Collimation Problems?

[lukebl]

Interesting, umadog. I see your point, but why do various tutorials insist, therefore, that you do need to ensure that the focuser is square to the tube?

[TheThing]

Possibly the same reason that they insist you see all of the mirror clips. An error which has crept in to be repeated?

TheThing

[lukebl]

Possibly the same reason that they insist you see all of the mirror clips. An error which has crept in to be repeated?

TheThing

Perhaps this dates from a time when secondary mirrors were smaller, which would make the visibility of the clips more important?

[TheThing]

Yes, possibly. Good thinking.

TheThing

[umadog]

Here is a lively debate on the matter. http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/4320978/page/0/view/collapsed/sb/5/o/all/fpart/2/vc/1

My reading of it is that a focuser that is reasonably square can make it easier to ensure that you get the secondary rounded and centred (which is what actually matters). If you're currently able to get the secondary where you need it then you don't gain anything by squaring the focuser in the OTA (indeed, it may well be square already).

If you're seeing out of focus stars in part of the CCD field then that must mean that the focal plane of the primary isn't coincident with the CCD. The purpose of the primary and secondary tilt adjustments is to achieve that. So if those have been done right, you should be good. If you're still not good then it may be that the CCD isn't sitting square in the focuser. It seems unlikely that a tilted focuser is at issue because if your alignment signatures are right then by definition the primary focal plane should be "squared" with the focuser, whatever direction the focuser is pointing.