>
>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)
>


Um, how do you equate eyelens diameter to rear port aperture? And how the
heck does the eyelens (part of the EP), and exit pupil (determined by the
EP) figure into a calculation of illumination at the focal plane?

Is this book (or this formula, in particular) dealing with SCTs? The issue
here is the rear port restriction, and the answer (max partially illuminated
field) depends heavily on the distance from the rear port to the focal plane
at the EP field stop. With a typical EP and 2" diagonal, that's around 6".
With an f/10 system and a 1.5" restriction 6" ahead of the focal plane,
illumination falls to 0% at roughly 6"/(f ratio) larger than the restriction
diameter, so the illumination covers a 2.1" circle, or 1.53° at 2000mm focal
length.

Taking into account the fact that the last rays to be vignetted as you
approach the edge of this circle are coming from the opposite edge of the
primary and are therefore more steeply angled than those at the edge of an
on-axis f/10 light cone, the actual illuminated circle would be even larger.
But of course a 2" EP is limited to using only 2" of this circle, even
less due to the thickness of the barrel walls, so the max field is about
1.38°.

-John

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