OK, so you have a telescope and a collection of eyepieces ranging from 6mm up to 45mm but you have no idea how much the bloody things will magnify! Welcome to my world. I keep on forgetting how to do the calculation and what the focal lengths of the Astro2 telescopes are!

One might ask, ‘why are eyepieces labeled with focal lengths instead of magnifications?’. I mean it would be much easier if they did that, right? Unfortunately they can’t because the magnification is not just a function of the eyepiece but also of the focal length of the telescope it is used in. Hence, the same eyepiece will give a different magnification on different telescopes.

Astro2 have the use of two telescopes so that means that there are two focal lengths I’m going to have to start remembering:

• Meade 10" LX200 – 2,500mm
• Meade ETX90 – 1,250mm

Now that we have the focal lengths of the telescopes, how do we get the magnifications for each of the eyepieces?

M = f_o/f_e

Where f_o is the focal length of the objective (i.e. the telescope) and f_e is the focal length of the eye piece (make sure the two are in the same units, usually mm).

This means that the eyepiece I use most often for Astro2 events (26mm) gives a magnification of 96X on the big LX200 and 48X on the little ETX90. It also means that the maximum we can magnify for Astro2 events is about 415X and the minimum is about 60X with the LX200 and 30X with the ETX90.

Mind you it should be pointed out that there is a limit to how far you can magnify with any scope till the image quality just gets too poor to use and that maximum is approximately the diameter of the primary lens/mirror in mm so for the LX200 that gives us an optimum magnification of 250X (i.e. about a 10mm eye-piece) and for the ETX90 about 90X (i.e. about a 13mm eye piece).