Test Stands

To test a telescope mirror in the shop, you need to hold the mirror so that the test jig can see it, so that it is adjustable in position and placement, and so that it does not inflict a change in the shape of the mirror due to stress from gravity or the test stand itself.

The mirror needs to be held so that it is pointing uphill from horizontally just a tad, therefore the edge of the mirror, not the back of the mirror, is supporting most of the weight of the mirror. There are two standard ways to do this so that the shape of the mirror is not altered. Using pegs or straps seems to induce a shape error, so I use wiffles at the center of gravity of the side of the mirror. The mounts for the wiffles need to be mounted 90o from each other, and the wiffles are 22.5 degrees apart, so there are 4 of them altogether separated by 22.5 degrees, two on the left and two on the right. This seems to support the mirror with the least negative infliction.

I do mostly Hartmann testing now which tests for the shape of the figure and also for astigmatism, but Iíll write this up for somebody who is not doing that. One of the tests to do is an astigmatism test, and if it turns out that you see astigmatism in the optic, you would want to eliminate the test stand forces as the cause of the astigmatism. To do this you would rotate the mirror and see if the suspect astigmatism rotates with the mirror or stays in the same position. If it stays in the same position, your test stand needs changing [or your test method of astigmatism]. If it rotates with the mirror, then the mirror needs fixing. Also, if you are testing your mirror with a bath interferometer, it will see the astigmatism generated by the test stand. If you rotate the mirror into different positions and test with the interferometer, then the software will eliminate the warpage of the glass.

Your mirror test stand should have adjustments for tilt. Adjusting it for position is as easy as moving the test stand. Also your test jig will be able to go up and down. Your equipment should be built so that you donít have to go back and forth a jillion times between the mirror and the razor.

I have a fan on the back of my test stand to draw air past the mirror, and a cowling shroud attachment so I can direct that air past the mirror in a way that has experimentally show to bring the mirror to ambience quickly and repeatably.



Go on to figuring.