Re: ATM : Additional Secondary for guiding ( was offcenter secondary)

[Andy Saulietis <iss@pvtnetworks.net>]

Ulhas writes,

> Have following the discussions on placing a second secondary (!) for
>guiding.
>Why not make a hole in the main secondary -ala Cassegrain- and place a
>second flat behind the first ,inline and perhaps on the same diagonal
>support. this should avoid all problems. Surely a small hole would not cause
>any significant light loss.

Unfortunately, the center of a secondary newtonian scope
is also occulted by the secondary mirror; the converging
image is a hollow cone. You'd have to increase the size
of the secondary hole significantly to get a usable
brightness & fov size image.

Another way is to have the secondary reflect
maybe 80-90% of the light for imaging with the
rest passing thru the secondary to a tertiary
that folds the guiding image outside the
tube. The guide image would be 10-20% the brightness
of the main but you could use the entire field
and guide on axis if needed. This technique also
allows the guide image to be analyzed for active
optics correction techniques.

Another possibility, for small fov CCD imagers, is to
have the center part of the secondary 100% reflective
(the part used by the CCD), the rest transparent &
going to the tertiary. The transparent part could
be a separate optical glass 'diagonal' to which the
CCD secondary is attached. The advantage would
be more light for the guide image, with a 'blind'
spot in the center.






Andy Saulietis
ISS Enterprises
39 Silver Fox Trail
Mayhill NM 88339
505-687-3067 voice
505-687-3021 fax, call voice or e-mail to set up
e-mail: iss@pvtnetworks.net