Posted 10 May 2013 - 01:08 PM
John, you're absolutely right -- the RossNullXP OSLO file is somewhat of an enigma. If you try to enter it by hand the results don't match, and the only parameter that cannot be manually changed is -- the light source (pinhole) size which, as you noted, is ridiculously small. I would be interesting to find out how the XP alters that number.
In reality, when testing optics, the light source image can be reduced to that of the diffraction limit and no amount of reduction beyond that will make any difference. The test will also be most sensitive at or about the diffraction limit. Anyone who has done the Ritchey Common test for flats will know that. Attaining such a small light source that's bright enough can be achieved by using a well polished steel ball bearing or a microscope objective, using a monochromatic light source (superbright LED plus a narrow-band filter), and the simple lens maker's equation.
However, DAVIDG is right in that getting the error down to 1/10 wave is the least of your priorities when performing a Ross null test. I would personally never trust the Ross results as anything more than a ballpark figure that tells you if you're going in the right direction.
If I have a choice I will not do the Ross test at all, but if you have a large mirror and probably no matching flat for autocllimation purposes, then the Ross is something to consider.
You're also spot on when it comes to Ed Jones' conjugate null test. There was a thread about it not so long ago. Unfotunately not too many have seen the great benefit it offers as an alternative. Of course, the biggest problem are the air currents.
So, for big mirrors, there is still the Foucault test as the only test that can offer qualitative and quantitative results -- but even here we have to thread lightly. The quality of the testing equipment itself, the experience of the tester performing the test, and many other factors play a role in just how reliable the test results will be. It pays to invest in a good professional micrometric stage, for sure, but then you still have to deal with shadows and subjective judgment calls.
besides, the Foucault test is just not best suited for mirrors in the f/3 range. Slitless testers used by many are not very sensitive and this can be demonostrated by its inability to differentiate a spherical mirror from one that departs from true sphericity by a small amount.
That's why, to those who plan on making big mirrors, I say: invest in (or just make) an autoclllimation flat that's bigger than anything you'll ever make -- it will pay itself off many times over, and when you've fulfilled your telescope bucket list you can sell it, recoup the money, or even make a profit. There's always going to be someone who's looking for a suitable optical flat.