Quote:
Originally Posted by Don Pensack
Having overhauled many of these, I can tell you that centering the secondary likely won't center it in the corrector unless the secondary holder is, itself, dead center in the corrector.
It always seems to help to center the corrector plate in its cell, but note that there is a caveat to this: the primary itself may not be centered in the tube and the corrector needs to be centered on the primary, not the tube of the scope.
You only center the corrector if the primary is also centered. Otherwise, if the primary is off 1mm in a particular direction, offset the corrector in the same direction.
Then the alignment of the secondary: If the primary mirror's baffle does not have a front end (as seen through the back of the scope) that is concentric with the outline of the secondary (indicating the baffle, which is always perpendicular to the primary mirror) isn't pointed at the secondary mirror exactly), then the secondary must be off-center in the same direction the primary baffle indicates.
In the collimated scope, the corrector is off-center to match the primary, and the secondary is off-center to match the pointing of the primary baffle.
If you're extremely lucky, everything is centered in the tube as well.
When I worked on my old Meade 8", the only error was an off-center corrector, but everything else was perfectly aligned with the tube. And centering the corrector (and then re-centering the secondary) made a BIG difference (improvement) in the star images.
The final result was a tiny pinpoint with 1 or 2 diffraction rings around the stars.
But, from those I've seen, I was lucky. I guess they were making them a lot more slowly in 1993 than they are today.
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Hi Don,
Many thanks for your reply. Hearing from somebody who has a lot of experience in overhauling Meade's, your knowledge is is invaluable. I'm running this thread in the cloudynights forum in parallel, and will copy over your comments. All the people following this discussion in both forums will certainly appreciate them.
As I have said to other people, my 12" F8 LX850 on first light produced circular stars edge to edge, until I started tinkering with it ( O why O why). I now have the collimation back to about 99.8% of its original collimation. I will do a lot of measurements of exactly where all the components lay with respect to each other, "before I fiddle with it again".
Cheers
Peter