2006-12-09 15:48:42 · 5 answers · asked by Robert H 1 in Science & Mathematics ➔ Astronomy & Space
Porthole glass has been popular for scope mirrors for a long time, provided it is thick enough (no more than about 10:1 Dia./Thickness, otherwise gravity will complicate your mirror mount) and it has been annealed. Pyrex is better and doesn't cost much, about $30 for an 8". I have to tell you, properly grinding and polishing your own is a pretty big undertaking, tho one many have done successfully, including myself. Feel free to email for more info, I used to be very into this hobby and hope to someday finish the 18" blank I started 4 years ago.
PS - The idea that glass is a liquid with a finite rate of flow is actually an urban legend based on glass panes in centuries-old buildings looking as if they have flowed. Actually, that's the best panes of glass they could do hundreds of years ago, prior to invention of the float method of glass manufacture wherein molten glass floats on top of molten liquid tin to make a nearly perfectly-flat pane. Glass has no real flow at sub-molten temperatures.
2006-12-09 15:59:27 · answer #1 · answered by Gary H 6 · 1⤊ 0⤋
No porthole glass will be too thin,. The mirror will have to be ground into a parabolic or spherical shape. To do this and still have some mechanical thickness left over, you'll probably want to start with a blank that's about 25 mm (1 inch) thick for a reasonable size diameter.
2006-12-09 15:57:05 · answer #2 · answered by Gene 7 · 0⤊ 0⤋
Yes, lots of people have used porthole glass for mirror blanks. Just make sure the glass isn't tempered or laminated.
2006-12-09 16:00:09 · answer #3 · answered by injanier 7 · 1⤊ 0⤋
I think the main problem is that it is glass, and, based on my shipboard experience, not the highest quality glass at that. It will be heavy, will have defects, different regions in the piece may react differently to changing temperatures. Glass is a liquid and it flows (very slowly, but it does). You may get "biscuiting" a lot faster than objective lenses that are made of optical glass.
However, if you are willing to try, go right ahead. My cousin and I tried to grind, then aluminize a piece of wood. It worked for a few days... but we only got poor quality images.
2006-12-09 15:56:49 · answer #4 · answered by Raymond 7 · 0⤊ 0⤋
You'll have to grind it. And isn't porthole glass kinda thick - it might be too heavy.
2006-12-09 15:51:59 · answer #5 · answered by eri 7 · 0⤊ 0⤋