quote:

Hi Sam,

You have a number of good questions (none are rudimentary or
rediculous, BTW!) Let's see if I can address some of them.

1. I'm not sure of the exact sequence of the imaging train in your
system. TCFS should be between the telescope and the reducer, and the
distance between the reducer and the camera should be fixed to
Optec's specification. If that's not done, each time you refocus,
you'll change your optical system in a significant way. This will
include change in aberrations, and even different focal length. The
exact (fixed) distance between the FR and the chip is also critical.

2. CCDInspector and star-field images skewed in one quadrant:

This indeed could be a collimation problem. Another very good
possibility is a tilt in the system somewhere past the secondary. For
example, if the focal reducer and the camera are not properly aligned
to each other, or a filter is tilted relative to the camera and/or
the reducer. If so, no amount of collimating the secondary will fix
this problem.

I would suggest you try to simplify the equation by removing the
reducer, and even the TCFS to see if you still get the skewed stars.
I suspect this will go away (note that you may need to recollimate
once you remove these accessories). I also suspect that the glow in
your flats will go away as well.

3. Circular glow close to the middle of your flats is most likely a
reflection caused by either a filter or the reducer. It could also be
an aberration caused by improper spacing of the reducer and the chip.
It is offset from center likely because of a tilt between the two.

The fact that the glow in the flat looks similar to the CCDInspector
curvature map could be just a coincidence, but could also be a
telltale sign of optical aberrations introduced by the reducer.

Again, check that you have the reducer properly spaced from the chip,
and not too far away from the back of the SCT. Check that the camera
is square to the reducer. If possible, use all screw-in
connectors/adapters. With thumb-screw, push-fit adapters it is
notoriously hard to get the components exactly square to each other
and the optical path.

It may help if you post an example of what you're seeing: a star-
field image, a CCDInspector curvature map, and a flat-field image.

...

Regards,

-Paul (Author of CCDinspector)