what to look for when setting back distance up on an imaging train

[iwols]

hi guys just wondered what to look for when setting back distance up on an imaging train ,is it the sharpness of the focus?

[symmetal]

I assume you're referring to using a field flattener or coma corrector, and the optimum correct distance between it and the image plane.

You set the back distance to get round stars at the corners of the image and this should hopefully be the same distance to get best overall focus as well. You can get good focus near the centre of the image at any spacing distance, but as you move further from the image centre the spacing distance becomes more critical which is why the corners of the image show the spacing errors the most and are used after making small distance corrections to see if they look any better.

Field flatteners give elongated stars at the image corners at incorrect spacing while coma correctors will still show some coma when incorrectly spaced. I've never used a coma corrector so assume that's what you'll see. 😉

If you have tilt in the imaging train then you'll find not all corners show the same star shape errors. One corner may look good while another has misshapen stars. Altering the distance will improve the misshapen stars corner, but the previously 'good' corner may then start showing errors.

Alan

[ollypenrice]

Have a look at the first post in this thread: 

Olly

Edited July 13, 2020 by ollypenrice

[newbie alert]

This is one of those things my head won't grasp... in my mind the light cone caused by the lens should be what the focal length distance is to the sensor.. I know the spacing needs to be at the distance so the sensor is fully illuminated.. just can't grasp how the 2 combine.. and it's said that a quad don't need a spacing distance... brain scramble!!