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Hi Werner
Welcome to the hobby! Mirror making is engrossing and from a quick read of your previous messages you are obvious enthusiastic.
I gather making flats is your first project and you want a flat to be able to null test a mirror. I think that’s a great idea but making an optical flat as a first project may be a bit of a challenge - partly because not that many amateurs do it and so you don’t have a lot of expertise to draw on. Stefan is the obvious exception here and it’s great he’s following your thread.
I don’t want to discourage you but I’d like to suggest a simpler first project. If you built a long focus smaller mirror say a 6 inch F10 you would learn the basics of mirror making and have a lot of expertise available to you on web sites such as stellafane and through this thread. A 6 inch F 10 can be null tested with a simple ronchi or Foucault test as the mirror can be left spherical so you avoid the difficulties of figuring and testing a parabola which many first timers find challenging. A well made long focus Newtonian is a versatile instrument.
After you have this experience you, you’ll be able to tackle more difficult projects. I could supply you with a 6 inch Pyrex blank if you decide to do what I suggest.
Rod
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