MN66 Collimation - BINGO!

wrote:
Dan wrote:

I tweaked a little more as you (Andrea) suggested and now have what appears
to me as perfect or near perfect collimation. A little difficult to tell
with the seeing but I feel confident that the scope is quite well
collimated.

Only issue remaining is that at low power I see slight oval patterns to out
of focus stars; horizontally when outside focus and vertically inside focus;
i.e., foci line up horizontally /vertically. Adjusting the focuser doesn't
seem to change things. Do I need to make very fine adjustments? While the
adjustments I made were not coarse they were not on the level of the fine
tweaking I did with the secondary mirror screws.

Thanks for the help, it's greatly appreciated.



Dan,

Glad you sorted out the collimation issues. As for the the oval
pattern, ignore that. It is NOT astigmatism (!!). It is due to the
severe illumination fall-off of the MN66 ad the edge of the field. Some
paraxial rays make it and some don't. Don't worry about it and start
observing.

Andrea T.


Andrea - could this effect also be caused by using a very small
secondary? I notice the same problem in my 8" f/8 and had thought that
it must be due to minor astigmatism. But the secondary is as small as I
could make it (ma 1.55" - right at the limit). Could the loss of light
at the edge, if the secondary weren't in precisely the right spot, cause
this problem? The in-focus image with this scope is awfully sharp.

Thanks - Chris