ASI 2600mc:
Trying to understand what is happening. I originally was guiding with a guide scope on the C-8 HD and some told me I had elongated stars because of the guide scope not being in sync with the C-8. OK I purchased an OAG but still had elongated stars…….hummmmmmmmmm.
The C-8 was a little out of collimation so I adjusted it to it now ‘looks’ to be collimated (small donut hole well centered). Still elongated stars so I started looking at back focus, and I understand that for the C-8 HD it is 133.35mm of back focus needed. OK I then measured each piece of the imaging train and discovered I need 7mm spacer.
17.5 Camera
17.5 Filter Holder
2.5 Spacer
19.5 OAG
1 Spacer
19 Spacer
7 Spacer
50 Celestron Extender Tube
134mm from the end of the treads on the C-8 HD to the CMOS sensor of the ASI2600mc camera. Still elongated stars. Guiding may not be perfect but the elongated stars length is the same with a 10, 30, 60, and 180 exposures. 
Any suggestions or opinions
Kenn
