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Thanks Stephen for the congrats. I get a little astig sometimes too, but according to a thread at Cloudy Nights, as you mentioned, the edge support makes a difference. I use packing tape, approx 10mm wide for lower 180 degrees of mirror. I have large air vents in the bottom of the mirror cell for cooling and can check that the tape is sitting dead centre. This seems to help for me. I notice temperature seems to move the glass around a fair bit, I guess as various stresses come in to play as I only used thick sheet glass from Sydney Glass, but it doesn't seem to make a lot of difference to the image, however as it moves from say under corrected to over corrected during the evening there is image deterioration if you are observing planets. As I want this for looking at faint fuzzies I am not worried.
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