Comments on SCT and diagonal orthogonality

Derek Wong <dawong@...>

From Todd Gross' post, copied with permission from Roland:

The biggest obstacle to getting the polar alignment and subsequent
accurate object acquisition is tube orthogonality. It is not a problem
with alt-az systems like the Meade, but as soon as you go equatorial,
you have problems. I know people who could not get the images to fall on
their CCDs with the Meades until they shimmed up the fork arms to the
point where the optical and mechanical axes coincided. I know it sounds
like I'm trying to pass off the problem, but this is not the case. If
you can get ahold of Sam Brown's classic book on telescopes, in there he
explains, in plain easy to understand language and diagrams, all about
equatorial mounted telescopes, and how you tune up the mounts and how to
use polar scopes. If you can't get a copy, I can fax you the relevent

You will have two problems with your SCT, one is the non-orthogonality,
and two is the focus shift. If you have a Televue or Lumicon diagonal,
you will also compound this problem because these diagonals are not
exactly 90 degrees, and the image will shift out of the center by a
significant amount when you rotate the diagonal as the scope swings from
one side of the mount to the other (this is where an alt-az mount has a
distinct advantage). Also, if you tweak the collimation on your SCT,
your orthogonality is gone with the wind. What it means is that you
will have to use a seperate polar scope to bring the mount in. You will
not be able to get images to fall in the center of a medium to high
power field as you switch sides, and will have to center a known star in
the field and press the recal button (#9) before going to a faint
object. This is the only way you will have pointing accuracy with an
SCT, unless you go through a complete mechanical tune of the entire

Roland Christen

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