Re: ATM Troubleshooting double image from 10 " f6 Dob.

["Bob May" <bobmay@nethere.com>]

Ok, I've gotten the fact that the scope can resolve a star into two seperate
round blobs, not a elongated blob.  This means that the mirror isn't
astigmatic as it would be one blob that changed direction of it's long axis
as you go from one focus to the other of the mirror (the image goes from a
thin line in one direction to a big blob and finally to a thin line in the
other direction!) but rather you are getting two distinct images of the same
object and probably are able to get an Airy Disk on each of the images.  A
superbright star may show a tiny very faint filament between them but then
again, probably not.
The two images are always in the same orentation relative to the tube and
moving the EP or primary doesn't make any difference.
Masking off half of the mirror makes one of the images go away.
Now, take and rotate the mask 90 deg. and the two images should both dim
equally or nearly equally.
This is a result of the secondary that isn't flat but rather has a fairly
sharp angle between the two halves of it.  Rotating the primary in it's cell
doesn't change the image direction so it's left to be the secondary and
since you're getting two good images and no light between them, you have to
have a secondary that is shaped like ^ in some direction or another.  The
must be a fairly sharp edge or there will be light connecting the two images
together.
Bob May
http://nav.to/bobmay
bobmay@nethere.com