ATM Null Test similar to Artificial Star Testing

[Adam Smith <ajus@xport.on.ca>]

Henry E. Paul published a test in "Telescopes for Stargazing" (1965,
Amphoto) that involved using a distant point source. The notion was
basicly the same as Dall's null test, (or autocolimation, for that
matter) except using a pinhole from 40-120 feet away instead of the
large flat and pinhole at Focal point. The test was based on work by
Spoelhof (I think, the book is at home), and showed a formula for
calculating minimum required distance for various physical tolerances,
along with examples for various apertures/focal ratios. The figures were

for small mirrors and "normal" focal ratios (7.5-8) and the longest
distance published was approx. 120 feet (off the top of my head) which
gave a physical tolerance of 1/15 wave for an 8" f7.5. I think 6" f8 1/4

wave was about 36-40'. Paul also mentions the idea of using a flat to
fold the testing distance in half (which halves the size of flat
compared with that required for auto-collimation, an important point).
The testing procedure is similar to foucault, but shows null at an
approximate parabaloid (long ellipse really: the pinhole is at the other

focus), instead of null for the sphere. The pinhole gets more distant
rapidly with larger aperture and/or smaller focal ratio. The knife edge
is at the focal point instead of the R of C.

If anyone is interested I could attempt to paraphrase the test more
accurately (with book in hand, for example). I don't think I can publish

the material without going back to Amphoto for permission, but I suppose

if enough people were interested I could try that as well.

Regards,

Adam Smith,
(new subscriber to the list)