I’ve tried quite a few eyepieces since I bought my first real telescope, always in the hope of wide fields, bright images, a wide range of magnifications and little cost. I sell on those which I no longer have a use for, so I don’t currently own all the EPs listed below.
I measure focal length by projecting a grating through the eyepiece. Note that I am not an optics expert, nor do I possess proper laboratory equipment. These measurements are not guaranteed to be accurate, they are merely my best attempts. I would welcome advice on how to improve my methods.
The “field stop diameter” of an eyepiece is the best measure of how much sky you will be able to see in a given telescope. This true field of view is calculated from the field stop diameter and the telescope focal length as follows:
tfov = (fsd / fl) x 180 / π
Some eyepiece manufacturers state the field stop diameter, although it may be hidden away on an obscure part of their web site. On some eyepieces the field stop is a narrowing of the barrel and can be directly measured. In other cases it can only be estimated1 from the stated apparent field of view and focal length.
The “focus position” is a measure of how parfocal the eyepieces are. I measure it using a small refractor. Each eyepiece is used to focus on a distant TV aerial, and then the focuser drawtube position is measured with a vernier caliper. The “zero” position is measured with a screen in place of the eyepiece. Positive values mean the focuser is further out. This can be “corrected” with parfocalising rings. Negative values mean the focuser is further in. This can make the eyepiece unusable with some telescopes.
Another effect of the “focus position” of an eyepiece is to vary the gain of a Barlow. A positive value means the eyepiece’s focal plane is nearer the Barlow lens, which reduces its gain. A negative value increases the Barlow’s gain. The effect is greatest with the popular “shorty” Barlows. Stephen Tonkin has written a good web page on Barlow gain calculation.
- The field stop diameter of an eyepiece can be estimated from its focal length and apparent field of view as follows:
fsd = fl x AFOV x π / 180 ↩︎