Re: ATM Performance with Spherical Aberration

[Anthony Stillman <atmer@flash.net>]

Joe Hobart wrote:
>The optics/scope were ordered as a classical cass, but the maker said
>he did not remember that and said he made a spherical secondary and
>an 80 percent corrected primary, which in my opinion does not match.

Get it in writing!

Joe,

The Dall-Kirkham design calls for a spherical secondary and a prolate
ellipsoid primary.  Is your primary a prolate ellipsoid?  Perhaps, 80
percent from a sphere to a parabaloid would be a prolate ellipsoid.  Is it
the correct prolate ellipsoid?  Perhaps, you'll need to test it.  You can
perform a null test using differing conjugate distances.

f/long Dall-Kirkham's are nice instruments.  If the optics are good it will
give you good performance.  Mel is of course correct about baffling.  This
is a problem with all Cass designs.  A cass performs best under dark skies.

Anthony

PS If you would like real help, write me off-line.