From: Aart Olsen 
To: Guy Brandenburg 
CC: atm{at}shore.net, novac{at}his.com
Reply-To: Aart Olsen 


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{at}prairienet.org

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