Re: ATM f ratio

[Anthony Stillman <atmer@flash.net>]

>... What are you using for a definition of "useful physical field"?


Ron,

I was trying to distinguish between the angular field and the physical
field.  For example an 8" f/10 might be designed to have a useful physical
field of 3/4 of an inch, likely limited by the size of the secondary, and
its angular field for its 80 inch focal length would then be about half a
degree.  The coma limited physical field diameter of a parabaloid is
determined entirely by its f/ratio.  The diameter of the parabaloid has
nothing to do with it.  Hence, the useful uncorrected physical field  at
prime focus of the 200" Hale is the same as a 2" diameter f/3.3.  And the
useful angular field of the 2" f/3.3 is a hundard times larger.


Anthony