billhinge

I have read that too but never believed it, surely it would imply that both mirror and corrector were astigmatic and have to be placed to cancel each others astigmatism?

I never made a sct but I have made 3 mirrors in the past and I do have some atm books e.g. rutten http://www.willbell.com/TM/tm6.htm

If you take the corrector off you will see that there is some slight play on the shims holding the corrector and that it is only held in place by the tension of the screws at the front

surely after some years you would expect a minute amount of movement which would make any astigmatism obvious?

Also how would the factory align both optical surfaces accurately in a mass produced process?

How could anyone ensure quality control if both surfaces were not optically perfect

I could be wrong but I don't believe it