Ok, so moving along here. Today I'll step through the procedure in as much detail as I can stand. First the basics:
The mirror is unmasked obviously, but the setup otherwise is exactly the same as for Foucault testing with a Couder mask, only in place of your eyeball you use a camera. The camera is NOT attached to the stage but wants to be secured in space behind the KE, focused on the mirror, and attached to a tripod or similar support so it doesn't move at all.
It doesn't matter if the tester is fixed or moving source, but for correct analysis in FigureXP a fixed source tester MUST have the KE where the center of the mirror shows a null aligned longitudinally with the light source or the analysis can show significant error for fast mirrors. For a moving source (usually slitless ) this condition will always be met. If you use SixTests for analysis (you might want to because it doesn't appear to have any limit on the number of zones that can be entered, whereas FigXP tops out at 15 zones) you can have a misalignment longitudinally between the source position on KE, but you'll need to know what that is. Easier just to line it all up in the first place.
For this test to work correctly you need a mirror with a good diffraction edge, smooth, with no defects of figure of revolution. But you're wasting your time if those conditions aren't met so I won't mention them again.
Setup is to have the KE aligned as above on the center null of the mirror, the gauge ideally reading zero at the same position, and the axis of the tester aligned with the axis of the mirror. I accomplish the latter by pushing the tester forward a bit and then running the stage back and forth while adjusting its axis until the position of the KE on the mirror center doesn't drift left or right - then dragging the tester straight back while keeping the same center position. Then, at least the way I test, I run the micrometer back until the very edge of the mirror is nulled.
This takes the amount of travel that FigXP reports as the "ideal knife edge value" for the outer zone. But because the zones tested this way can be very narrow - and you don't know what they are until after doing the test - just get it to where the edge is illuminated and note that value as the starting point.
Next you take your eye away and put the camera in its place. It doesn't have to be close to the KE, so long as it captures the entire face of the mirror (or at least all the way across the center).
Using the camera viewfinder (or computer screen or whatever you have) adjust the setup until you see good contrast and the diffraction ring on the mirror. This is the first exposure, so capture it.
Now, iteratively, you want to step through the caustic depth of the mirror with the shadows changing (like in posts 2 and 3) from edge balanced to the central null.
You'll want to parcel out the expected travel of the KE longitudinally (the value of the outer zone) into the number of "zones" you'd like to capture, whether 10, or 100, for analysis. Just divide the value of the outer zone by the number of images desired. That will give you the incremental travel between each exposure, and due to the way that correction increases on a paraboloid as you move out, it won't be an even traverse across the FACE of the mirror, but it will be an even traverse across the SLOPE of the mirror, which matters much more for nailing down the surface correction.
So you just calculate out those steps, move the stage by the correct (always the same) amount to the next "zone", and capture another exposure. Save them or rename them with some scheme that makes sense.
Having done that, analysis comes next. To do this you need to know a couple things - the # pixels across the diameter of the mirror in each image (which will remain the same because the camera doesn't move at all) and the # pixels across the "zone" for each exposure. You already know the actual physical optical diameter of the mirror (let's hope). In some sort of paintshop program, open each of these images. I'll demonstrate it with the second exposure from the series above, test02.jpg:
When I measure the diffraction ring in pixels I get 428. You can of course increase the contrast to make this easier to read.
Determining the "zonal" value is a little more complicated. It's measured between the spots across the diameter where the gray values are identical. For a monotonically increasing curve like a paraboloid should be there's exactly three of these spots - left, right, and center.
Fortunately an easy visual method exists to make this measurement.
First open the image capture you're working with, and (important!) crop the image to the exact edge of the mirror on both sides:
Make a copy of this and mirror reverse it:
Then you take the difference between them - because the two (three) locations across the mirror have the same value, the difference there becomes zero, whereas it's positive everywhere else:
OK, well there may be zero values in there but they're hard to see. The final step clears that up. Using "color corrections" in IrfanView crank the gamma up and then adjust the contrast (or use any image manipulations program to do the same thing):
Here's the result:
Now you can measure the pixels across the mirror, since the only values that remain zero after that last step are the ones that were already zero. Alternatively, of course, all of this can be determined in software from the image files, which is how you'd do it in a fully automated tester.
You can see that there are 388 pixels across that diameter, so the diameter of that "zone" is 320mm (the mirror diameter) divided by 428 (the pixel diameter of the diffraction ring) times 388, or 290(.1) mm, and the radius is 145(.0) mm.
Edited by mark cowan, 26 May 2016 - 08:43 PM.