Karthik Subramanian, karthik !remove spaces! 301176 !+! bas !AT! gmail !DOT! com
The objective of this article is to provide the first-time telescope buyer with the minimum information needed to ask the right questions before investing in a telescope.
Please note that this article only covers equipment for visual observation. It does not cover equipment for astrophotography.
If you’ve already made up your mind that you want to buy a telescope, then jump straight to the Vendor Information page. If not, read on.
This guide by itself is probably not enough to help you buy a telescope; but it should definitely help you get started. It is necessarily short on many details. Like its title implies, it makes many assertions without proof, or even evidence. If you find that you have questions, please do ask on the BAS google group. In addition, Star Ware is a good resource on this topic.
No originality of content is claimed, except perhaps for the presentation.
… not just yet, anyway.
A quality telescope is an expensive instrument. Should you choose to buy a telescope and decide later that amateur astronomy is not for you, you’re stuck with the scope until you find someone who will buy it from you.
Many beginners approach a telescope with very high expectations. No matter how expensive the telescope is, it cannot do certain things.
For one, what you see at the eyepiece will not be anything like the colourful photographs you see taken from the Hubble Space Telescope. In fact, most of what you see at the eyepiece will be images in varying shades of grey. This isn’t really the telescope’s fault; the human eye cannot perceive colour in very dim images. Cameras can, however - but that’s the subject of another discussion.
For another, your telescope will likely not show you the massive images of the planets that you’re used to seeing in photographs. Jupiter will probably appear smaller than a single whole piece of black pepper.
So what can you see through a telescope? This site gives a good idea of what to expect at the eyepiece.
A word of caution: the sketches in the link above show you views that can be achieved with a telescope. The views are real, other people besides the original sketcher(s) have seen them, /and/ documented them. However, please do keep in mind that it takes a lot of practice to actually achieve these views.
Inspired by the sketches, you might take your newly-acquired scope to your rooftop and hunt for say, the Andromeda Galaxy. In all probability, you will see just a grey blob. Many factors affect the view in your telescope - some, like the equipment, and its care, are in our control. Others, like dark skies (or the lack thereof), or the weather, are not entirely so.
Please try and look through a telescope or two, to appreciate what you’re getting into before you put down your money. One way of doing this is to attend star parties. Another is to just sync up with your friendly neighbourhood BAS member, most (if not all) of us will be happy to show you our telescopes.
If you really want to buy something right away, consider buying a good pair of binoculars. Why binoculars?
This page has more to say about binoculars.
It is usually a good idea to get familiar with some telescope terms, and what they mean, before buying a telescope. The Telescope Terminology page should help. The rest of this guide assumes that you are familiar with at least some telescope terminology.
More aperture is a good thing to have. With more aperture comes more light-gathering capability, brighter images, and the ability to resolve finer details. More aperture also brings with it more bulk and weight.
Longer focal lengths yield a smaller field of view. Shorter focal lengths give a wider field of view. Longer focal lengths also result in a longer telescope, except in the case of Maksutov- and Schmidt-Cassegrain telescopes.
When budgeting for a telescope, keep in mind that you might also need to buy a few accessories. The minimum setup you need is a telescope OTA, a finder scope, a mount, and one or more eyepieces. Depending on the telescope, you might also need a star diagonal. Most beginner telescope packages contain all of the above, in which case you probably do not need to buy anything more.
The prices mentioned below are current as on the time of writing (Nov 2015).
The prices mentioned below are necessarily approximate, and are meant to give the reader a rough idea of the cost. Please contact your vendor for up-to-date and accurate pricing information.
The ultimate buying decision is that of the buyer. Please do not take the recommendations below at face value - instead, use them (and this guide) as an aid to do your homework before arriving at a buying decision.
If budget is your limiting consideration, then the buckets of your interest are roughly Rs. 13K, and Rs. 23K. If your budget is smaller than Rs. 13K, and you do not already possess a pair of binoculars, you are encouraged to read the binocular page.
A budget of around Rs. 13000 will fetch you a 70mm Equatorially mounted refractor. In the same bucket, you might be able to find a 70mm Alt-Az refractor for a little less, or an 80mm Alt-Az mounted wide-field refractor for a little more.
