ICY SCIENCE: SCIENCE SPACE ASTRONOMY Spring 2014 | Page 9

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Equations for telescopes
So what is the first thing a relative or friend will ask you when you
get a new telescope? Oooh lovely, how many times does it magnify?
10x, 100x, 1000x? It’s a tricky question to answer, as you probably
know, because it’s, well, its not important really. Well, I mean it is,
the purpose of a telescope is to make small things bigger after all.
However, it’s pretty easy to get it to magnify up to almost any power
you like by putting the right lens in.

So the magnification is a straightforward equation. It’s the focal
length of the objective (that’s the big lens or mirror) divided by the
focal length of the eyepiece.

Now the eyepiece usually has it written on it. I often use a 26mm eyepiece, and my telescope has a 127mm
main mirror. So I know f_ebut not f_o- but that’s ok we can work it out. The f-ratio of the telescope is the
focal length of the objective, divided by the diameter of the objective. So we can work that out.

ICY SCIENCE | QTR 2 SPRING 2014