How to Determine Which Eyepieces to Use with Your Telescope

Every scope Celestron manufactures includes the following information in its specifications chart: the minimum useful magnification and the maximum useful magnification. This information provides the theoretical limits for each telescope model. These limits assume a night of good stable atmosphere, good transparency, a scope that has been allowed to cool to ambient air temperature, and well collimated optics (for Newtonians and Cassegrains).

If you choose an eyepiece that causes the telescope to exceed its highest useful magnification, the image will be magnified but will not carry or enhance any more details. This is generally defined as 60x per inch (25.4mm) of aperture. If you choose an eyepiece that provides magnification lower than the lowest useful magnification, the exit pupil becomes larger than what the human eye can support. Your view will become vignetted (encircled by a black ring) and you’ll never see the entire field of view. The lowest useful magnification is 3.6x per inch (25.4mm) of aperture.

For this article, we’ll use the AstroMaster 130EQ as an example. This telescope’s focal length is 650mm. (To find the focal length of your telescope, look at the telescope’s nameplate or retaining ring.) The aperture is 130mm.  The scope’s highest useful magnification is 307x and the lowest is roughly 19x. 

To determine the focal lengths of eyepieces that fall within the magnification limits, you must take the focal length and divide by the magnification. 

For the AstroMaster 130EQ, the highest useful magnification is achieved with a 2.1mm focal length eyepiece. The lowest useful magnification is achieved with a 34mm focal length eyepiece. Celestron offers several options for eyepieces that fall within these limits.

A 2.3mm X-Cel LX eyepiece will give you 282x, which is close to the highest useful magnification of the optics. An Omni 4mm eyepiece with a 2x Barlow lens will exceed your highest useful magnification at 325x.  It’s unlikely that any eyepiece will be a perfect match, but with the right combinations of eyepieces and Barlows, you can achieve numbers that are very close.

Remember, the performance of your telescope will always be limited by your seeing conditions. Choosing an eyepiece with too much magnification on a night with poor seeing conditions will only magnify the turbulence in the atmosphere, providing poor quality views.

To determine the magnification of a given eyepiece when paired with your telescope, use this equation: