Re: ATM projector lens ==> finder scope?

[Aart Olsen <aolsen@prairienet.org>]

For a lens with such short BFL, instead of a negative Barlow lens in
front of the focus you can more easily use a positive achromat behind
the focus to reimage the focal plane farther back.  A pair of achromats
(positioned symmetrically, crown side to crown side, nearly touching)
performs better than one lens because the aberrations of the two lenses
largely cancel, but try one lens first because the primary lens has its
own aberrations and a single lens might help cancel those.  You can get
a range of magnifications by adjusting the object and image disances. 
The image will be erect, which some people like for a finder and others
dislike because it is different from the main telescope.  Using a
positive lens to reimage and invert like this is the equivalent of the
concave secondary in a Gregorian telescope.

-- 
Aart Olsen
aolsen@prairienet.org