"Amateurs should not fool around with alternate nomenclature caused by incomplete information or lack of knowledge."
I agree 100%.
That being said, how about the null test layout with concave reference inside the ROC of the mirror being tested? Parameters, such as diameters, spacings etc? How does sensitivity compare to a test with an AC flat?
I agree, but it is also true that even those quite familiar with optics are not always familiar with the works of D. D. Maksutov, and it is also more a habit to call it (incorrectly) The Waineo Null Test (WNT).
Any conjugate test, including the double-pass autocollimation test (ACT), will be sensitive to accurate spacing.. Likewise, accurate alignment of all elements is just as important. The tolerances vary according to the focal ratio of the mirror being tested, but very few conjugate null test tolerances will be as generous as those of the ACT.
The real advantage of the Mak Null Test (MNT) is that it is a better test than the Ross Null Test (RNT), because the quality of the nulling Maksutov sphere is known, whereas the quality of the Ross null lens is not -- because for most ATMs its' difficult to accurately test the concave surface of the Ross lens. Also, using a reflection surface avoids any substrate inhomogeneity problems present in refractive optics, especially bargain basement lenses. But the quality of a reflective surface has to be better than that of the reractive.
It is easy to show that a for moderate apertures the MNT is adequate for mirrors as fast as f/3. For example, the Mak Null Test can correct an 8-inch f/3 paraboloid to about 0.16λ ptv, but a 32-inch f/3 only to about 0.7λ.
By far the best nulling compensator for all diameters, including very large (6 meter) and fast mirrors (down to f/1.25) is the Offner test, and the tolerances are tight but not too tight. People often mention the The Hubble Space telescope fiasco as an example, but that unfortunate error was caused by gross negligence. If anyone is interested in the story of this event here is a link
As can be seen in this article, a technician not only used a wrong reflection as reference but to "fix" the problem inserted an ordinary hardware washer as a spacer in the nulling compensator! This is at least 1.2 to 1.5 mm when tolerances range in the microns! It's very unlikely for a meticulous worker to do that under ordinary circumstances or outside ATMs' capabilities.
The Offner can be both refractive and reflective. High precision and high-quality melts are required for the refractive model, and the reflective version looks very much like a miniature MNT!
Edited by MKV, 15 December 2015 - 09:58 AM.