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darryl
03-18-2017, 03:22 PM
Was thinking again about doing some spin casting to produce a parabolic surface. Normally I'd be using a resin of some kind, probably epoxy, and starting with a pre-shaped mold to reduce the quantity of resin required. I've done this before and it's turned out quite well, but a question comes up- because this involves spinning a liquid, would the coriolis effect have any bearing on the surface produced? This is just an academic question, posed out of interest.

I know that spinning mercury mirrors have been used- I wonder if any effects have been noted. And if not, then why not? I'd think that if there were some effects, it would be easily seen in the optical quality of the mirror.

The Artful Bodger
03-18-2017, 03:32 PM
As far as I know coriolis effect is to do with the change in angular velocity that comes with a change in spin diameter, example when the fancy skater draws here arms in and hence spins faster or the rotation of air masses in weather systems when there is a change in lattitude. My guess is no detectable change due to coriolis effect.

polaraligned
03-18-2017, 03:46 PM
I know that spinning mercury mirrors have been used- I wonder if any effects have been noted. And if not, then why not? I'd think that if there were some effects, it would be easily seen in the optical quality of the mirror.

While I don't have an answer to your question, telescope mirrors made from borosilicate are often produced by spin casting (up to 8+ meters dia). They get these mirrors to within a small fraction of an inch, but not nearly to optical quality. Spinning them pretty much saves them the rough grinding stage.

Ironwoodsmith
03-18-2017, 08:05 PM
So does spinning on a horizontal plane produce a parabolic profile in a liquid?

chipmaker4130
03-18-2017, 11:38 PM
I don't think it would have any notable effect. To me it would be like an MRI machine being affected by the earth's magnetic field. It's there, of course, but the relative strengths at the point of concern are very widely spread.

tomato coupe
03-18-2017, 11:45 PM
As far as I know coriolis effect is to do with the change in angular velocity that comes with a change in spin diameter, example when the fancy skater draws here arms in and hence spins faster or the rotation of air masses in weather systems when there is a change in lattitude. My guess is no detectable change due to coriolis effect.

The skater example would be an illustration of conservation of angular momentum.

J Tiers
03-18-2017, 11:49 PM
The mercury mirror was good enough for Robert W. Wood, casting should be good enough for you.....

BTW, MRIs ARE affected by external fields.... Apparently when light rail was run past medical facilities at U of Minn, the varying fields as the trains pass caused a loss of detail.

darryl
03-19-2017, 12:24 AM
Spinning liquid in a horizontal plane does produce a parabolic surface. My first attempt made a very shallow curvature, so it's hard to tell, but speed it up and the parabolic shape is obvious. For a rough look-see, take a glass that's somewhat tall and stir some water in it to get it spinning. At the least you'll see that it's not a spherical shape.

I was expecting there to be some effect on the curve produced when you spin a liquid, but if it doesn't affect the optical quality when used as a mirror (mercury) then it must be an almost non-existent effect.

Evan
03-19-2017, 01:34 AM
Is the mercury mirror still in operation in your vicinity?

darryl
03-19-2017, 02:31 AM
I do not have a mercury mirror, nor do I know of any near here. I was referring to projects I've read about and seen pictures of. I don't recall reading anything about optical distortions except for what could be introduced by passing trucks, etc. I would imagine that the debut of lightweight glass mirrors and multiple mirror telescopes, plus the fact that a mercury mirror is destined to be only horizontal would have killed interest in it. Not to mention that mercury seems to have some sort of health hazard associated with it :)

Evan
03-19-2017, 02:08 PM
The mercury mirror is not far from you at the "University of British Columbia's Malcolm Knapp Research Forest, about 70 km (43 mi) east of Vancouver"

https://en.wikipedia.org/wiki/Large_Zenith_Telescope

http://www.astro.ubc.ca/lmt/lzt/

Heh, nice size too, 6-metre aperture.

PStechPaul
03-19-2017, 02:59 PM
At first I thought this was about using a spinner type fishing rod (http://www.cabelas.com/product/fishing/fishing-rods/spinning-rods|/pc/104793480/c/104764680/sc/104836680/cabela-s-fish-eagle-reg-spinning-rods/2430567.uts?destination=%2Fcategory%2FSpinning-Rods%2F104836680.uts) and reel (http://www.cabelas.com/browse.cmd?categoryId=734095080&CQ_search=spinning+reel) for casting the bait, hook, line, and sinker. :D

More details on liquid mirror telescopes:

https://en.wikipedia.org/wiki/Liquid_mirror_telescope

flylo
03-19-2017, 03:24 PM
I can't answer your question about casting but it has to be considered in long range shooting. Here's a simple explanation for those who haven't dealt with it. As the world turns:o
https://stratus.ssec.wisc.edu/courses/gg101/coriolis/coriolis.html