CALCULATING EYEPIECE FIELD OF VIEW Last updated: 15 March 2004

Date: 3/12/04, 12:35
From: Dave Wallace (d_wallace@ecrm.com)
The field of view may be calculated if you know
(1) the focal length of the eyepiece in question,
(2) the apparent field of view of the eyepiece in question, and
(3) either the focal length of the telescope or the magnification
provided by the eyepiece and telescope.

For Meade eyepieces, you have data for items (1) and (2) -- see their
product descriptions.  Example: the 26 mm Series 4000 eyepiece has a
focal length of 26 mm and an apparent field of view of 52 degrees.

If you don't have the magnification OR the focal length but have the
aperture and focal ratio, you can obtain the focal length (and the
magnification) from that.

Here are the formulae:

V = Ve / m.     (V is field of view; Ve is apparent FOV of eyepiece; m is
magnification)
m = F / Fe.     (m is magnification; F is focal length of telescope; Fe is focal
length of eyepiece)
F = A * R.          (A is aperture; R is focal ratio)

So, depending on what you know, use either

V = Ve / m

or

V = Ve * Fe / F

or

V = Ve * Fe / (A * R)

Alternatively, measure the time it takes in minutes for a star at
the celestial equator (DEC = 0) to go completely through the field of
view when the telescope is not tracking.  Call this T.  Your field of
view is then:

V = 4 * T           (V is in degrees and T is in minutes.)

For more than you *ever* wanted to know about this, I have a freeware
program available to calculate all this stuff.  It's at
http://wa1gsf.home.comcast.net/astronomy_blog.html#downloads (requires
Windows operating system, unless you want to port from the source code)

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