Posted here are two difference images of the dispersed (med-res) FCLS source on slit 1 and the central decker arc. 4.6µm filter, ND out, -1V applied bias, 31 ctdelay, grating @52.5 degrees.
I took noise the way Bob does, and found that in areas off the illuminated band the noise in a fowler-4 is 3.3 DN with the motors enabled, 0.9 DN with them disabled. The two jpegs posted here are difference images from adjacent 1 sec integrations in the FITS files. Same stretch is applied to both.
Worth noting that the noise is significantly higher (4.3 DN) if the cal source cables are not connected....this fits with Matt's observation that unterminated motor channels can add noise. I'll look at the manual to make sure we're running the motors in 'current reduction' mode, which is the factory default on the MID-7604...
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| Motors disabled |
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| Motors enabled |
3 comments:
Went back to the motor driver amplifier to see whether I could improve matters by monkeying around with the front panel DIP switches on the driver boxes. Turns out: no.
I tried setting the current to its smallest value, and then to its biggest value, no joy. There is also a control that reduces the current to the motors when they aren't being driven, but that had no effect either. Set everything back to factory defaults.
Do you know why the illuminated region has so much more signal than unilluminated in the difference? If the only change between the two frames was moving the FCLS off the slit, I would have expected there would be very little light left in the difference except for the point source.
As for possible cures, can we set the ctdelay so a frame is in phase with 60 cycle around 1 second? Then the noise might subtract. We are probably there already trying to get to exactly 1 second frametimes.
Re 60 cycle, this interferences is in the tens of kHz...60Hz looks like horizontal bars in these frames....
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