>
> Just to be devil's advocate:  are you sure you're not adjusting the black
> level so that the differences are magnified to 50 grey levels?
>
I checked the black levels -- in each case I have about 5 units or
so of dark signal. Gain is set so that there is no saturation, and the
peak signals are at about 200 for the "green" channel (pmt2) , and
about 150 in the "orange" channel (pmt1).

> Maybe something is wrong with the shape of the pinhole(s)?
>
The iris apertures on PMT1 and PMT2 were 0.7mm (round aperture -- the MRC-1024
has no other shape). I had just aligned M4, M5, M6 prior to checking the
uniformity.

The iris on PMT3 was 2.5mm, because I was using a green fluorescent block
which was not a strong emitter below  605nm. Having a wider aperture often
"improves" the result by causing the system to be more tolerant of
misalignment.  Again , black level was checked and gain was set so that
the signal was between 100 and 200.

Thanks for the suggestion.

--aryeh
Aryeh Weiss                          | email: [log in to unmask]
Department of Electronics            | URL:   http://optics.jct.ac.il/~aryeh
Jerusalem College of Technology      | phone: 972-2-6751146
POB 16031                            | FAX:   972-2-6751275
Jerusalem, Israel                    | ham radio: 4X1PB/KA1PB