Curtis noticed while looking at the blackbody and gate valve that the detector often exhibited lower response after we filled the wells appreciably. The first diagnostic was that for a fixed grey-scale level, the first frame of the second integration when we were looking at appreciable flux would have "white patches" at the pixels that saw the most flux. The OTPAT below is N3 S1 N2 D0.
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| Frame0 during first integration with greyscale = 8000 to 11500. Note pixels near center where flux is starting to accumulate |
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| Frame0 during 2nd integration with same greyscale. Note pixels with most flux (black above) are white here indicating fewer electrons in the wells than above or even general pixels. |
Added a throwaway to the end of the OTPAT (N3 S1 N2 D0 T0) and saw that the behavior went away.
Took a series of 8 stares at blackbody (260K) with the N3 S1 N2 D0 pattern (file bb.sci.20412.fits) and with N3 S1 N2 D0 T0 pattern (bb.sci.20413.fits). You clearly see the same offset in the D0 set for every integration after the first and you clearly see more normal behavior in the D0 T0 set. However, if I do the ND subtraction as we typically do while observing for each of the 8 integrations in both of the sets and compare how these NDs vary depending on OTPAT, I see that only the first integration is different.
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| Top Panel: Row 600 for 8th integration with N3 S1 N2 D0 pattern (white) and with N3 S1 N2 D0 T0 pattern (red). Clear difference especially at the highest flux level. Bottom Panel: Each integration minus the 8th integration for Row 600. White is N3 S1 N2 D0 pattern 1st minus 8th. Green is for integrations 2 through 7 minus 8th. Red is for N3 S1 N2 D0 T0 pattern 1st through 7th minus 8th. |
Since adding a throwaway costs us a frame each time through the OTPAT and we are always going to be doing like-for-like subtractions when doing the nod differences, I think we probably don't need the T0 patterns even though something funny is going on. Flat's might be a different matter since the photon flux between black and sky is so different. Comments?
1 comment:
Sorry, but when you say 'the first frame of the second integration when we were looking at appreciable flux would have "white patches" at the pixels that saw the most flux.', do you mean that the number of counts in a single scan is smaller in the most brightly illuminated areas, or is the artifact apparent only in a difference set? I have a hard time imagining a mechanism that would make the absolute signal in the most brightly illuminated pixels smaller than in the dark areas if the flux is still present. Probably, that is not what you're saying.
Also, when you refer to an 'ND subtraction' I take it that you mean the difference between the first non-destructive scan and the reset scan, yes?
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