Astronomy for the Beginner or User of Small Telescopes
User Eyepiece Data
Set your scope parameters and EP Field of View and Submit.
FL, DIA, Exit Pupil in mm, Apparent and True FOV in degrees.
The 1X, 2X, and 3X in resulting table
indicate no Barlow, 2X Barlow, or 3X Barlow.
If you have the typical box of eyepieces, and perhaps a few telescopes
of dissimilar focal lengths, no doubt you sometimes tear out your hair
trying to figure out what magnification any given eyepiece provides with
any given telescope.
Perhaps this web page will provide some solace. Just select some
parameters about your telescope (diameter and focal length) and eyepiece FOV
(sorry, the program currently assumes your collection all have the same field
of view), then click Submit. A table will appear showing data for
common eyepiece focal lengths and other data based on the telescope and
eyepiece field of view information that you provided.
If your eyepieces have different field of views, it's easy enough to make
a new table, just select a different FOV and re-submit. Maybe print out a table
for each telescope and stick it in your eyepiece box for easy reference. If
you don't know the apparent FOV of your eyepieces, the following chart can
be used as a rough guide.
|Type ||Typical FOVi (deg)
|Kellner ||40 - 50
|Modified Achromat || 40 - 50
|RKE ||40 - 50
|Orthoscopic || 40 - 45
|Plossl ||50 or more
|Konig ||60 - 70
Focal length, diameter, and exit pupil parameters are in millimeters, and
the True Field of View (TFOV) parameter in degrees. The Exit Pupil tops out at
5mm, as that's typically about the maximum eye pupil diameter of person's eye
when in the dark. Cells indicating exit pupil sizes greater 5 mm and related
fields of view are shaded in the table. The shaded cells indicate that the
related eyepiece focal lengths are likely giving imgages being limited in FOV
by your own eye pupil size.
Exit Pupil and True FOV are computed for the 1X, or no-Barlow configuration.