# Stargazing Liquid Mirror Telescope: How to create a parabolic shape

1. Aug 13, 2007

### jasc15

I recently read about a proposed liquid mirror telescope to be constructed on the moon. My question is one of mechanics, hence this not being in the astronomy section.

My question is regarding the shape of the surface of a rotating liquid. The design will have a dish containing a thin layer of mercury (~1mm) which will be rotated to generate a parabolic shape. Is the curve of a rotating liquid affected by the shape of its container? I suppose if it is thin enough, the friction between the liquid and its container would have an effect on the shape of this curve. If the liquid is sufficiently deep (i.e., the effect of friction on the container has a negligible effect on the surface) i would imagine that a rotation would produce a purely parabolic shape since the horizontal force would be proportional to the square of its velocity, whereas the vertical force is constant. If my logic is correct, why does the design of this mirror require a thin liquid layer?

2. Aug 13, 2007

### mgb_phys

To reduce the amount of material - mercury isn't nice stuff to have around.

The idea is to use the parabolic curve produced by a spinning mirror to just give the final high quality surface - instead of requiring years of polishing that a glass mirror needs. Producing a roughly parabolic coarse surface for the bottom of the container is fairly easy.

I don't know about using it on the moon but there are a couple of liquid mirror telescopes used on earth - they are a cheap way to make a 10m telescope as long as you onlt want to look up.
A very clever system is used by Roger Angel at U. Arizona, they spin a furnace containing a full size (6-10m) molten mirror blaank to give a parabolic surface which is then polished to give the final smooth reflective surface/
This saves a lot of grinding away of the glass needed to get to the rough parabolic shape.

3. Aug 13, 2007

### jasc15

So the idea of designing a thin dish doesnt have to do with the shape, but to reduce the necessary amount of material?

The article i read mentioned that it was to get a more accurate parabolic shape (though, unless i am wrong in my previous post, simply spinning a liquid in a container should create a parabolic surface).

4. Aug 13, 2007

### mgb_phys

I think the author misunderstood - failry common when you try and report technical stuff to the general news media.
The purpose of the mercury is to simply and cheaply produce a highly accurate surface.
The reason to start with a roughly parabolic shape is to reduce the amount of material. Although there could be secondary effects such as convection currents or coriolis force which make a thin layered liquid telescope more accurate or more controllable than a deep tank.