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purchasing a telescope – things to avoid December 9, 2009

Posted by aquillam in Astronomy.
Tags: ,

‘Tis the season for spending money we don’t really have on gifts we hope turn out to be worth it. If “telescope” is on your list of things to buy, there are a lot of guides out there to tell you what you should look for (including an early post of mine)

So how about a guide to what NOT to look for.



The primary gimmick I see is amazing powers of magnification. Recently, my husband spotted a tiny telescope in a pharmacy chain with “See the Moon at 150x!” splashed across the box. A quick search for a telescope will bring up many cheap ‘scopes which include magnification in the title.

The primary reason this is a gimmick is that any halfway decent (and some not so decent) ‘scope can do a wide range of magnifications. Just change the eyepiece. If you can’t change the eyepiece, skip the ‘scope.

Additionally, if the telescope or the viewing location aren’t good, more magnification is actually a bad thing. Looking across the neighbor’s house in winter, you’re likely to have a lot of heat shimmer (“bad seeing“). Add more magnification, the heat shimmer gets magnified to the point where Jupiter is a smeary blob and there’s no hope of making out bands.

extra or “free” items

The next thing to look out for is supposed bonus or free items. For example, the same tiny telescope I mentioned above also  said it included a tripod, for free!

With very few exceptions (and all of them either very cheap or very expensive), you should expect the following with a telescope: the optical tube assembly (OTA, the main part of the ‘scope); 2 – 3 eyepieces; finder; and a tripod. A good telescope will also include some sort of guide like a planisphere or Starry Night Backyard software.  If the tripod (or extra eyepiece, finder, etc) really is added on as an extra without increasing the cost, it probably means they cut some corners on the OTA.

Bad equipment

cheap optics

A bad objective lens or primary mirror makes any telescope worthless, so you should always try to look through a ‘scope before you buy it.  If possible, you want to look at something high contrast. Look for a crisp, clear image all the way to the edge and all the way around.

bad and non-standard eyepieces

Some cheap ‘scopes come with tiny, plastic eyepieces.  Not only will these give less than stellar (so to speak) images, they are also hard to look through and limit the expansion possibilities of the telescope.  1.25″ diameter eyepieces are standard for most ‘scopes so if you decide you want to expand there are a lot of choices. And if you upgrade your ‘scope, you can still use the eyepieces.

Also, don’t be sucked in by the Barlow lens.  Barlows have their place, but they aren’t a replacement for a good set of eyepieces.

frustrating tripods

A bad tripod can make an otherwise great ‘scope unusable.  The tall, spindly tripods sometimes packaged with scopes in the $50 – $150 range are much too flimsy.  One wrong step (or one over-excited 6 year old) in the dark and you can wind up with a hollow tube on a bi-pod.  A tripod that wiggles all the time isn’t helpful either, and one that’s too hard to set up will never get used.

bad finder

Every scope should have some sort of finder.  On the cheapest scopes, it may just be a couple bumps or notches like a rifle site, or a hollow tube.  More expensive ‘scopes will have a mini-refractor for a finder, and it should be adjustable so you can align it with the ‘scope.  If the finder isn’t adjustable, make sure it really does align with the ‘scope, and isn’t easy to knock out of alignment.  Some finders mount on a thin metal piece that screws into the OTA.It can be next to impossible to get this aligned with the OTA, and if the ‘scope gets banged around or dropped (remember the flimsy tripod?), it can break right off.

Bottom Line

Any of these things will make your ‘scope worthless.  It’s not so bad if you only spent $25 on it, but it’s very bad if you spend $500 on a computerized telescope and the tripod collapses the first time you have it out. Which just leaves me to reiterate my advice from my other post about choosing a telescope: consider binoculars.



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