Magnification from a telescope

One of the most common questions clients ask us is how to get more magnification from a telescope.

Just about everything optical these days comes with a zoom lens, so people think nothing of zooming in and out. So how do you do this with a telescope?

Short answer: you change the eyepiece.

Warning: I’m about to get a bit technical. I’ll use pictures to illustrate, but I won’t be offended if you don’t read on!

All telescopes work in the same basic way. The main scope bends light into a focus, and then you use an eyepiece to look at that focus. The main scope might use a lens or a mirror, but the effect is the same – all the light squashed into a focus. The distance between a lens and its focus is called the focal length. The shorter the focal length, the more powerful the lens.

Have a look at my little hand-drawn diagram. I’m starting to get a reputation for this. 

Magnification of a telescope is given by the focal length of the objective (main) lens or mirror divided by the focal length of the eyepiece. So if you’ve got a telescope with a 1000mm focal length, and you’re using a 20mm eyepiece, you’re working with 50x magnification.

I’ve illustrated this with four saxon Plössl eyepieces on a scope with a 500mm focal length. I found a bollard in our car park with a label on it. Then I put a saxon ScopePix onto the eyepiece. Here it is. Geeky, huh?

Using all this, I took four photos of this bollard using the scope and 25mm, 20mm, 10mm and 6.3mm eyepieces, so the magnifications came out at 20x, 25x, 50x and about 80x.

You’ll notice the difference in the magnification. You might also notice that the quality of the image gets lower as the magnification rises. This is because the amount of light coming into the telescope is fixed. Squeezing more and more magnification out of this raw material starts to be a problem.

I normally prefer the wider, brighter. clearer view you get through a longer eyepiece.

Bottom line: swap eyepieces to change magnification, but don't go too far.