fringetesting

Testing the Schiefspiegler secondary mirror by observing interference fringes:

I received a set of mirrors for making a 5 inch Schiefspiegler telescope from Mark Harry, a veteran mirror maker who lives in the New England area. As received, the mirrors should perform well. The only reason that I went ahead and continued polishing was to try out some of that synthetic pitch, and see the effects of polishing strokes and techniques. For details about the lap see: Making a Lap

The testing here is very similiar to testing optical flats for flatness. Here we are concerned with the concave primary mirror and the convex secondary mirror having similiar mating curvatures. When the secondary is placed on the primary, interference fringes can be seen. If the curvatures exactly match, then the fringes will appear as straight lines. If the curvatures are slightly different, then the fringes will appear as concentric rings. A convenient measure for how well the curves match can be made simply by counting fringes as shown below. The photo on the left has "1 fringe" curvature difference. The photo on the right has "2 fringes" curvature difference.

When making a Schiefspiegler secondary, it's ok if the fringes are not straight lines. What is important, is that they appear as nice circular rings with no abnormal bends or odd patterns. Then you know that the surface is a nice sphere.

How to tell if the radius of curvature of the secondary is shorter or longer than the radius of curvature of the primary:
Apply slight pressure to the edge of the glass in order to get the fringe circles centered. Then with another finger, apply some downward force at the center of the glass. If the fringe circles all move inward, the radius of curvature of the secondary is longer than the primary. If the fringe circles all move outward, the radius of curvature of the secondary is shorter than the primary.

The effects of polishing:

Here is the fringe patterns of the Schief mirror set as received from Mark Harry. Mark wrote that the grinding was done with the 3 inch secondary on top of the 5 inch primary. There is only about 1 fringe difference in the curvatures. Also the secondary shows a very slight turned up edge of about 1/8 wave at the edge, if the black lines are adjusted for a "best fit" the problem would be approx. 1/16 wave overall. Very nice by anyones standards!.
curvature difference = 1 fringe

Here I polished for 15 minutes with a W stroke and the mirror on top. The center of the convex secondary deepened quickly(reducing its curvature some) and now more fringes are seen. The turned up edge is now gone, but a slight turned down edge may have started to form.
curvature difference = 2 fringes

I polished for 15 minutes with a W stroke with the lap on top, hoping to increase the curvature some.(to match the primary) Since there is more glass around the circumference than the center area to remove, progress is slow, but in the right direction.
curvature difference = 1.75 fringes

I polished for 30minutes with a W stroke with the lap on top, hoping to increase the curvature more.(to match the primary) A slight turned down edge remains.

curvature difference = 0.8fringes

I polished for 30minutes with a straight center over center 1/3 stroke with the lap on top, hoping to reduce the turned edge some. Now the fringes appear quite circular. Even with the lap on top, the curvature decreased slightly, increasing the number of fringes slightly. I think I should consider the glass finished!

curvature difference = 1.2fringes

Results:
The center deepens quickly with the mirror on top.
The outer edge can be worn down, but this requires much more time with longer strokes and a W stroke.
Acculap pitch works just fine!

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