Quote:
Originally Posted by xelasnave
If you did put in a glass disk of high quality ... Alex
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Alex you're quite right that would eliminate the spikes of course but a disk of plane parallel optical glass to quarter-wavelength wavefront error is not cheap in the size needed for a reflector. Strain, bubbles, ripples, curvature and wedge (non-parallelism) just to name a few defects in plate glass that just don’t work well in a scope.
I did see one 8" newtonian made this way once, in the mid 1980's - the owner asked me to look at it, unsure if it was a schmidt-newtonian or just a newtonian with flat disk (it was the latter, and the glass was uncoated). Optically the scope was OK and it looked like a DIY affair made by someone competent with a machine-shop, though I have no idea who made it.