I have recoated the mirror. the frosty patch is gone, but I'm still not 100% pleased. There is a faint haze, like looking though cirrus, when looking directly at the mirror under strong lighting. Looking down the tube it isn't visible and through the eyepiece (at the local radio tower red beacon, was so far too cloudy for stars) the performance looked no different to the previous coating.
I suspect the haze may be costing a few % in reflection and extra scattered light.
disturbingly, more tiny scratches are visible under strong lighting. I'm worried that perhaps my chalk supply is also containing some silica. It is labeled pharma grade so should be ok I guess. I will check this by dissolving a sample with dilute hydrochloric acid, which should dissolve calcium carbonate but not silica (or alumina if it was contaminated with clay).
If so, bugger. At this stage I will not regrind and refigure!
Quote:
Originally Posted by hamishbarker
Yes, first mirror. pretty crazy to take on such a monster. Originally I was looking for a 16 inch blank but saw stephan's trademe post. I have a spare 22 inch blank completely unground. I liked to think it was my insurance against accidentally dropping the one I was working on, although not sure my wife would let me start again. :-) I don't think I will use it to make a matching mirror for a 22" bino (although never say never).
She is happy that the test rig has left the living room at last, and the polishing bench will soon leave the spare room.
I'll take some pictures of the coating and post soon.
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