Sure. I cast a 15” bronze mirror blank using a Plexiglas model formed by slumping a ¼” sheet into a parabolic antenna dish in the oven…
There is actually a company that produces large (10s of feet in diameter) lenses by spinning molten glass slowly so that it forms a sphere.
I wonder if you could form a lens by spinning plaster of paris while it hardens to form a parabolic base mold, then use it to cast a lens…maybe also spinning the mold while the molten glass hardens in it. Don’t know if you would still need to grind such to get a suitable surface, tho.
Thanks Lief. I'm not so much worried about the curve or sagitta as the precise focal point is marginally irrelevant in my case here. I'm more concerned with how did they physically grind them. DaVinci shows a several foot diameter grinding stone with a machine to work it across the surface. Curious if anyone has seen anything else.
From: Historical Recreation in the Kingdom
of Calontir [[log in to unmask]] on behalf of David McNeal [[log in to unmask]]
Sent: Sunday, October 13, 2013 10:38 PM
To: [log in to unmask]
Subject: Re: [CALONTIR] Grinding optics
The measurement of determining the curve is called a diopter. I don't know the formula to determine the exact parabolic curve you may want, but Master Craig might be able to point you in the correct direction. Mundanely he is an Eye Doctor (I can't remember if he's an Optometrist, or an Ophthalmologist). He ought to know those formulae or be able to point you in the direction for them.
P.s. If nobody pipes up with his contact info, I think I know who might have it. PM me in that circumstance. Hope this helps.
On Oct 13, 2013 8:10 PM, "Hall, Hayward" <[log in to unmask]> wrote:
Does anyone have references to period grinding of optical glass or metal, specifically concave mirrors. I've see DaVinci's grinding machine (too difficult to produce), various post-period techniques, and sphere production for eyeglasses, but nothing really relating to the large concave "burning mirrors" described both in antiquity and in late period. Birringuccio talks about casting them but really doesnt go into how to produce the parabola.
I'm currently employing techniques used by current Amateur Telescope Makers however, while primitive, I don't know their origin.