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Why is glass still the norm for amateur telescopes?
(lemmy.world)
Googling says thermal expansion. It's 8x worse.
This is exactly why. Some numbers:
- Polycarbonate thermal expansion coefficient - 65 * 10^-6 per degree C (source)
- Glass thermal expansion coefficient - 9 * 10^-6 per degree C (source)
Most amateur telescopes are used shortly after dusk when the temperature is changing more rapidly than deep in the night when the temperature might be more stable. Also, the coatings are not as resistant to more dramatic expansion cycles (remember that you need those mirrors to stay very smooth). Given the high price and low volume of the goods, it doesn't make sense to cheap out on a critical component like this and give yourself a bad reputation among a very small but dedicated community.
Low thermal expansion glasses/ceramics can have coefficients of thermal expansion of 0 (+/- tolerance). For example, SCHOTT ZERODUR can be 0 +/- 0.02 * 10^-6 per degree C.
(amateur)
Also, I can make thermals irrelevant easily with a small heating element and PID loop running on a $0.50 microcontroller. Many people run such elements already to avoid issues with moisture.
Which is more complexity.
You haven't really presented any compelling reasons to use poly.
Whats the advantage that outweighs the negatives?
The OP said cost. A 16" glass mirror is $1400. Adding $20 of microcontrollers, heaters and sensors to a $100 plastic mirror could be extremely interesting.
There is also a weight savings which makes the rest of the design easier.
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.
They are used for cheap mirrors such as some parabolic makeup mirrors, cheap reflectors for looking around corners, etc. Polycarbonate and acrylic mirrors are available, but offer much worse optical properties. They cant be as easily polished so you will have to cast them into a mould. Their low stiffness means they are easily distorted. Thermal expansion significantly affects focusing and distortion of optical systems.
Because you haven't done it yet. Get to work
Plasma sputtering is hard. I have a good vacuum pump but it is still not in this class of stuff and while I'm good at electronics, that is a project that is beyond my skills experience scope. I'm maybe more capable of the surface stuff as I come from automotive class paint perfection. While nowhere near optical precision, I have defeated myself, in that I can apply any amount of tedium required to get the job right despite the emotional toll it takes to get results.
this post was submitted on 01 Jun 2025
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