I purchased one of the inexpensive 6" F5 mirrors on e-bay for just under $24 (not including $10 shipping.) I tested it using the various equipment I have around.
Overview: The mirror is a plate glass 6" F5 sphere rather than a parabola. It is a reasonable sphere with an RMS error of about 1/18 wave (on the wavefront.) As a parabola isit is quite bad, with an RMS error of 1/3.5 wave (on the wavefront.) The P-V error is just over one wave on the wavefront or 1/2 wave on the glass. The Strehl ratio is 0.044, which is very poor.
Tossed into a telescope, I would expect that the mirror would give very modest results-strickly low power, fuzzy views. However, it is a thin mirror and a reasonably good sphere. Using Alder's method one might be able to distort it into a good or even very good paraboloid.
Here are some pictures:
Ronchi test with an 80 line-per-inch filter near ROC--outside of focus. (I always work outside of focus so I don't get confused.)
Knife-edge test:
Both show slight parabolization. The knife-edge test shows a very narrow TDE (bright ring on the left edge) that should be masked before putting it into a telescope.
For full interferometric test results, see the following two files:
The first page analyzes the mirror as a sphere, and the second analyzes it as a parabola. The deviation from the sphere are modest, but the deviations from a parabola are severe.
The mirror undergoes potato chip distortion on the test stand. I have removed the resulting astigmatism in the analysis and have not attempted to do a more precise analysis. Note that both interferograms give essentially the same correction of -0.149, close to a sphere (0) and a long way from a parabola (1).
For those of you who are interested, here are the interferograms. This is a coated mirror, so I use a "nylon filter" to reduce the returning beam intensity. The interferograms have a lot more noise than with an uncoated mirror. These are listed in the same order as the analysis shown above.
A description of the interferometer can be found on this page.
