P3D Re: Stopping down a mirror based lens system

[John Ohrt <johrt@xxxxxxxx>]

Eric Goldstein wrote:

> > The FOV is consistant with the stated f/ ratio
> 
> Help me out here, John... By FOV, do you mean field of view? And if so,
> what does field of view have to do with f/ratio or stop?


Typically a cat or a mak is quoted something like a 4" f/8.  This
implies a focal length of 32 inches, and the observed field of view will
be consistant with that focal length.  I agree that so long as you
follow this practice, field of view is strictly a function of focal
length, which is readily deduced.

If you were to quote an f/ratio consistant with theoretical
transmission, you would also have to quote other factors to deduce the
FOV.  Some people have been doing this in this group.  We are free to do
what ever we want.  But it seems to me that there would be far less
confusion if the effective transmission (a value less than 1.0 for all
lens systems!!!!) and modulation transfer function (to mention my
fondest dream) were quoted for consumer optics, or even "professional"
optics.  Technical and scientific optical devices usually quote these
numbers and are proud of them.

To astronomers, knowledge of the FOV is necessary to determine whether
the scope is appropriate to observe the desired phenomena.  (remember
that I mentioned field wideners, which effectively alter the f/ratio).

By the way, Celestron and Meade are famous for their ability to build
cost-effective cats.  Cats require an "aspheric" corrector plate which
are very tricky to make.  Makutsovs, if my memory is correct, use only
sperical surfaces which can be ground to very high accuracy.  There are
variants of the Makutsov such as the Makutsov-Newtonian which have
smaller central obstructions and less complex processing and assembly.


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