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| A Comparison of a Good Mirror to a Mirror with Figure Problems |
The following is a comparison of two 15” F/4.5 parabolic mirrors. I am writing this in response to some questions posted on the Yahoo Torusmirrorgroup. Specifically, someone wanted to know what a bad mirror looks like. I have made some detrimental polishing runs on a scrap mirror (nasty chip on the back side) to obtain the following images for comparison purposes. To get the bad results I made some runs with a sub-diameter polishing lap, applying way too much pressure and with the stroke set to result in a turned down edge. These runs also resulted in surface roughness and a bit of a central depression. The images on the left are 15#10, as posted on our web site in the test images section. It has a smooth surface with a good figure and no turned edge.
The two Ronchigrams above were taken using a 60 line/inch grating roughly the same distance inside the center of curvature of each mirror. Note that the fringes in the Ronchigram on the right are somewhat mottled. This is due to surface roughness or micro-ripple. Note that the curvature of the fringes on the left is more monotonic (smoother). The curvature on the right is not as regular. This shows that the mirror’s “parabolic” figure is not regular. The figure problems are: a slight central depression, an overcorrected outer zone and a turned down edge. The TDE in this image is indicated by the quick inward curvature of the fringes at the edge of the mirror.
The Ronchigrams above were taken with the same grating just outside the center of curvature of each mirror. The curvature of the fringes outside Rc is the inverse of the fringe curvature inside Rc. The TDE in the image on the right is indicated by the outward hook in the fringes. Note that there appears to be a slight outward hook in the fringes on the left. This is due to diffraction, not a turned edge. Have a look at the following link for an excellent explanation of diffraction effects in the Ronchi test.
http://www.users.bigpond.com/PJIFL/ronchi_diffraction.html
The above images are Foucaultgrams of the two mirrors with the knife edge near the 70% zone. [A quick note: The image on the right was taken with the usual Foucault knife edge. The one on the left was taken with the 60 line/inch grating, using one grating fringe as the knife edge. This accounts for the light spilling out on the right side of the mirror. In the future I will use only the standard Foucault knife edge to take these shots.] In the right image note the swirl marks in the mirror running clockwise from the center out. These swirl marks result from too high polishing pressure. You can also see the central depression in this image.
The two Ronchigrams above were taken with the mirrors in double-pass auto-collimation with a precision optical flat. The flat has a 2” hole in it’s center and we are testing through the hole. This configuration is a null test so you want to see straight fringes. No problem on the left. On the right you will notice that the fringes bow slightly inward inside the 70% zone. The central area of the mirror is slightly under-corrected. Closer to the edge the curvature of the fringes goes inward indicating that this are is slightly overcorrected. The overcorrected edge transitions into the TDE, represented by the quick inward hook.
Following are some additional Foucaultgrams of the bad mirror with the knife at different positions to bring out specific errors.
Central Depression |
Zone Inside 70% |
Turned Down Edge |
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