ATM semantics - Prime Focus

[Barbara & Matt <B.Smerald@worldnet.att.net>]

Regarding ATM semantics - Prime Focus:

   It seems to me that the confusion over the term "Prime Focus"
  could be clarified by realizing that it arises from comparing
  apples to oranges. The apple, in this case, being a Primary
  Mirror's prime focus, while the orange is the prime focus of the
  optical system it is used in.

   In my book, you must distinguish between the two. Primary mirrors 
  intended for optical use, other than flats, each have their own prime
  focus. This prime focus is separate, and may be very different from,
  the prime focus of the "system" in which it is used, eg: SCT,   
  Coude, etc. 
 
   The case of the Newtonian can be best understood by thinking of the
  prime focus of its Primary as not being "reflected", but "directed" 
  to a point outside the tube, where it is more readily accessible. 

                                         Matt