ATM semantics - Prime Focus

[Bratislav Curcic <epabcc@epa.ericsson.se>]

Jeffrey S. Medkeff wrote 

> 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.

I've seen this logic already applied several times before, and I can
say that I totally disagree. A PRIME focus of an SCT telescope is where
everybody expects it to be - behind the primary mirror. What is
'secondary optics' in an SCT ? We have (1) corrector (2) concave
spherical mirror and (3) convex spherical mirror (in commercial
incarnation of SCT scope). NONE will form a proper image on its own,
without the other two. So there is really only one focus, and it sounds
logical to call it PRIME focus, as that is the _intended_ focus for
this particular telescope.
We _may_ talk about prime focus in a telescope using paraboloidal mirror
(e.g. Newtonian). In that case we can dispense with secondary and place
film there. But even then, Newtonian is MEANT to have its prime focus at
the side of the tube, and that's what I would call 'prime focus'.
Even 4m AAO (used as an example by Steve Lee in his definition of prime 
focus) CANNOT work without the corrector as it has hyperboloidal primary.
So there _is_ secondary optics involved. Should we call these "Ross" or
"Wynn" or "Gascoine" foci then ??? How about telescopes that have CONVEX 
primary mirrors ? What's definition of 'prime focus' for them ?

Bratislav