ATM Focault KE/LS Offsets

["Bennett, Thomas D" <thomas.d.bennett@lmco.com>]

I had some concern about the accuracy of my fixed-light-source Foucault
tester which has a knife edge (KE) that is positioned about 2" behind and
1-1/2" laterally from the light source (LS). A "poor man's ray trace"
spreadsheet I wrote to analyze this
(http://www.pe.net/~grezes/SBVAA/foffset-parab.htm) indicated that for a
perfect 8" F6 mirror (ROC=96) with mirror center located at (0,0); a LS
location at (0.75, 95); and 4 mirror zones centers located at 1.585",
2.535", 3.19" and 3.74" radii from the mirror center resulted in KE
locations of (-0.766065, 97.03572); (-0.766383, 97.077435); (-0.766688,
97.117358); (-0.766997, 97.157954) for each zone. Feeding these 4 KE
y-values into Tex.exe indicated that this mathematically perfect mirror is
about 1/45 wave OC. Similar runs for perfect 8" F5 and 8" F4 mirrors
resulted in Tex.exe outputs of about 1/25 wave and 1/10 wave OC
respectively. Assuming that my spreadsheet is correct, is this accepted
knowledge that offsets of this magnitude between LS and KE on Foucault
testers is not significant unless relatively fast mirrors (faster than F4)
are being tested? Also, since I plan on building a Caustic tester for fast
mirror - seems like KE/LS offsets will be more critical for this type of
tester. Thanks - Tom