How to Choose the Right Telescope

When I was younger, even living in a major metropolitan city, you could still see the stars at night fairly easily from your own backyard, or any place that wasn’t exposed to the blaze of downtown. Now our cities, and even towns, are so over-lit (from an astronomer’s viewpoint) that seeing everything is a struggle. The first time you take someone out to “find dark” the look of sheer wonder on their faces as they see the stars for the first time is great.

In the city now you might make our twenty or thirty of the brightest stars; in the country, surrounded by farm fields on a dead-end road you feel like you can see all 200 billion (just in our own galaxy). Of course you’re confined to just a few thousand really, but it’s just so many more that it feels that way.

Our galaxy is a bit of a heavy weight, with between 100 and 300 billion stars (including the low-mass ones). Most of the galaxies we can see are smaller, but a rare few are even bigger. Let me caution you not to take pride in the fact that our galaxy is so large. A microscopic parasite living on the eyelash of the world’s tallest human is about one trillion times more qualified to take pride in the size of his human host than we are to brag about the size of our galaxy.

If the galaxy is so big, what can I see?

Everything else there is. Pragmatically that would be stars, nebulae, galaxies, some local planets, and moons, asteroids…even the occasional comet. Of course, it’s easier with a telescope.

What should I look for in a telescope?

First, join a local astronomy club, if there is one, and learn to use other people’s telescopes. That will be the best guide as to what you want for yourself. When you do finally decide to purchase (or ask Santa) for your telescope, you’ll have a good idea of what you need.

  • First you need a wide aperture. The bigger, the better. This is the place where light enters the telescope. The large sizes increase your ability to distinguish between two objects near to each other. Naturally the more light you can gather, the brighter the object will be. Four inches would be adequate to start. The bigger you go, the better it gets, but the cost ascends swiftly, too (except for Dobsonians, which I’ll mention shortly)
  • Forget the magnification for the most part. Anything beyond 2 ½ times your aperture (in millimetres) is virtually useless. If you have a 100 mm (4”) aperture then you’ll max out at 250x before things start to get uselessly blurry. My 7” (180 mm) is good for about 450x, though on the coldest night of winter, with thick, heavy, still air I might toss in a 250x and a 2x Barlow lens for an effective 500x magnification, to check on the icecaps of Mars or try to see the Martian moons Phobos and Diemos orbiting that planet (it works by increasing the focal length, without altering the focal point and is often called a doubler but can come in triple power or more).
  • Decide on whether you want a refractor, a reflector, or a catadioptric. Good refractors with limited (or no) chromatic aberration will be expensive $1,000-5,000, but a half-decent hobbyist’s model could be had for $200. They will use lenses to focus light like a magnifying glass.
  • A remarkably good reflector can be had for about $200-700, with fewer elements between you and the ultimate image, which will render brighter, higher-contrast, large aperture views with no chromatic aberration. Their light amplification is accomplished by reflecting light between curved mirrors until it arrives at the final eyepiece for magnification.
  • Catadioptric telescopes combine the best of both worlds, with shorter physical lengths (while retaining long focal lengths – hence the name folded-telescope), lighter weight, and faster set-up & take-down.
  • The last choice is something you can build yourself called a Dobsonian telescope (or “Light Bucket”) which is still essentially a reflector, but on a large scale, and is terrific for Deep Sky objects like nebulae and distant galaxies. It is surprisingly light and transportable for something so large.

I could mention brand names and personal preferences, but really, you’ll learn more from joining a local club. They’ll happily teach you more than I could ever cover in such a short article. Do a little research and find some fellow stargazers locally. If you have to drive to arrive, get there before dusk. If you’re arriving late remember to turn off your headlights and drive slowing with only your parking lights. The last thing you want to do is ruin everybody’s night vision by sweeping the crown with your headlights. Be one with us… Join the Dark Side…

  1. Vipin August 24, 2015 at 8:41 am

    I think ESO are planning to have some kind of citoetomipn to get the public involved in naming the ELT – but of course Council would still have the final word. I just want something that’s easy to pronounce, saying E-ELT 20 times in every talk I’m giving at the moment, and watching half the audience roll their eyeballs or snigger at the dumbness of the name, is really tedious….