[HN Gopher] Making a concave mirror using 15th century technolog...
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Making a concave mirror using 15th century technology (2018)
Author : Tomte
Score : 73 points
Date : 2022-12-24 09:20 UTC (13 hours ago)
(HTM) web link (wp.optics.arizona.edu)
(TXT) w3m dump (wp.optics.arizona.edu)
| 082349872349872 wrote:
| 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?
| tejtm wrote:
| to be honest, I had to search to refresh the old grey cells
|
| https://en.wikipedia.org/wiki/Anamorphosis
| tejtm wrote:
| Apologies, you know it is a good nerd snipe when you go off
| on the question before noticing it has already been answered
| for hours.
| kwkelly wrote:
| 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)
| dekhn wrote:
| Wow, I'm a fan of dutch golden age art and I've never seen
| that. Fantastic!
| vstuart wrote:
| Very cool. You can 3D transform the image in Gimp to view the
| skull:
|
| https://i.imgur.com/DxY3uEo.png
| 082349872349872 wrote:
| _Anamorphosis_ , thanks, and same time frame:
| https://en.wikipedia.org/wiki/Anamorphosis#Renaissance
| bodhiandphysics wrote:
| 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)
| kragen wrote:
| late 19th
|
| early 19th, as a powder
| bodhiandphysics wrote:
| At extreme cost
| kragen wrote:
| yeah, brass or speculum metal (a high-tin bronze) were much
| more practical options until the late 19th century when
| aluminum became cheap
| 082349872349872 wrote:
| even in the early 01900s aluminium was apparently much
| more expensive than now; an ex of mine had inherited a
| set of "silverware" that was aluminium and had been
| originally been bought as conspicuous consumption.
|
| https://en.wikipedia.org/wiki/History_of_aluminium#/media
| /Fi...
| Treebeard123 wrote:
| John Dobson used this technique to great effect to grind his own
| parabolic mirrors to build reflector telescopes. If you're not
| familiar, I highly recommend reading about his "Dobsonian"
| telescope design and its impact on amateur astronomy.
| anadem wrote:
| Yes, the grinding process is surprisingly easy. My son and I
| met John when he was showing one of his telescopes in San
| Francisco one evening in the '80s, and he gave us one of his
| instruction leaflets and an eight inch glass from a porthole,
| which we ground into a mirror for our own Dobsonian telescope.
| He was a generous and kind, humble man; at that time he was
| living in an ashram in SF.
| WalterBright wrote:
| 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.
| ledauphin wrote:
| 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).
| meltyness wrote:
| 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.
| Someone wrote:
| https://en.wikipedia.org/wiki/Rotating_furnace discusses that.
|
| I think most mirrors cast this way still would need treatment.
| For example, the mirrors of the Giant Magellan Telescope (https
| ://en.wikipedia.org/wiki/Giant_Magellan_Telescope#Mirro...) are
| being cast using this method to get a rough (for modern
| astronomy. For example https://www.techbriefs.com/component/con
| tent/article/tb/supp...: _"after the casting, the surface
| "roughness" is about 2.5 millimeters, or a tenth of an inch, on
| average. "The polishing and constant measuring are what turn
| this amazing piece of glass into a mirror," he said. "By the
| time we finish polishing, it will be accurate to better than 25
| nanometers"_ ) shape for them. That way, there's a lot less
| glass to pour, the remaining glass cools down faster, and less
| glass has to be removed afterwards.
|
| (For the GMT, an additional complication is that six of the
| seven mirrors will be off-axis. I don't know whether they spin
| them off-axis, or accept that more grinding will be necessary
| to give them their final shape)
|
| Instead of glass that solidifies, you can also use a container
| with mercury or another reflective liquid.
| https://en.wikipedia.org/wiki/Liquid-mirror_telescope:
|
| _"Liquid-mirror telescopes are telescopes with mirrors made
| with a reflective liquid. The most common liquid used is
| mercury, but other liquids will work as well (for example, low-
| melting alloys of gallium). The liquid and its container are
| rotated at a constant speed around a vertical axis, which
| causes the surface of the liquid to assume a paraboloidal
| shape. This parabolic reflector can serve as the primary mirror
| of a reflecting telescope. The rotating liquid assumes the same
| surface shape regardless of the container 's shape; to reduce
| the amount of liquid metal needed, and thus weight, a rotating
| mercury mirror uses a container that is as close to the
| necessary parabolic shape as possible. Liquid mirrors can be a
| low-cost alternative to conventional large telescopes. Compared
| to a solid glass mirror that must be cast, ground, and
| polished, a rotating liquid-metal mirror is much less expensive
| to manufacture."_
|
| Disadvantage is that you can only point the mirror straight up.
| Also, there's evaporation of the mercury.
| 082349872349872 wrote:
| How steady? Does a massive honking flywheel suffice, or does it
| require control?
| meltyness wrote:
| 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.
| buescher wrote:
| I recall seeing the technique used in the Amateur Scientist
| column in Scientific American, with epoxy or casting resin.
| The verdict was the result was "maybe for infrared"...
| found it: https://optica.machorro.net/Optica/SciAm/LiquidMi
| rror/1994-0...
|
| If you search around, you'll find people have kept trying
| and have got somewhat better results in the meantime. Fun
| stuff.
| Sharlin wrote:
| With very simple tools (and quite a bit of patience) it is
| possible to make spherical and parabolic mirrors that deviate
| from the true shape by (much) less than one wavelength of light,
| which is rather impressive.
|
| https://en.wikipedia.org/wiki/Foucault_knife-edge_test
| kragen wrote:
| the candle and knife edge needed for the interferometry are
| indeed quite widely available in human history, but the
| necessary knowledge of the wavelike behavior of light is only
| about 200 years old
| perihelions wrote:
| (If the navigation is confusing, this is a subsection of chapter
| 3 (in the ToC sidebar). The parameters for [Lorenzo] Lotto's
| hypothetical telescope are derived in chapter 5, from an analysis
| of one of his paintings (!)).
| mnw21cam wrote:
| Nice.
|
| On an aside, I'd recommend the YouTube channel Huygens Optics
| https://www.youtube.com/@HuygensOptics for all sorts of stuff to
| do with grinding decent optics.
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