So What Can That Telescope See?

Jack Kramer

When we show off the sky to friends, relatives, or the public, someone will likely ask what sorts of things we can see through our telescope. Of course, some specific answers depend on the size of the telescope, but there are generalities that apply to almost any instrument. Here are probably the two most frequently asked questions.

How far can you see? That's probably the easiest question to answer. Just about any response will do! When we look at deep sky objects, we're talking about light years, and cosmological distances are hard to grasp for many people. But let's just take the galaxies M81/M82 in Ursa Major, an interesting pair that most amateur astronomers have observed a number of times. At about 7 million light years, they are in one of the closest galaxy clusters beyond our Local Group. You've no doubt observed many galaxies well beyond the M81/M82 grouping, so it's valid to say simply that you can see "millions of light years". In case someone is a stickler for details, you could add that one light year is 6 trillion miles, give or take a few. Heck, with just our naked eyes we can see the planet Saturn at 880 million miles, and well beyond to stars that lie thousands of light years away.

Can you see the Apollo landers left on the moon? You probably get this question often. The obvious answer is "no". According to NASA, even the Hubble Space Telescope couldn't see them. The moon averages 238,855 miles away and at that distance, the smallest things Hubble can distinguish are about 200 feet across. The average amateur-size telescope might resolve an object on the moon that is around two miles wide. Again, this depends on the telescope size and optical quality, plus seeing conditions.

Published in the September 2007 issue of the NightTimes