Here is how we have done the Ross Null test for 15 years at the Delmarva Mirror Making class so this is first hand experience. The results of mirrors figured this way have been crossed check using Double Pass Autocollimation which we also use at the class and and other test methods and the agreement is excellent. Many of the mirrors made have won optical awards at Stellafane when judged by 9 optical experts over a number of years.
First the Ross Lens is mounted in an independent holder. So it can be adjusted in tip, tilt and height. The bottom of the stand that holds the lens has a straight edge. This edge can be placed up against a stop to fix the distance of the mirror to lens in place. The offset from the front edge of the base of the lens holder to the vertex of the lens is measured and is a constant for that lens/holder combination. You adjust the Mirror to Lens spacing accordingly to take into account the off set of the holder.
1) Using the Ross Null software calculate the distance from the mirror to the lens.
2) Set up the mirror and your tester just like your doing a standard Ronchi test.
3) Place the tester about 6" behind were the lens would be positioned.
4) Align the mirror and tester just like your going to do a standard Ronchi test. Since your not at the radius of curvature of the mirror you'll see many more Ronchi lines then in the standard test method. The key is to get everything lines up and so you have a symmetrical Ronchi pattern. You do not have the Ross Lens in place at this point.
5) Measure the distance from the mirror to were the Ross Lens needs to be with the correction for the Ross lens holder. Place a heavy object like a book so the edge is at this exact position.
6) Move the Ross lens into place so the front edge of the Ross lens holder is against the stop ( ie heavy book or piece of wood etc) Something heavy enough that it won't easily move.
7) Now the lens is at the correct distance and you can slide it side to side without changing the mirror to lens spacing
8) Look thru the Ronchi screen and position the lens so you see the Ronchi lines coming thru the lens. Adjust the tip. tilt and height of the lens so the lens is centered on the outline of the mirror and the Ronchi pattern of lines you see is symmetrical up down and right left. So you can slide the lens side to side, raise it up or down and/or twist it to get the pattern to be symmetrical.
9) Now move only the Ronchiscreen/light source forward or back just like your doing a standard Ronchi test so you reduce the number of lines to 3 or 4. Now you can go from inside of focus to outside and examine the shape the Ronchi lines.
10 ) You want to figure the mirror so you have three straight Ronchi bands and it test just like a spherical mirror You interpret the lines like you do when testing for a spherical mirror ie a hole is hole a hill is hill etc just that your reference surface your trying to achieve is parabola and not a sphere.
Remember to calculate the distance in the software using the wavelength of the light your testing at. Usually it is in the red. I use a red LED but when doing critical testing I also look thru a narrow bad interference filter that pass 632nm. Until your down to the very last stages of figuring and have what looks like very straight bands the filter isn't needed.
If you never used a Null test before, what your going to find it is much more difficult to achieve a null figure on the mirror then what you think so issues with getting the exact spacing, known the exact focal length of the lens, the exact refractive index don't come into play until you are real close to having that null. It is not like if these these parameters are not known to 3 decimal places or more that you will see the Ronchi bands go from dead straight to bowing like crazy if your off by a small amount.
Hope this helps. Happy to answer any questions
Edited by DAVIDG, 10 October 2019 - 03:57 PM.