Why is glass still the norm for amateur telescopes?
Why is glass still the norm for amateur telescopes?
If we can do plasma deposition for cheap mirror reflective sunglasses with polycarbonate, why aren't these materials commonly used?
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Polycarbonate has one of the lowest ABBE values of any of the optically used plastics, so it scatters light pretty badly the thicker it is. As compared to eyeglasses, telescope lenses are really thick, so the poor light transmission would ruin the clarity. On top of that, poly expands a lot under heat, and so any coatings on the lenses will eventually start to craze and delaminate. Glasses don't need to last more than 5 years before being replaced, so it's not as big of a deal. If your telescope became unusable in that time, you'd be furious.
Poly, while being impact resistant, is not nearly as scratch resistant as glass, and is nowhere near as chemically stable. Didn't realize there was dirt on your cleaning cloth? Ruined scope. Cleaned it with regular window cleaner? Ruined scope.
There are other resins that would be better, but nothing nearly as simple and durable as glass comes close to the optical clarity.
Eyeglasses would still be made from glass if they weren't so heavy and potentially dangerous, Not to ignore that nobody wants to wait two months for custom lenses to be made.
Source: I'm an advanced optician running three offices and a lens lab.
Why not use first surface reflection so that the optical properties are irrelevant. The primary large scope is a reflective Dobsonian mounted light bucket which is a Newtonian reflector on an azimuth base. I've had Smith and Oakley sunglasses with great durability for the metalized coatings that have lasted a decade and a half without issues. I've also had some cheap ones that wiped off with soapy water well over a decade ago.
If it wasn't so physically demanding I would absolutely try grinding my own mirror. The old school DIY technique is silvering. Surely a plasma sputtering coated polycarbonate is better than the old Brashear's or Rochelle salt silvered mirrors.
So what's the advantage of poly over glass here?
As an eyeglass wearer, the advantage is weight (comfort) and safety, with a tradeoff in clarity and for me, plastic is a bitch to keep clean and scratches much more easily.
None of this seems to matter for a telescope, so why bother?
I think they are referring to using the polycarbonate as the structural part of the mirror, so its optical properties aren't relevant. The rest of your points stand though.
Ah, got it. I'm pretty focused on lenses just due to my day to day.