Here are the images seen on the test, captured with my videorecorder at the eye position of the foucault tester. The mirror is a 12.5" Cassegrain I am making, thus there is a 2.75" hole in the center, which appears black in these photos. At this point, this mirror needed alot of TLC. It has the traditional ATM bugaboo - a turned down edge (TDE). There is also a raised zone inside that, and a bit of a turn down edge near the center hole.
Foucault test images, without mask over mirror:
A Ronchigram of the same mirror:
more Test images as progress was made...
Photos of other mirrors:
This is an oblate sphereoid, combined with a turned edge.
The Ronchi bands will turn outward when beyond the radius of curvature and curve inward when inside the radius of curvature.
In this case, where the bands curved outward for just the outermost section of the mirror: the outer bands show some parabolization while the center is flat. When inside radius of curvature, then center would also look flat, and the outer bands would curve inward.
Another image of the same mirror.
General Rules to Interpretation:
When outside ROC, the Ronchi bands are more sensitive to the edge zones, when inside ROC, the bands are more sensitive to central zones. For more information on testing and interpreting Ronchigrams, see our software and manual.
How good is a mirror, really? Besides doing a star test, the real test is an interferrogram. This is typically made with a LUPI using a high quality laser and microscope objectives. Click here to see some examples.