[ATM] WG: Focault (sic!) Help

[Jeremy Royston]

The Foucault and especially the Ronchi test are not very sensitive to
astigmatism, and you need a certain amount of experience to see it.
The easiest way to see astigmatism is to set up your mirror as for the
Foucault test, use the reflection of an LED (high power, narrow beam)off a
clean small steel ball (4-6mm) as a point source. Look at the returned
immage with a short focus eyepiece (about 5mm). If there is no astigmatism
the image is round inside and outside focus.
The test is described in Texereau
If you want a quantitative test, use the 2D Hartmann test, my hobbyhorse.
See http://home.earthlink.net/~burrjaw/atm/odyframe.htm The measurement
needs no more time than with Foucault, needs no experience, and gives you a
picture of the whole mirror 

Jerry

Berthold wrote:

... snip
The only aberration worthwhile looking at right now would be astigmatism
which could be detected by Foucault and Ronchi at this point, but demands a
little bit of experience in order to be correctly identified. If you work by
hand, rotate your glass diligently and support it well, it is unlikely that
astigmatism is an issue.

Regards

Berthold