Telescopic mirrors are parabolic(almost concave) reflectors whose main aim is to gather as much light as possible. The light incident on the mirror is focused by the mirror's curvature at a single point on its axis. This is the focus of the mirror.
Telescopic mirrors are typically made of glass as it has a low coefficient of thermal expansion. This means that during temperature variations, the shape and exact curvature of the mirror would not be affected.
Telescopic mirrors need to be very precise and are thus ground to precision. This is a very lengthy and time-consuming process. Making mirrors manually, by grinding it to a finish is even more so. It took me 22 hours of grinding and 4 hours of polishing to make a small 4" mirror at IUCAA. So, I wanted to make another mirror, but better this time and also in an innovative way. The ideal shape for the primary mirror of a reflector telescope is parabolic. So I thought of spin casting the mirror.
The main idea in spin casting is to heat the substance till it melts, place it on a rotating platform and spin it as it cools slowly to give it the desired curvature.
A simplified model for this is as shown
In this simplified model when steady state is achieved, no matter which material is used(glass, aluminium, etc.) the focal point will be at g/(2w^2) in metres. Viscosity would not matter as the fluid would not be moving at the steady state.
The equation of a cross-section of the mirror would be a parabola
where w is the angular velocity in radian/sec and g is acceleration due to gravity.
For my requirement(focal length~1m) the angular velocity was 28 rpm.
I used a system of pulleys to reduce the rpm of the ac motor from 722 to 28 rpm. My uncle had a fancy 3-phase ac motor speed controller that could very accurately control the rpm of the motor by changing the frequency of the ac supply(variable frequency ac motor driver). I made an optical tachometer using infrared sensors and an arduino board to measure the exact rpm and compensate for any discrepancies. So, all that takes care of the curvature. Now for the setup:
I made a small turntable out of plywood and attached a ceramic tile on it using steel plates for insulation. Using similar bent steel plates I made a holder for the crucible/mirror cell. For the crucible I used a 2 inch section of a steel pipe of diameter 120mm with steel base welded on. It was crude but that did not matter as the mirror's surface would never come in contact with the mirror cell.
What I thought was that on heating the glass, it would become very fluid and on spinning it, it would acquire the exact curvature. In fact I thought the surface finish would be so good that I probably would not have to grind it at all. I knew of a liquid mirror telescope at an observatory that uses spinning mercury(liquid) as the primary mirror. This re-enforced my faith in spin casting. However things did not work out the way I had planned...
More in a later post..
Telescopic mirrors are typically made of glass as it has a low coefficient of thermal expansion. This means that during temperature variations, the shape and exact curvature of the mirror would not be affected.
Telescopic mirrors need to be very precise and are thus ground to precision. This is a very lengthy and time-consuming process. Making mirrors manually, by grinding it to a finish is even more so. It took me 22 hours of grinding and 4 hours of polishing to make a small 4" mirror at IUCAA. So, I wanted to make another mirror, but better this time and also in an innovative way. The ideal shape for the primary mirror of a reflector telescope is parabolic. So I thought of spin casting the mirror.
The main idea in spin casting is to heat the substance till it melts, place it on a rotating platform and spin it as it cools slowly to give it the desired curvature.
A simplified model for this is as shown
The equation of a cross-section of the mirror would be a parabola
For my requirement(focal length~1m) the angular velocity was 28 rpm.
I used a system of pulleys to reduce the rpm of the ac motor from 722 to 28 rpm. My uncle had a fancy 3-phase ac motor speed controller that could very accurately control the rpm of the motor by changing the frequency of the ac supply(variable frequency ac motor driver). I made an optical tachometer using infrared sensors and an arduino board to measure the exact rpm and compensate for any discrepancies. So, all that takes care of the curvature. Now for the setup:
I made a small turntable out of plywood and attached a ceramic tile on it using steel plates for insulation. Using similar bent steel plates I made a holder for the crucible/mirror cell. For the crucible I used a 2 inch section of a steel pipe of diameter 120mm with steel base welded on. It was crude but that did not matter as the mirror's surface would never come in contact with the mirror cell.
 |
| Mirror cell/crucible |
 |
| Optical tachometer |
What I thought was that on heating the glass, it would become very fluid and on spinning it, it would acquire the exact curvature. In fact I thought the surface finish would be so good that I probably would not have to grind it at all. I knew of a liquid mirror telescope at an observatory that uses spinning mercury(liquid) as the primary mirror. This re-enforced my faith in spin casting. However things did not work out the way I had planned...
More in a later post..
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