add an extra lens or any optic at all inside a telescope it affects the quality of the image by a great deal. This is because when we are viewing the night sky we are dealing with very small amounts of light that have travelled a huge distance and through our atmosphere to reach us. A high focal ratio, say 1400/150 as I've seen for some Sebens, which equals f9.33, is also going to give you that narrow field of view. You should be aiming for a focal ratio of about 6 or 6.5 in a general purpose scope. Being able to see more is better than being able to magnify more.
I should point out there there are designs of telescope called Schmidt-Cassegrain and Maksutov-Cassegrain amongst others that also use catadoptrics. This is why those really expensive scopes have that same kind of shape. But those are semi-professional instruments in a whole different league and even they suffer slightly for the nature of their design. At the lower ebay prices (and aperture sizes) catadioptric designs just do not work well.
This is also where the quality of the optics come in. You know those 99p sunglasses you see by the seaside? Rubbish?
An exaggeration perhaps but the same is basically true of telescopes. I can promise you this, the quality of the optics is the most important part of any telescope, or any optical instrument and this is without exception. You can have the biggest scope in the world but if it uses terrible mirrors you will see nothing!
The size of the aperture determines how much light-gathering capability your telescope will have. Forget about magnification, this is the key thing. This is where the cheap scopes usually snare buyers. They have large apertures, as big as 150m which promise much. But for the reasons I have already discussed namely quality of optics and an effective focal length, and making sure the scope is a bona fide Newtonian rather than Catadioptric design, are going to be much more important things to get right. After that, you want to get the largest aperture you can afford that is practical for you and what you will be doing with your scope.
If you want to pack up your scope and take it with you to a dark site (which you will find yourself wanting to do if your scope is good quality and promises much even in light-polluted skies) then you need to get realistic about how big you can go. This will probably not be tough anyway since beyond a certain size of aperture the price curve for telescopes rises considerably. Scopes over 200mm are not that portable. It is important to note though that even a modest increase in aperture can gather a lot more light. For example, Celestron replaced their 114GT model (114mm aperture) with the 130GT (now 130 SLT). It sounds like a modest increase, yet the 130mm gathers 60% more light!
If you live in a dark area and have a large back garden and somewhere safe and dry to store your scope within a few yards of said garden, then you are very lucky. Not only do you have the checkboxes ticked for anything up to a 10 or 12 inch scope but I will assume you can also afford one! The rest of us, as I said, need to get real. If you can afford 8 inches (200mm), great, if you can't, don't even waste time thinking about it, anything over 80mm by a decent manufacturer will be good and over 100mm is where you want to be aiming. You can always buy a bigger one.
Remember, it's easier sell a smaller scope made by a reputable brand than a bigger one by an unknown.
There are other points to be made here too. Many experienced amateur astronomers will tell you through slightly gritted teeth how they use their largest telescopes much less than their more portable ones. Simply, they take a lot of setting up and "fiddling" to get going. You will be surprised how long it takes to set up even a modest scope of the 100-200m range. Take a step back and think how long it will take you to move the scope, where you will store it, how long to set up the tripod, etc.
Larger telescopes are also quite heavy, and the tripods they sit on must needs be even heavier. Do not buy something you are not comfortable moving about. If you drop the optical tube the chances are the mirror will shatter and it will be essentially worthless.
A really important factor that is overlooked by many newcomers is the mount that your scope sits on. There are different types available so as with earlier do some google research on equatorial and alt-azimuth. Avoid big scopes on rickety aluminium tripods and flimsy mounts - these will be highly unstable at high magnifications and are badly affected by the wind.
I personally would recommend the extra investment in a mount that has electric motor and gears and also a "goto" computer system if you are new to astronomy. Google these terms now if you like. Having a telescope automatically track an object in the sky under its own power is extremely useful and these kinds of scope usually have much better mounts than non-powered versions. They are more expensive and there are some hardcore astronomers who insist that you should learn the star charts and be able to manually guide a scope and track your target, but I personally believe that a newcomer can lose interest quickly this way. Letting the scope take much of the guesswork out of observing is a legitimate way to get people interested and prevent wasted sky-time. Besides, if you get hooked, you will soon be linking your laptop up to your computer-driven telescope and then you will be using software that will teach you the constellations etc. anyway!
I really would recommend the extra investment - if you are serious about achieving good results you will need to stop thinking about getting a "bargain" for £100 or so or less and start thinking more about £200 to £400 all in. I know it's a lot of money, but it's the price you must pay to have equipment worthy of keeping your interest. Trust me, it's worth it :)
Some final key points to remember;
- Read as many opinions as you can from experienced amateurs before buying! (V. important.)
- Buy from a good manufacturer with an established reputation (Celestron, Meade, Orion etc.)
- Magnification is achieved with eyepieces and is the least important factor, not the most.
- Choose a focal RATIO of about f6.
- If it's too cheap to be true, it usually is.
- Get the largest aperture you can afford but budget for a very good mount and tripod too and don't sacrifice on these!- Think about where you will keep it and use it.
- Ask yourself what you think you will interest you most. Refractors are a little better at solar system objects and reflectors do a little better at DSOs (Deep Sky Objects).
- Be realistic - your telescope will never give you the views that Hubble gets but it will be more magical and the light will be all your own!
- If you just want to look at the skies and aren't fussed about knowing what you see - consider a Dobsonian. Otherwise newcomers should consider a Goto scope to really enhance their learning curve.
- You can find just as much good stuff at reasonable prices on Ebay as you can average stuff at cheap prices. Choose the former!
And above all, have fun. It's meant to be enjoyable! :)