A Simple Mirror Test

A Ronchi Tester and application for the masses

Tony Gondola, BigEye Optics

 

Anyone who owns a telescope will at some point be faced with the question  of optical quality. There are many ways this can be tested to high accuracy by an optical professional or skilled ATM  but for most people, a relatively simple test that doesn't in most cases even require removing the mirror from the OTA. is the best answer. You could use the well known star test but it requires good seeing and is notoriously hard to interpret, especially when multiple errors are present. The good news is there is a simple test that can tell you most of what you need to know about the surface of your mirror and that's the Ronchi test. All it requires is a replicated film grating, (100 to 125 lines per inch is about right) and a light source.  The test can be applied at focus using a star as a light source but doing it that way reduces the sensitivity of the test and makes it subject to seeing effects. It's much better to do it as a bench test at the radius of curvature. The sensitivity is higher, seeing isn't a factor and the result can be photographed or studied in comfort and leisure.
 
The first step is to make the tester. This consists of nothing more then a few pieces of scrap wood, the film grating mentioned above and a light source. The light source can be just about anything but a small  LED key chain light keeps things cheap and simple. The following drawing shows the basic arrangement:
 
 
Once you have the tester sorted out you'll need to set the test up as shown below:
 
 
Even if the mirror is uncoated the test is bright enough to be done in normal room light.  The only thing to be careful of is to find a spot that won't be subject to a lot of vibration or air currents. Doing it in front of a heating vent for instance would be a bad idea.

To line up the tester place it a distance from the primary mirror that's twice it's focal length.  If you have a 6" F/8 which has a focal length of 48" (6 x 8) the tester would be placed at a distance of  96" from the primary. If you're testing with the mirror in the tube it will be best of the secondary mirror and spider is removed for the test.

Once the tester to primary distance is about right turn on your light source and locate the return beam. If the mirror is coated you can probably find it by placing a white card in front of the tester (while not covering the light source of course.). Another method is to step back from the tester a few paces and bob your head around until you see the reflection of the light source in the mirror. Once located adjust the mirror and/or the position of the tester to place the return beam so that it shines through the top half of the grating. Once the beam is positioned place your eye behind the grating at the position of the return beam.  When everything is lined up you'll see the mirror fully illuminated and crossed by a series of vertical curved lines. If you don't see any lines try moving the tester a little closer or further from the mirror until they appear. To give you a general idea of what you'll see here's a simulated result with a 10" F/6 parabolic mirror. The left image is outside focus, the right side image is inside focus:
 
 
Even if this is the first time you've ever done this test there are a few basic things you can look for right away. Adjust the tester distance slightly outside focus so that you see 4 to 6 lines.  Look carefully at the lines at the edge of the mirror. they should remain constant right up to and over the edge. If they hook sharply outward the edge is turned down. If they hook sharply inward the edge is turned up. 

After you've checked the edge move the tester towards the mirror until the lines disappear and the mirror is more or less uniformly illuminated. You're now at focus, look at the surface carefully, it should look fairly smooth without any major apparent roughness.

Now push the tester back and forth outside and then inside focus while carefully observing the angle of the lines. If you see a change in this angle so that it flips one way and then the other as you move the tester through focus then the mirror has a significant degree of astigmatism.

For the last gross test carefully look at the lines both inside and outside focus. They should curve smoothly without any abrupt bumps or zigzags. If you see a defect that goes through all of the lines at the same radius from the center, you're looking at a zone error. If it's bad enough it will also be visible in the roughness test you performed at focus.

If all of the above tests are passed without any notable errors then the chances are good that you have an excellent mirror. Note that we haven't actually ascertained if the mirror has the proper amount of correction but chances are, if the person who made the mirror went to the trouble to avoid all of the above errors, the overall correction is probably close to ideal. To get a feel for how close it really is you'll want to use a Ronchi generation program to make a set of patterns for both inside and outside focus and then compare that to what you actually see. This test is a very sensitive gauge for mirrors in the F/5 to F/10 range. For faster mirrors it won't be as accurate but it's still useful. When you set up the tester to replicate the same number of lines that you see in the generated patterns  it should be a very close match for slower mirror and an exact match for faster mirrors. This is one area where a follow up using the star test would be a good idea. If the patterns that a star makes inside and outside focus at high power come close to matching then your overall correction is very close to ideal.

 
If you feel the need a more experienced eye on all of this you might consider photographing the test results and then posting them to a good ATM group for comment and analysis. To do this simply place your camera so the lens is looking through the grating just as your eye was. You can modify your tester to carry the camera or just simply mount it on a tripod.  Adjust it's position so that the mirror is fully illuminated and then zoom in to fill as much of the frame as you can.  Focus on the mirror (not the grating) and have at it. You may have to adjust the exposure from normal settings, especially if the room is dark. Just try different settings and check the results.
 
If you want t learn more about the Ronchi test and it's interpretation, here are some useful links:

http://members.aol.com/RonWin20/

http://www.users.bigpond.com/PJIFL/ronchi_index.html

http://telescopemaking.org/ronchi.html