Tumbleweed Observatory's

Astronomy Hints




Astronomy for the Beginner or User of Small Telescopes

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User Eyepiece Data

Set your scope parameters and EP Field of View and Submit.
The 1X, 2X, 3X indicate Barlow Multiplier.


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
Erfle 60
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.