Re: ATM Calculation of Prime Focus.

[Jeff Medkeff <medkeff@sinosa.com>]

At 02:49 PM 9/14/97 -0700, you wrote:

>I am building a 4x5 sheet film camera for my C-14.  Is there a 
>calculation for how far the film plane should be behind the scope to be 
>at +Prime Focus+?  Or does prime focus just mean +to project an image on 
>a film plane without the use of an eyepice+?

I not sure this will really answer your question, but I do want to ask if
you really mean to be building a 4x5 film camera for the _prime_ focus of
your SCT. That would be a large obstruction indeed! Do you mean instead you
are building a 4x5 sheet camera for use at the Schmidt-Cassegrain focus of
your scope?

Prime focus has traditionally meant the placement of the film at the
location of the focus of the telescope without the use of secondary optics,
i.e., the film or imager sits in a cage inside the tube roughly where you
would expect to see a secondary mirror. Most amateur astronomers take
pictures at the Newtonian or Schmidt foci of their instruments; in fact, I
have never heard of an amateur taking a picture at actual prime focus.
(Though I guess I am about to if they did.) And many amateur astronomers
who take pictures at the newtonian focus of their scope claim they are
taking pictures at prime focus.



--
Jeffrey S. Medkeff  _|_  If a little knowledge is a dangerous thing,
Sierra Vista,        |   where is the man who has so much as to
Arizona              |   be out of danger?