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Hi Steve
The actual mix isn’t crucial. Roughly 75% concrete should be OK. If you are in a hurry a glazier might cut a rough circle in 12 mm glass for you and you could use that. You might need to smooth the edges with a carborundum stone.
Changing the lap is no easier / harder no matter which way you make the facets. At F9 you shouldn’t need to tinker with the lap anyway.
Not sure what you mean by refraction spikes?
F9 is a nice choice for a first mirror - if you can cope with the long tube. My first mirror was a 6 inch f10 sphere. In retrospect it was poorly made but the tolerances were so loose that I got an acceptable result. I had a lot of fun with it. I remember being astonished that it actually worked when I put it together. That was nearly 40 years ago.
Rod.
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