If you’re willing to stretch your budget roughly twofold, a good “middle path” choice would be a 6” Dobsonian telescope. This should typically set you back by around Rs. 23000.
As a rule of thumb, do not buy a refractor with optics sized smaller than 70mm. You will read that any telescope is a worthwhile investment, even if it’s “just” a 50mm refractor. This statement is true. The trouble with smaller (and cheaper) telescopes is that they are usually a marriage of decent optics with useless mounts. Thus, it takes some experience to extract goodness out of these telescopes; this can be a frustrating experience for a beginner.
In a similar vein, avoid reflectors sized smaller than 4.5” (114mm).
The ideal starter scope according to many is a 6” Dob. This is indeed a good recommendation. Before you go ahead and buy a Dob, please try and see one for yourself. A person of typical build should have no trouble in carrying a 6” Dob, in two pieces at the most. However, you should verify that this will indeed work for you before investing in a Dob. If portability is your overarching concern, you might want to consider a small (5” or thereabouts) Mak or SCT.
“Starter” mounts are usually underprovisioned. Buying a well-provisioned mount will let you buy a second (bigger) telescope in the future without having to buy a mount for it. (Assuming that you do not want to use both telescopes simultaneously.) It goes without saying that a good mount will also inflate your budget. It will also be a rather heavy piece of metal.
GoTo mounts are capable of providing some of the conveniences of an equatorial mount, without weighing as much. Most of them work with reasonable accuracy after being aligned. Note that GoTo mounts need power, either in the form of batteries, or AC. Some will not work at all without power.
All telescopes do not perform equally well on all kinds of targets. Broadly, one could speak of Planetary Observation and Deep-Sky Observation.
Long focal lengths are suited for Planetary targets, since the narrow FOV (and high magnifications) will work well to show details on the Moon and Planets. This is also good on Double Stars.
Short focal lengths give a wide field of view, and are suited for Deep-Sky targets.
Here too, a good “middle path” choice is a 6” Reflector. Most available 6” Dobs seem to be figured at f/8 or thereabouts, making them slightly on the longer side.
Refractors need the least maintenance.
Reflectors need to be collimated fairly often. Collimation is a simple procedure, however, and is nothing to be afraid of. A badly collimated reflector will perform badly.
Short-tube reflectors with spherical mirrors and a corrector (Barlow) built into the focuser (the Bird-Jones or Jones-Bird design) are difficult to collimate. You can recognize them by comparing the stated focal length of the telescope to the physical length of its tube. If the tube length is a lot lesser than the focal length, the scope is a Jones-Bird. Note: There is nothing inherently wrong with the Jones-Bird design; it is just that most low-end scopes using this design are badly done. See References 5 and 6 for more details.
Maks and SCTs usually do not need to be collimated in the field. Collimating a Mak is more difficult than collimating a Newtonian reflector.
Collimation Tools
Newtonian reflectors need collimation every now and then. Most reflectors come with a collimation cap, which should be sufficient to perform satisfactory collimation. More expensive collimation tools like Laser Collimators and Barlowed Lasers are available as well.
Cleaning Tools
For most purposes, an air blower (of the photographic kind) and a soft camel-hair brush should do. Do make sure to perform the “cheek test” on the brush - poking at your cheek with the brush should not cause any bristly feeling. If it’s soft enough for your cheek, it’s OK to use it on your mirror as well.
More complicated cleaning might be needed on an infrequent basis. This is detailed elsewhere.
Buying “Toy-Store” telescopes which are marketed solely on the basis of their magnification. In other literature, these are also known as “Department-Store Telescopes”.
Trusting reviews that do not explain why (or why not) a telescope is “good” or otherwise.
Telescopes with a focuser barrel size other than 1.25” or 2”. 0.965” Eyepieces are hard to come by.
45-degree diagonals. They’re good for terrestrial viewing, but are uncomfortable for astronomy.
Please consult the Vendor Information page.
Check the OTA for scratches and dents.
Check to see that the mirror (or lenses, as the case may be) is reasonably clean, and doesn’t have any visible spots or scratches.
