[lost_in_space]

OK, It's late and I guess I was being lazy. I should have known this.
Google is your friend.

define:"prime focus"
Definitions of "prime focus" on the Web:

• The point in a reflecting telescope where the mirror focuses incoming light to a point.
  astronomy.nju.edu.cn/astron/AT3/GLOSS_P.HTM
• A reflecting telescope's primary mirror focal point. Astronomers place cameras at prime focus to record bright, wide field images.
• www.ccastronomy.org/terminology.htm

• The focal point of the objective mirror or lens.
  www.opticalvision.co.uk/content.asp
• When the eyepiece is removed from a telescope and replaced with a camera body it is said to be at prime-focus. It is the point at which a telescope’s optics direct incoming light.
  www.dirtyskies.com/index.php/glossary/