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Subject: Re: [M]: Piggyback mount -Reply
From: Ric L Ecker
Reply To: mapug@shore.net
Date: Fri Mar 14 00:38:21 1997

>Of course, there is some parallax between the camera and the OTA but
>if
>your camera can measure that you have a mighty fine camera :-) Heck,
>it's
>tough to measure the parallax between the center of the Earth and your
>polar axis.
>
>---
>Bill Arnett billa@znet.com
The biggest problem with piggy-back is not field rotation per say
(this can be maintained with polar alignment) its flexure. Its the same
problem that you can have with a guide scope. A piggy-back with a 300mm
or longer lens will have a lot of flexture that will cause field rotation
if the lens is not properly held. A camera with a long lens that has its
only support by the camera will have quite a bit of flexure. A test on
the pole star will show what will happen. Align the scope on the pole
star and the camera on the same star (use cellophane tape crosshairs on
the focus screen of the camera or the inner prizm)(something that has a
reference) rotate the scope 180 deg. and see if the camera is still on
the star, it won't be. This also a good time to align the forks of the
scope if the scope is also out of alignment. I use the Losmandy
dove-tail and rings to support my larger lens which has zero
flexure......................ric
PS you can also use dial indicators and run the test during the day
without a star..

. '\|||/' ric ecker
co-president of
. (@@) Oceanside Photo and Astronomical Society
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