The Caustic Test, From Amateur Telescope Making Book Three
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There is an Excel spread sheet that seems to be an accessory program for the above chapter of 'ATM Book Three'. The Excel spread sheet is by Ken Hunter. The link for it is at http://www.efn.org/~mbartels/tm/software.html.
I tried to use my home built tester in place of the 'ding bat' described. I can tell you that the 'ding bat' will need to rest on something better than the average camera tripod to get repeatable measurements to less than .001 inch.
Bill Thomas uses film ( or CCD ) to capture images of the caustic. His test bypasses some of the difficulties of the standard wire test used for caustic measurements.
Anyway, my idea was to use the grating lines in place of the left/right drive on the 'ding bat'. Then I would need to write a computer program to work the caustic formula forward many times until it found a radius on the mirror that satisfied the fixed grating line locations. Then the mirror mask instead of having regularly spaced holes would have holes as prescribed by the computer program.
As
a first step I masked off all but one line in the 100 line per inch grating on
its lower half using vinyl electrical tape and a toy microscope. Now the light
source is a single slit instead of a multi slit. Doing this seemed to improve
the resolution of the Ronchi test, particularly when working in the sensitive range
between minimum and maximum radius of curvature of the mirror.
Another
Tester called the DAFT tester uses an eyepiece to view the image of a pin hole
( not a slit ). A string is placed in front of the mirror mask openings
to produce a diffraction pattern. If I understand the description, the
test finds the same locations longitudinally ( to and from the mirror ) as the
Foucault test. So if you have a sturdy Foucault tester already it can be
adapted to the DAFT test. The home built tester
design given in this program is not sturdy enough to carry the extra lenses
required. See: daft.htm The link for this
article is at: http://nav.to/bobmay Be
advised the link has popups but otherwise its a good link.
The Dall Null Test, From Amateur Telescope Making Book Three
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Another way to test precisely is to put a lens between the tester and the mirror. This can cancel the spherical aberration caused by testing at the center of curvature. The test is then a 'null' test as if the mirror were a sphere. There is a company, Ceravolo Optical, that sells the null test lenses.
There is no point in setting up the above tests unless the mirror is already corrected to better than half wave since correcting to a half wave can be done more easily using a Ronchi program printout. If you have more time than money then best to just wait for good testing nights and use an LED for an artificial star and the knife edge, and eyepiece tests to finish . Likely windless, overcast nights will be best. Be advised that different combinations of eyepiece and Barlow can produce spherical aberration. People that use the eyepiece test all the time usually have a favorite high power eyepiece for testing.