To: atm{at}shore.net
From: tkrajci{at}san.osd.mil
Reply-To: tkrajci{at}san.osd.mil


>From: "Jack Schmidling" 

>Now, I am contemplating making a new astrograph and doing an F5 instead of
>F4 to solve the coma problem.
>
>How do I calculate the coma free field based on focal ratio?

Jack, I'm not sure, but I think coma scales by the square of the
f/ratio...so an f/4 mirror has a coma free field (at the focal plane)
that's roughly 2/3 the size of a come free field in an f/5 mirror.  (Or,
conversely an f/5 mirror's coma free field is about 1.5 times larger than
for an f/4 mirror.)

Some other folks know this scaling rule better and can correct me if I'm wrong.

However, there is still astigmatism and field curvature - but I don't know
their scaling rules...and it's too late at night for me to think clearly.
Some ray tracing analysis can help determine how large your 'tolerable'
field size is.

Tom Krajci
Tashkent, Uzbekistan

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