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So, here I was today twiddling my thumbs waiting for the fine grits and wondering how I overshot on the sag so much. It was down to below 3 and still had the fine grits to go I was starting to worry. It was then that I noticed the bar attached to the unistrut looked a bit askew, so I measured it and found it at least an inch too close to the centre of the mirror. As I was grinding it must have worked itself closer and closer without me noticing, thus grinding chunks out of the centre. After a bit of a think I decided if I was going to change to chordal strokes it would be advantageous to go the whole hog and aim for f3.5, or f4 even, as I have plenty of meat on the edge and I have read lots of bad things about f3 mirrors. After a couple of sessions with #80 the sag has started moving backwards .010mm. At least I'm keeping to form and making everything twice!
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