Check that the mount is working fine, and slews smoothly.
(This test applies to any tripod-mounted telescope). Focus on any object of your choice. Rap the mount near the OTA (or the tube rings) with your knuckles - not very hard, but with just enough force to make the view in the eyepiece shake. (This shouldn’t be too difficult). Count the seconds until the view in the eyepiece settles down. A settling time of 3-5 seconds is good; anything more than 10 seconds is not. If your scope takes too long to recover from vibrations, you might have to do something to make the mount more steady.
The Star Test The idea behind the star test is to focus the telescope on a bright star using reasonably high magnification. Then, adjust the focus inwards until the star is out of focus - you should see a pattern of concentric rings. Adjust the focus outwards, until the star is out of focus again. You should see exactly the same pattern of concentric rings as before. If not, your scope needs collimation.
The star test is detailed more comprehensively in Star Ware, and also in Gary Seronik’s No-Tools Telescope Collimation.
Freshly-unpacked Refractors and Catadioptrics should not need collimation. The best thing to do in such a case is to return the scope.
If you bought a reflector, and the primary mirror is not centre-dotted, then you might want to do this. Centre-dotted mirrors are easier to collimate than the ones without. A good reference for centre-dotting is 8.
No harm done. If you are satisfied with your purchase, and are able to use it well, skip this section. If not, read on - it might be possible to salvage the situation.
I bought/was gifted a small (50mm/60mm) refractor by a well intentioned friend/relative. It shakes like crazy and I can’t seem to find anything in the sky apart from the moon.
Don’t throw it away!. Not just yet, anyway. Small (and cheap) refractors, as mentioned earlier, are usually not a bad deal, except for the mount. This is especially true of the lower-end Celestrons and the like. Typically, the tripod/mount that these telescopes come with are light and flimsy. In some cases, it is possible to make the tripod/mount more stable by counterweighting the tripod. It is also possible to make use of pieces of teflon sheeting to smoothen the motion of the mount head.
If your refractor shows interesting colours of the rainbow on the moon, and you’re not really able to see the moon except as a blob (albeit a blob with some features) then the optics are probably made of plastic, or low grade glass. This is usually the case with Toy-Store scopes. They are usually a bad deal.
I bought/was gifted a low-end Jones-Bird reflector that doesn’t work well
The most popular Jones-Bird reflector in this location seems to be the Celestron AstroMaster 114.
The primary complaint with a low-end Jones-Bird is usually that the images are “soft”, and it is impossible to bring the instrument to focus. This could be caused by either of two reasons:
The instrument is out of collimation (and is difficult to collimate), or,
It refuses to come to focus, regardless of the state of its collimation. This usually happens because of a badly positioned corrector, or a bad corrector.
Another complaint could be that the image is blurry throughout the range of the focuser. For instance, the moon remains a white blob, and it is impossible to resolve any details - even craters - on its surface.
Some background is available in Reference 6.
To troubleshoot:
Check that it is possible to unscrew (or otherwise remove) either the corrector alone, or the focuser along with the corrector from the OTA.
Sometimes the corrector is secured by it being glued in place. This could make it impossible to remove the focuser/corrector without risk of damaging either.
If this is the case, the best thing you can do is to try and collimate the scope with the corrector in place. This, as we have already seen, is fairly difficult to do. You might want to try using a laser collimator if you can get hold of one.
If it is possible to remove the corrector, please try and remember if you removed the focuser/corrector at any time in the past. If you did so, and inadvertently reversed the corrector, the telescope will not come to focus.
Remove the corrector. Check the state of the telescope’s collimation. If it is out of collimation, try and collimate it. See either Star Ware or Gary Seronik’s Beginner’s Guide to Collimation for simple collimation instructions.
Replace the corrector, and see if you can now achieve focus. If not, try reversing the corrector and check again. If this works, you are good to go. If it is not possible to reverse the corrector, then your best bet is to return the telescope to the seller.
If, after performing all of the above steps, the views in your Jones-Bird are no better than before, then you should return the telescope.
If you still have questions, please feel free to post on the BAS Google Group.