A telescope for Christmas

SkyCaramba weekly astronomy blog for the week ending December 24, 2011

If you’re buying a telescope for someone for Christmas, here’s some useful information to make sure you get a truly good deal. There are all kinds of purported bargains. You need some quick tips to separate the questionable advertising from the offers of truly good deals.

 Generally speaking, you get what you pay for. Telescopes that go for under $100 and are sold in the toy section aren’t going to be great scopes for astronomy. I may buy one for a child under ten years old who still needs to prove he can take care of such things before anyone spends money giving him something more sophisticated and expensive.

Ignore any telescope that’s purported to offer views with magnifications in the hundreds. You have to be watching from a mountain top or even higher to get a good view through Earth’s atmosphere at 350x. Even on the clearest, stillest nights, you’ll notice a little bit of a wavy motion in your view at the highest magnitudes.

If that doesn’t compromise a good view, the difficulty tracking an object at such high magnifications will. Add to that how most astronomical objects aren’t bright enough to be seen easily blown up so much. Finally, physicists say magnifying an image in visible light more than 400 times doesn’t yield any more visual detail anyway.

The best deal will probably be a telescope with standard size eyepieces, the part the eye looks through. The two main sizes are 1.25” and 2”. If you can get just a few eyepieces, you can save a lot of money. The person getting the gift can buy more eyepieces later. Different size eyepieces offer different magnifications. If they aren’t included with the telescope, they’ll be described by their focal length. Divide the focal length of the objective lens on the telescope by the focal length of the eyepiece to calculate the magnification you’ll get from it. In other words, a 700mm focal length objective lens and a 70mm focal length eyepiece give a 10x magnification.

If you do have a lot of money to spend, I suggest getting a few filters. A moon filter lets the user look at the very bright moon comfortably. A telescope made for letting in a lot of light so you can see the stars will let in enough moonlight to hurt the eye a little. There are filters for safely viewing the sun too. And there are colored filters for making planetary features stand out.

Telescopes that are wide let in lots of light and provide bigger images. Those are best for people with very dark skies. They cost more than narrower scopes. So if your gift recipient lives in a city and can’t get to rural darkness much, a bigger scope may not be ideal.

There are many more things you can think about that could affect your telescope purchase decision. If you need more help, try to shop at a store where you can talk to a salesperson who wants you to be happy with your purchase. If you’re satisfied, you’ll likely shop there again for accessories.