The Lake County Astronomical Society

A Newbie's Telescope Quality Check

Jack Kramer

Most newcomers to amateur astronomy start off with a modest telescope. Normally, you want to actually see a potential purchase, so not knowing where to go, it's tempting to buy a telescope at a department store. I've also encountered a few people who picked up telescopes at garage sales. Merchants who specialize in telescopes are definitely the best source, but if you're enticed by what looks like a bargain somewhere else, at least be a smart shopper. Often the primary mirror or lens is decent enough, and since checking an optical system can be a bit complicated, here are a few other tip-offs to the overall quality of an inexpensive telescope.

Soda Straw Finders - Most instruments come with a finder telescope - a small low power, wide-field scope that allows you to easily find an object and place it within the crosshairs so that it will also be in the much narrower field of view of the main telescope. The problem is that the finders on many entry-level telescopes are so small and have such poor optics that come nighttime, it's virtually impossible to see anything faint in them. You might be better off using a plain old soda straw! One solution is what are called 1x finders, (Telrad, Orion EZ Finder, Rigel Quickfinder, etc.) which project a dot or reticle pattern on the sky. There's no magnification, but you at least have a direct view of what you're aiming at. The first thing to do with any finder is to line it up with the main scope. This can be done during daylight hours using a small target object at least a block away.

Staying Put - This is high on the list of frustrations. When you get some object in the field of view and let go, the telescope moves and the object disappears. So you try to cope by aiming above or below the object, hoping that when the telescope settles back, the object will again be in the field of view. This is a symptom called "backlash" - the tendency of a mounting not to stay where it's aimed. If the axles are tightened sufficiently for the telescope to stay put, then too much effort is required to move the telescope, the motions of the mounting become jerky, and you still have a problem getting anything in view. The problem is that the mounting is entirely too small for the weight of the telescope. At night with a small and often faint object in the narrow field of view, the problem really comes home to roost. You can check this for yourself during daytime by using a high power eyepiece and trying to get a small, faraway object in the field. A good mounting should move with only a slight effort and when you take your hand off the telescope, it should stay where you stopped.

The Shakes - It's not uncommon for even decent instruments to tremble in a wind or when you're focusing. This becomes especially apparent at higher magnification. To completely eliminate this would require a mounting so solid and heavy that it's a back-breaker just to set up. Clearly, that's not practical. But a good mounting is solid enough that if you give the scope a sharp tap while looking through the eyepiece, it should stop trembling within about two seconds. The cause for excessive shaking can be due to many factors - loose screws, loose shafts in the mounting, tripod legs that flex too easily, poorly balanced telescope, etc. But alas, the problem too often is that the mounting and/or tripod is simply too flimsy for the telescope it's supporting. One solution is to suspend a heavy weight from the bottom of the mounting so that it hangs between the tripod legs. Believe it or not, this does work. Replacing metal legs with ones made of wood also helps because wood has a higher vibration-damping factor. One advantage of the Dobsonian mounting is that it's very steady.

I Can't See a Thing - You want to get a good view of a planet or other object, so you pop in that eyepiece and Barlow combination that gives you 500x. Assuming you can even get the object in your field of view, you see a stunning view of something that looks like a ball of cotton. One problem is the poor quality eyepieces that are packaged with many entry-level scopes. But even worse is the fact that these high power eyepieces give magnifications way beyond what the main optics are capable of delivering with clarity. Dump any eyepiece that gives a magnification in excess of 50x per inch. This means that a 60mm (2.4") refractor should use no more than about 120x, and then only under the best of atmospheric conditions. Assuming the telescope optics are okay, the single biggest improvement you can make is getting a couple of better eyepieces.

Plastic - Finally, one of the best indicators of telescope quality is the material used in its construction - the more plastic, the poorer the quality. The problem is that plastic wears much more quickly than metal and can distort over time. Beware of plastic parts anywhere on the mounting or tripod ... because of the stresses here, they're prone to breakage and premature wear. A plastic focuser can wear to the point that it will be almost impossible to focus on an object. Adjustment screws on plastic finder brackets strip the holes in the plastic after extensive use and will no longer hold the finder telescope in position. Well, you get the idea - the more metal, the better the telescope.

Learning one's way around the sky is hard enough for the newcomer to observing. A telescope should not be a further impediment. So don't waste even a little money.