Physicopoeia

φυσικοποιΐα
THINGS I WISH I'D KNOWN WHEN I STARTED TEACHING PHYSICS

What sort of telescope should I buy?

Physics teachers are sometimes approached by keen pupils (or parents or fellow teachers or, in extreme cases, complete strangers) wanting to pick your brains for advice about what sort of telescope or binoculars to buy to do some basic star-gazing. I wouldn't say I'm a specialist in this area at all, but I've been lucky enough to work with a couple. What follows is a summary of their responses to such questions:

 

TELESCOPES

Cheap telescopes are often sold rather disingenuously by boasting of the magnification they offer. This seems very tempting to those starting out, thinking that high magnification is the name of the game. In fact, even a very rough home-made telescope can easily provide more than enough magnification for almost any amateur stargazing, and you quickly get to the point where increasing the magnification does not improve the quality of your view because of the way the optics work. You get a bigger image, to be sure, but it will be less and less focussed as you increase the magnification so it's actually quite disappointing.

 

Instead, what you should look for is the size of the objective – the thing that the light hits first (either a lens or a curved mirror, depending on what type of instrument you’re considering). The bigger the objective is, the greater the ‘light-grab’ will be, so you will be able to see fainter objects more clearly. This is why professional telescopes are huge: the idea being to maximise the size of the objective, all other considerations being secondary. Here’s a guide to what to buy as you start out:

  •  If you’re buying a refracting telescope (e. where the objective is a lens), you should look for something 75mm or bigger.
  • If you’re buying a reflecting telescope (e.where the objective is a mirror), you should look for something 150mm or bigger.

  

BINOCULARS

Many amateurs prefer binoculars to telescopes. Using both eyes can make for a more relaxing experience, and it’s very easy to aim them at different objects compared to re-aiming a telescope. Binoculars are described by a pair of numbers:

 magnification x objective diameter

Both are measured in millimetres (e.g. ‘10 x 50’). As with telescopes, go for a nice big objective size rather than worrying too much about the magnification. Be wary of weight though – the heavier they are, the more stable they will be whilst you're looking through them, but they'll also get hard to hold for long periods of time. Starting with a pair of, say, 8 x 30s would be fine to start with. Graduating on to a pair of 20 x 70s would be a good next step.