So … you’re interested in getting into backyard astronomy, and you’re thinking of buying a second-hand telescope - where do you start? Used telescopes come up for sale all the time on eBay, GumTree, garage sales, second-hand dealers, etc, as well as on Astronomical forums such as Ice In Space http://www.iceinspace.com.au/forum/forumdisplay.php?f=27 and Australia Astronomy Buy and Sell http://www.astrobuysell.com/au/index.php . Many of these telescopes were bought by other novices (just like you!) who bought a telescope and then discovered they didn't have the knowledge or interest to sustain the hobby - you can benefit from their mistake by buying a telescope at half the recommended retail price or less, but you need to be able to pick the gems from the junk.
How does a novice go about evaluating a used telescope to decide whether to buy?
Do you have any experience with telescopes, or do you have a friend who has some knowledge? It is a LOT easier to buy a used telescope with confidence if you know what to look for. If you have no prior experience with telescopes, you may have no way of knowing whether the telescope which is offered for sale is any good at all, or is in fact a complete dog. I am not going to offer you any tips for how to judge the quality of a telescope other than by looking through it and assessing the quality of the image, which is ultimately the only thing that really matters.
If you don't have any experience with telescopes, and can’t inspect with a friend with some experience, the following tips may not make much sense.
First and foremost, do your research so that you know what sort of telescope you are looking for, and the relative strengths and weaknesses of each type. Do you want a long focal length (high magnification) or short focal length (wide angle) telescope? Do you want maximum aperture or maximum portability? Will the telescope be used mainly for visual or photographic use? Do you want a GoTo mount, or will manual pointing be OK? A lot of second-hand telescopes are on the market because the owners simply don’t know how to use them, so you may not get much useful advice from the seller.
Note that there is no single “best” telescope, they all have their strengths and weaknesses, and buying any telescope is therefore a compromise. There is a truism that “the best telescope is the one that you use the most”, so think about where you will store it, where you will use it, and how you will move it around.
A big Dob will give jaw-dropping views of deep-sky targets, but can be cumbersome to store and move; a modest refractor or Cassegrain (80 mm - 150 mm aperture say) on a computerised GoTo mount can be a much more portable instrument, but will lack the light gathering power of a big Dob. Intermediate sized Schmidt-Cassegrains (200mm - 250mm aperture say) are considered to be wonderful all-rounders, but are relatively expensive. If you succumb to the backyard astronomy bug, it is inevitable that you will contract Aperture Fever, and you will be on an endless pursuit for bigger and better telescopes. I would suggest that a first-timer should consider a quality instrument which is convenient and portable as a first telescope, as it is likely to be used more often than a big heavy cumbersome “light bucket”.
If possible, I take a couple of my best eyepieces with me for the inspection, a 10mm - 20mm diameter chrome ball bearing (which I’ll explain below), and a torch. A lot of used telescopes are sold with only one or sometimes no eyepieces, and it is pretty hard to judge the condition if you can’t look through the telescope! Sometimes, the eyepieces that come with entry-level telescopes are quite ordinary quality, and a decent eyepiece will show the telescope to its best advantage. I would take a good wide-field 10mm and 25mm eyepiece (or thereabouts), and possibly a 6mm and a 32mm as well.
I prefer to inspect in daylight hours if possible, because it's a lot easier to judge the condition in good light, and you can get a good feel for the optical quality by looking at a distant building or tree. For astronomical use, the ultimate test is a star test, and if I was spending a lot of money, I would want to do a night-time inspection before sealing the deal, if possible. However, Murphy’s Law will probably conspire to make it cloudy when your dream telescope is offered for sale, and you won’t want to let it slip away. A star test can be simulated in full sunlight by putting a chrome ball bearing in the sun about 20 metres away and pointing the telescope at it. The image of the sun on the ball bearing should be a brilliant pin-point of light, which simulates a star in broad daylight. If it is cloudy, or at night, shine the torch on the ball bearing to create an artificial star image.
Ultimately, as long as the optics are good, the telescope should be usable. I wouldn't buy a telescope with poor optical quality unless it is to get parts for tinkering - if you don't have good knowledge and technique, it can be very difficult if not impossible to get a good image from a telescope with dirty misaligned mirrors and lenses.
A little bit of dust on the outside of a refractor or Cassegrain, or on the primary and secondary mirror of a Newtonian, is not necessarily a big problem, but I would walk away from any telescope which has a lot of dust inside the closed tube of a reflector or Cassegrain, or any telescope with a lot of fungus on any of the optical elements. I would also avoid any telescope which shows a lot scratches on the lenses and mirrors. It is possible to clean dust and even fungus from lenses and mirrors, but they are also easy to damage if you’re not careful. It’s very important that you can re-assemble the telescope precisely aligned if it needs to be pulled apart for cleaning and refurbishment, so don't think about undertaking this unless you have done your research first.
If the telescope is clean and tidy on the outside, that indicates the optical quality is PROBABLY in fair to good condition, but check anyway. Shine a bright torch down the tube, to see how clean all the elements are. A little bit of dust is not a big problem, but thick films of dust or other gunk are something to be wary of. I’m not too worried if the outside of the telescope tube and mount have a bit of dust and even light rust (especially for an older telescope), but very poor external quality is again a sign that the telescope has not been cared for, and this could affect the optical quality as well.
If the telescope looks OK, set it up to point at a distant target (a tree or building on the horizon is ideal), and check out the image. Note that if it is a warm day in particular, the air will shimmer from thermal currents, and the image will wobble. Try to avoid pointing over hot surfaces such as roads, building roofs etc, as these tend to generate the strongest thermal currents. A distant target over trees will generally give a more stable image than over building roofs and roads, but of course, you will be limited by the situation where the telescope is offered for sale.
How does the image look - crisp and sharp from edge to edge, or is it blurry and hard to focus? If the general image looks OK, I would then use my ball bearing “artificial star” as the final test - you should see a brilliant pin-point when it is in focus, and this should turn into a nice-round donut with a black hole when you focus slightly in and out. If you can’t get a nice pin-point in focus, and nice donuts when slightly out of focus, this indicates an issue with the alignment of the optical elements (collimation). Unless you know how to collimate a telescope, I would walk away at that point. Collimation is quite easy for some telescope types (Newtonians and Schmidt-Cassegrains) as long as it is not too far out of alignment to start with, but much harder on refractors and Maks). If you know what you’re doing, you might even be able to collimate it on the spot, but if it’s badly out of collimation and you don't know what you’re doing, walk away.
Assuming the telescope comes on a mount, how does it feel - is it smooth and easy to balance, or does it grab and jerk as you move it? Does it hold position firmly when you find a target, or does it slip and droop? There is nothing more frustrating than trying to go stargazing on a mount which won't let you find and hold targets!
If it is a powered mount, does it work properly? Again, a lot of used telescopes are sold by people who have no idea how the powered mount should operate, so it is worth checking before you go whether they know how to power it, whether it has batteries, etc. If they don't know, it will be difficult to test the mount unless you are familiar with how that type works, and can bring along a suitable power supply to test it. It can be tricky to fully test a computerised GoTo mount in daylight hours, but as long as you can get power to the two drives, and move it left and right, and up and down, there is a good chance that the mount is more or less fully functional.
If the mount is in very poor condition (or missing altogether), most telescopes can be mounted onto any standard mount, so you can consider buying the telescope for the optical tube alone, and budget for a separate mount.
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