There are a lot of telescopes on the market, and not all of them are good. Sure, a good percentage of them are decent telescopes and even some of the weaker entries might be good enough for 90% of people’s basic viewing needs (such as of the Moon and Bright Planets). But there are a lot of telescopes that, even if functional, are simply not worth the money you pay for them. Here are some of the of warning signs of a bad telescope that you can determine without having to do full optical analysis:
1) Large magnifications listed on the box!
You’ll see this on a lot of department stores – they list huge magnifications on bright letters and those numbers are mighty big for a small telescope. Numbers like “640X” or “1280x” a all over the box, trying to impress the customer.
Fact is, a telescope, no matter how well made, should not be magnifying more than 50x per inch or 2x per millimeter of aperture. This is why advanced astronomers use bigger, wider telescopes.
This is another bad sign. You can get decent views of celestial objects in your telescope but trust us when we say they will not resemble the images sent back to Earth by NASA’s multi-million dollar space probes, orbiting telescopes, and Lunar Landers. We’ve seen the famous Pillars of Creation appear on more than one cheap telescope box. This is very amusing as the Pillars of Creation are just one small portion of the Eagle Nebula. And while the Eagle nebula is a very impressive thing to view in your own telescope it is probably too faint for most beginner’s telescopes and even in larger telescopes it looks a bit less detailed and colorful than the Hubble image.
3) Long Focal Length, short tube
This one is a lot more subtle, and unfortunately a lot of better telescope companies do it. You might spot a decent diameter reflector telescope with a fairly short looking tube. But when you look at the specifications it says the focal length is 1000mm (1 Meter) long! That doesn’t make sense! Yes you can get that kind of focal length in an expensive Cassegrain telescope but not from a beginner model unless….
…unless they cheat. To get more impressive magnification figures and also get away with using a cheaper mirror what the manufacturer does is they insert a fixed Barlow lens ( a negative lens used to increase the magnification of eyepieces) into the eyepiece holder. This is a poor way to cover for a cheaper mirror and it has the side effects of not letting you use the eyepieces and barlow lenses that you would prefer to use (since they already permanently shoved a piece of glass in the tube. In addition the telescope will also now be much harder to collimate (an alignment of the mirror that needs to be done with reflector telescopes from time to time)
4) Excessive Number of Accessories, all of them small, all of them plastic
Almost all telescopes will come with some accessories, and some of them may come with more. A typical loadout will be two eyepieces and with a new reflector some kind of diagonal (to ease in viewing through the telescope). Some other models and companies may add a few other items to “sweeten the pot” such as a barlow lens or moon filter.
But far too many cheap telescopes will pile on multiple accessories with plastic bodies and plastic lenses. These are piled on to make the telescope seem like a better deal or give it that higher magnification that you read on the box in #1. With the cheap telescopes you can get 3-4 bad eyepieces (ones that use 17th century optical designs – only ones made with plastic instead of glass) , a cheap barlow lens , a cheap ‘image erector (that brings the image right-side up and probably also increases the magnification to boot), and then for good measure a moon filter (made of cheap green plastic or glass instead of a neutral density filter) and in the worst cases a thread-on solar filter. If you see one of those, throw it away. Never use it.
5) Oh those Spindly legs!
OK, so most of us probably have some kind of small camera tripod that we used with our old auto-focus digital camera. They did the job, were inexpensive, and they collapsed into almost nothing when they were not in use. They were great for the job.
They were not designed to hold telescope tubes.
Many cheap telescopes use barely converted camera tripods to hold the tubes up. Now while there are plenty of strong camera tripod legs on the market these are not them. The legs bend easily, they shake with the slightest provocation, and have problems with balance. Look for a telescope with good solid legs. A good indicator is when the legs are collapsed the extensions slide in beside each other, not into the section above them.
6) Use of the term ‘professional’ on the box
OK, the use of the term “professional” is abused in a lot of fields, but in astronomy it is just plain wrong. First of all actual professional astronomers rarely operate optical telescopes. They will operate radio telescopes, or work with digital imaging telescopes, or infrared, X-ray, etc. The use of optical telescopes is pretty much relegated to amateurs. But do not feel bad about that term, in astronomy the amateurs do almost as much as the professionals in some ways. Most comets, for example, have been found by “amateurs”.