# Shape of Rotating Fluid Telescope

• posted

Somewhere I saw a post about building a really large reflector telescope by rotating a pool of water. I'm guessing the centrifugal force would form a bowl shaped surface. I'm also guessing a flat mirror aimed the light normal to the bowl.

Wouldn't the shape be y = cos(arc tan r) where r is the distance from the center of the bowl?

Isn't the primary mirror supposed to be a parabola, y = r^2?

For small y/r it's not too different than a circle or parabola so maybe that would work but you'ld need a high tower for the secondary reflector.

Or was there some way to computer enhance the distorted image?

I never did see the OP.

Bret Cahill

• posted

Dear Bret Cahill:

No. It is y=r^2, less effects like the departure of r along the tangent, when it really is constrained down very slightly.

Google with: parabolic mirror melt Magellan

David A. Smith

• posted

I neglected the fluid beneath the surface in my setup.

Come to think of it, the "frother" I use to whip semi sweet chocolate into my coffee creates something like a parabola.

Bret Cahill

• posted

Try:

It's mercury they use, not water.

-Paul

• posted

Since primary mirror size is limited by the width of the roadbed up the mountain, it seems like a liquid mirror could provide something much larger than the current maximum size with solid mirrors -- 27' at Mt. Graham in AZ.

All the components could be hauled up even a one lane road then set up on top of the mountain.

Bret Cahill

• posted

Dear Bret Cahill:

They don't always want to look straight up. They like to look around.

If you could work the vibration issues, you could cast the mirror in segments, having the "bucket" offset different distances from the center of rotation. One segment of parabolic spheroid is as good as any other. (And they play well together, too!)

And think of all the cookies you could bake in an oven that size...

David A. Smith

• posted

I asked a local astronomer -- we have zillions here in the desert -- if they ever looked at the sides of the local mountains. He said in early morning when the heat didn't distort too much and they could often spot a mule deer, etc. miles away.

Now when I'm out hiking I'm extra careful when I take a piss, crawling under a rock or waiting until I get to tree altitude -- 6000 feet.

Before I'ld only scan the ridges with binoculars for other hikers.

Bret Cahill

• posted

Mercury is being used in spinning mirrors. The figure is said to be ideal - parabolic.

Brian Whatcott Altus OK

• posted
< The figure is said to be ideal - parabolic.

I was trying to derive it to see if it is a perfect parabola even for large r at fast rotational speeds/large y:

At any point r from the axis of rotation the acceleration of the liquid is equal to r times the square of the angular velocity. The pressure at any point r will be the integral of the fluid density times the acceleration with respect to r from r = 0 to r = r so y ~ r^2.

This should hold no matter the plate shape. To cut down on the mercury and weight, however, the plate itself should be an almost perfect parabola.

Maybe they could build one several hundred feet in diameter and eliminate the bearings by using the earth for rotation. The location would be limited -- only the N. or S. poles -- and the secondary mirrors would be in low earth orbit. They might be able to get away with a less reflective but cheaper more environmental friendly liquid than mercury with tens of thousands of square feet of surface area.

Right now they seem to prefer to use two or more primary mirrors to collect more light. I'm not even sure why they need to locate them on the same mountain top. It's not like a few hundred or few thousand miles will make any difference with astronomical distances.

Astronomers do some much computer enhancing it seems like they could just saw a solid mirror up into pieces and reassemble it after they haul the pieces up the mountain. Even if they did a sloppy job they could just point it at something they knew very well like the moon, then adjust it with software.

I don't see why all this money is being spent on telescopes in the cheap info age. They need a curved shiny surface but that's all they need.

Bret Cahill

• posted

Dear Bret Cahill:

The sixty miles of progressively thicker atmosphere is problematic now. Adding more distance in dense atmosphere is not feasable for optical telescopes.

You may have heard of the VLBA, which uses radio telescopes on different sides of the Earth... They perform interferometry on the recorded signals to generate an image.

David A. Smith

PolyTech Forum website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.