Re: [ATM] Diagonal mirror size

["vladimir" <svla@isp.com>]

> What formula would one use for calculating the secondary (diagonal)
> mirror size?
>

If you search online under "diagonal calculator" there should be a few
of them coming up. If you prefer by hand calculation, the two
usually most important parameters are:

(1) fully illuminated field diameter, H=(Af-dD)/(f-d)

where A=minor axis, f=f.l., d=diagonal-to-focus separation and D=aperture 
diameter

and (2) field illumination (as a ratio number),

I=(a/90)-(c*sina/Pi)

where cos(a) = c/2 and c=(h-H/2)(f-d)/hf, "h" being the off-axis height in 
the image plane.
The formula is strictly valid for a minimum size secondary (Amin=d/F),
and for larger secondaries errs toward minus side (lower illumination).
For "oversized" secondaries, itt comes within a couple of percent by adding 
to the illumination figure
0.01 for every 10% increase in secondary size over the minimum.

For an 8" f/6, with the diagonal minor axis A=1.6" and diagonal-focus 
separation
d=8", fully illuminated field diameter H=0.32" and field illumination at .8" 
off-axis
I~0.59+0.02~0.61, or ~61%.

Vlad

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