ok. I have a celestron edge hd with .7x focal reducer for the edge hd. I have a zwo asi 294mc pro. I started out using the reducer, t ring (49.5mm , 11mm, 16.5mm,11mm spacers and the camera has 6.5mm to the sensor. acording to my calculation this is 104.5,, back focus, but my stars were way crazy looking. I ended up with the 11mm and the t ring this should calulate to 67mm of back focus witch is the picture below. This seems to give me the best looking image and plate solved worked in sharpcap. what do you think of the stars. or am I calculating this wrong. The pic was taken with sharp cap. at 4.2 second exposure, gain at 350, bining is 3, camera cooled to -15, total exposure is 63 seconds with 30 dark frames. I just do not understand why at the proper back focus the stars were oblong. I attached a picture of my collimation. I think it is pretty good it took me like 15min to get this far after installing bobs knobs.
Any help would be great