[ATM] sizing secondary mirrors,and the size of the 100% illuminated field

[Chris Todd <christopher.todd@gmail.com>]

I'm not sure if this is the right forum for this question, and if it isn't,
I apologize for wasting your time, and would appreciate being pointed in the
right direction. I ask this question here because I'm relatively certain
someone will know the answer.
 I am in the process of designing a newtonian reflector, and need to decide
on the size of the secondary mirror. I am aware of Dale Keller's Newt
program, as well as several diagonal size calculators available on the net,
and I (think I) understand the formula they all seem to be using (see the
bottom of the email).
 That formula includes a variable for the desired size of the fully
illuminated field, and many sources (e.g., Rutten and van Venrooij in their
book Telescope optics) state that a fully illluminated field of 10mm in
diameter is more than sufficient for visual observation (as opposed to, e.g.,
film astrophotography).
 On what basis did these authors reach the figure of 10mm? I understand that
illumination degrades gradually, and many authors seem to suggest that the
70% illuminated field diameter is the one you need to pay attention to,
implying that we can't detect a 30% difference in illumination across the
field. Are there biophysical studies to support this, or is this figure just
a general ball park guess? I guess I just find it hard to believe that a 30%
drop in illumination at the edge of the field wouldn't be noticeable, given
that variable star observers can estimate the magnitude of a star to within
.1 magnitude.
 And does the 10mm figure apply equally to both 1.25" and 2" eyepieces?
 Regards,
Chris Todd
 P.S. This is the formula I have seen used to calculate secondary size:
d = a/N + b - ab/f
where:
d = diagonal minor axis
a = distance from center of secondary mirror to focal plane
N = the scope's focal ratio (focal length/aperture)
b = size of fully illuminated field
f = focal length
Somewhat annoyingly (I am no math wiz), every source I found factored this
equation differently; the form above is from Rutten and van Venrooij, but
Dave Kriege, Allyn Thompson, and Charles Genovese (Best of ATMJ, vol 1, p
425) all factor the equation differently.
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