What is the largest (low magnification) eyepiece you can use with a telescope?

To figure out the minimum magnification of a telescope, just divide the aperture in mm by 5 or 7 (older observers have a 5mm exit pupil size, younger observers have a 7mm exit pupil size). Which reversed, is the same as dividing the focal length of the telescope by the focal ratio of the eyepiece to give you the magnification; and then divide the magnification into the diameter of the objective to work out the ideal exit pupil size.

eg A 100mm objective with an eyepiece of 20x magnification, would give 5mm, (an eyepiece with less magnification would require a larger exit pupil size). A 500mm focal length telescope with a 25mm eyepiece gives a 20x magnification. If the telescope has 100mm objective lens then the eyepiece exit pupil size is 5mm. So, A larger eyepiece would not be suitable for older observers as the light cone produced would be larger than the eye pupil size and the light wasted.

Another consideration is the apparent field of view using very low magnification eyepieces (over 32mm); with standard 1.25 diagonals and typical wide field Plössls the field of view may be restricted by the barrel size.

But the main reason to purchase a low magnification eyepiece is that the field of view is large. A 32mm, 52 degree, plossl eyepiece in a 500mm focal length telescope will generally give unrestricted views, and will be superb for tracking down objects and for star hopping.

Note: To work out the magnification that an eyepiece will produce divide the focal length of the telescope with the focal length of the eyepiece.

A f/5 500mm telescope with a 25mm eyepiece (500/25) give 20x magnification. A f/5 500mm telescope with a 32mm eyepiece (500/32) give 15.6x magnification. A f/5 500mm telescope with a 40mm eyepiece (500/40) give 12.5x magnification.