Tested my new refractor at my backyard last night.
Pointed the scope to zenith (90 degree), takes many 10 sec exposures with L filter.
Imaging with my STT-8300 CCD, guiding is on with very smooth guiding curve.
The focuser is tighten too.
Here are some diagnosis images:
(1) You can see egg shape of stars at the top-left and top-right corners.
(2) If I rotated the CCD to 90 degree, the egg shape stars are now at top-right and bottom-right corners.
(3) If I rotated the CCD to 180 degree, the egg shape stars are now at bottom-right and bottom-left corners
(4) If I rotated the CCD to 270 degree, the egg shape stars are now at bottom-left and top-right corners
I also point the scope to 45 degree and see if there is any flexure on the imaging train. The egg shape stars are still the same. I also tighten the focuser very hard so it should have nothing to do with flexure...