At 2004-04-12 14:33 -0700, Kal Krishnan wrote:

>Hi, I am a bit confused as to true FOV calculation... could you let me
>know if I am doing the right thing here?
>
>I have the LX-200 classic 8" f/10 scope. I have a Televue 2" Plossl 55mm,
>which says it has a 50* apparent FOV. (I am using the symbol * for "degrees")
>
>So, my real field of view is 50x55/2000 = 1.375*, right?
>
>If I get a f/6.3 focal reducer, do I then increase the true FOV to
>1.375/0.63 = 2.18*? Or, will I only get vignetting if I do that?

At 2004-04-12 20:30 -0500, John Mahony wrote:

>In fact, if you look for it, it's not hard to notice the vignetting you
>already get trying to use a (nearly) full 2" field, in a scope where the
>baffle tube and rear port are only 1.5".

According to good ol' W. J. Smith, "Modern Optical Engineering", 1966, p.
216, the true FOV (100% vignetted to 100% vignetted, rad) is the sum of
eyelens diameter (we'll use John's rear port diameter) plus exit pupil
divided by focal length. Changing all to metric,
1.5" = 38 mm, 8" = 203 mm, f = 2030 mm (for f/10)

(38 + 203*55/f)/f = .0214 rad = 1.23°

For f/6.3, f = 1279 mm => .0365 rad = 2.09°

-- Jim Burrows
-- http://home.earthlink.net/~burrjaw
-- Seattle N47.4723 W122.3662 (WGS84)

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