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Making a DIMM prism
A Differential Image Motion Monitor is a simple but acurate apparatus used by some professional observatories to monitor astronomical seeing in real time.
It consists of an aperture mask, placed on the front of a 10" or larger telescope, with two circular holes abut 50mm diameter placed as far apart as the aperture of the scope allows. One of the holes contains a prism that produces a deviation of 1 or 2 arc minutes. When the scope is pointed at a bright star, there will be two images of the same star on the sensor. Software monitors the distance variations between the two images and calculates the seeing.
Looks like someone here down under is having a go at producing a prototype, as I was approached to make a prism for it.
I started with a small slab of optical crown from my collection of old war surplus optical glass.
First, I had to reduce the thickness from 16mm to about 10.6 and then make it round. The final size needs to be 10 thick and 52 OD.
After the machining, I used aluminium laps and 240 grit carbo to get the thickness to 10.4, the diameter to 51.99 and lapped the edges at 45 degrees. Next, I will get the angle adjusted as I proceed with the fine grinding.
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