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Making a concave mirror using 15th century technology (2018)

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Re: Making a concave mirror using 15th century technology (2018)

#11

Post by dekhn »

kwkelly wrote: post #7
082349872349872 wrote: post #6

I forget the name, but some old paintings contain a visual equivalent of Rot-13, in that there are bits which look like odd blobs when viewed normally, but resolve to an image when viewed at the right angle or with a properly shaped mirror. Anyone remember the proper term for this, or which century it was popular?

Hans Holbein’s “The Ambassadors” [1] is the one that immediately comes to mind. It has memento mori which is heavily skewed. [1] https://en.wikipedia.org/wiki/The_Ambassadors_(Holbein)

Wow, I'm a fan of dutch golden age art and I've never seen that. Fantastic!

Re: Making a concave mirror using 15th century technology (2018)

#12

Post by meltyness »

You can show with fluid mechanics is that all you need is a container, molten glass, and a steady source of rotational motion while the glass cools into a parabola climbing the walls of the container, give or take a factor of two on the final shape depending on how you feel about density.

Re: Making a concave mirror using 15th century technology (2018)

#13

Post by 082349872349872 »

meltyness wrote: post #12

You can show with fluid mechanics is that all you need is a container, molten glass, and a steady source of rotational motion while the glass cools into a parabola climbing the walls of the container, give or take a factor of two on the final shape depending on how you feel about density.

How steady? Does a massive honking flywheel suffice, or does it require control?

Re: Making a concave mirror using 15th century technology (2018)

#14

Post by vstuart »

kwkelly wrote: post #7
082349872349872 wrote: post #6

I forget the name, but some old paintings contain a visual equivalent of Rot-13, in that there are bits which look like odd blobs when viewed normally, but resolve to an image when viewed at the right angle or with a properly shaped mirror. Anyone remember the proper term for this, or which century it was popular?

Hans Holbein’s “The Ambassadors” [1] is the one that immediately comes to mind. It has memento mori which is heavily skewed. [1] https://en.wikipedia.org/wiki/The_Ambassadors_(Holbein)

Very cool. You can 3D transform the image in Gimp to view the skull:

https://i.imgur.com/DxY3uEo.png

Re: Making a concave mirror using 15th century technology (2018)

#15

Post by bodhiandphysics »

kragen wrote: post #10
bodhiandphysics wrote: post #5

There is of course one thing here that isn’t 15th century technology! Aluminum, which was unavailable before the early 20th century. Brass or silver are better choices (or silver plate on brass)

late 19th early 19th, as a powder

At extreme cost

Re: Making a concave mirror using 15th century technology (2018)

#17

Post by WalterBright »

My high school physics teacher, Mr Tolby, always had a cadre of students grinding telescope mirrors by hand from glass blanks. A candle was used to measure the accuracy of them to amazing precision.

When the mirrors were done, he'd send them off to get an aluminum coating, and then the students would get a tube and have their own telescopes.

It really was remarkable how low tech it was, and what fantastic results could be achieved.

Re: Making a concave mirror using 15th century technology (2018)

#18

Post by meltyness »

082349872349872 wrote: post #13
meltyness wrote: post #12

You can show with fluid mechanics is that all you need is a container, molten glass, and a steady source of rotational motion while the glass cools into a parabola climbing the walls of the container, give or take a factor of two on the final shape depending on how you feel about density.

How steady? Does a massive honking flywheel suffice, or does it require control?

Height varies quadratically with radial frequency and radius, mediated by gravitational acceleration, (rv)^2/?2g. I'll venture you want a small curvature with a great deal of uniformity, the model also discounts variable viscosity with temperature and bizarre patterns that would appear with inconsistent anything, I suspect you're gonna want a control system, and probably also to shave the tails to acquire a lens compatible with the paraxial model of magnification.

Re: Making a concave mirror using 15th century technology (2018)

#19

Post by ledauphin »

WalterBright wrote: post #17

My high school physics teacher, Mr Tolby, always had a cadre of students grinding telescope mirrors by hand from glass blanks. A candle was used to measure the accuracy of them to amazing precision. When the mirrors were done, he'd send them off to get an aluminum coating, and then the students would get a tube and have their own telescopes. It really was remarkable how low tech it was, and what fantastic results cou…

my dad still has his from a different high school physics teacher. it's always been astounding to me that a parabolic mirror is something that can be made by hand (even though of course most things throughout history were indeed made by hand).

Re: Making a concave mirror using 15th century technology (2018)

#20

Post by kragen »

bodhiandphysics wrote: post #15
kragen wrote: post #10

Earlier quoted context omitted.

late 19th early 19th, as a powder

At extreme cost

yeah, brass or speculum metal (a high-tin bronze) were much more practical options until the late 19th century when aluminum became cheap
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