Re: ATM f ratio

[Anthony Stillman <atmer@flash.net>]

>Andy Mail <amail@scimar.com> wrote:
>>    What are the differences between mirrors with a high f ratio vs
>>    mirrors with a low f ratio?
>
Stephen Tonkin responded:
>...High f-ratio ... but will also give you a smaller FoV.
>


As Stephen knows, but no doubt ommitted for simplicity, its a little bit
more complicated.  If we are discussing a basic Newtonian then the FoV
(field of view) is more or less determined, as Stephen notes, by the
f/ratio.  A faster system provides more field.  However, the field of an
un-corrected Newtonian is limited far and away by coma.  And, the field of
a fast paraboloid is severly coma limited.  An f/4 without a coma corrector
has a useful physical field of about 1/16 of an inch.  That's regardless of
the primary's diameter.  Hence a 12.5" f/5 has a slightly larger useful
angular field than a 10" f/4.  As with everything else in telescope making,
its a trade off.

Anthony