[The_bluester (Paul)]

For those curious, the best way I have found to collimate this thing relies on the coma inherent in the SCT design, I am imaging without a reducer at this point as I was using the off the shelf Celestron one but I was never really happy with the sharpness and it produced some big reflections.

Collimation wise using short exposures (5 to 10 second subs) you adjust in focus so that the coma induced tails around the edges of the FOV all point away from the center, easiest was the stars right out in the corners as the coma is most visible there. This is not quite perfect but CCD Inspector (I downloaded the trial to have a closer look) called it within 4 pixels of bang on. It moves around a little as the focuser is racked in and out but it stayed fairly well put so long as you ran the focuser down with gravity then back to to focus before starting.

Given that even cropping the size down there is still some coma visible in the corners and there is spherocromatism visible that is related to the corrector plate of the SCT, I reckon these are getting close to the best it has got to give, not bad for a visual OTA re-purposed.

For those that are not familiar with spherocromatism, the SCT main mirror is a spherical form not a paraboloid and the corrector plate is there to effectively pre refract the light to make the system behave as though the mirror is a parabola not a sphere in form. Basically it is a thin lens of a single glass type so it suffers from chromatic aberration just like a single element refractor would. But in this case it is spreading the light on a radiant from the centre of the image circle, not as a fringe around bright objects, generally blueish to the middle and reddish to the outside.