The data tabulated below illustrate the degree to which bad pixels “develop” over time. At present, every night of UFTI observing begins with a sequence of four dark exposures; these observations are known as the “Array Tests” and are used to calculate the NDSTARE read noise. The fourth exposure, a 50-second dark, will be used periodically to check the accuracy of the bad-pixel mask being used on all data. The DR recipe REDUCE_DARK, when applied to this frame, yields a dark exposure corrected for bad pixels. Any remaining bad pixels will thus be either new or “transient pixels”. Results are tabulated below:
Bad-pixels remaining in a dark that has been “corrected for bad pixels”.
Typically about 0.3% of the 1,048,576 pixels in the UFTI array are flagged as “bad” by the bad-pixel mask. Initial results indicate that, over a short period of time (a few days) some pixels “migrate” across the good/bad cut-off; the bad-pixel mask nevertheless remains accurate to within a few percent for a period of a few months.
Making a Bad Pixel Mask (Starlink Software)
From 3 or 4 nights of UFTI data. Produce “QUICK LOOK” versions of the 50sec dark taken as part of the array tests (the 4th frame in the sequence) with ORAC-DR; these aren’t corrected for bad pixles (i.e. they are raw NDF frames). Take the average of these raw dark frames, and flag all pixels above 200 and below -20 as bad (should be a few thousand pixels). Finally, multiply the frame by zero so that the resulting bad-pixel mask has data values of “0” and “bad” only.
> oracdr_ufti 20010101 > setenv ORAC_DATA_OUT `pwd` > oracdr -list 4:4 QUICK_LOOK -nodisplay > oracdr_ufti 20010102 > setenv ORAC_DATA_OUT `pwd` > oracdr -list 4:4 QUICK_LOOK -nodisplay > kappa > add f20010101_00004_raw f20010102_00004_raw add_darks > cmult add_darks 0.5 av_darks > thresh av_darks av_darks_thresh -20 200 bad bad > cmult av_darks_thresh 0 bpm title=\"UFTI bpm, January 2001\"
The above sequence produces a bad-pixel mask NDF (bpm.sdf) that can be viewed with GAIA; note the pixel values (zeros and bad). To re-reduce your data in ORAC-DR with the new bad-pixel mask, specify the bpm on the command line, e.g. oracdr -list 5:500 -cal mask=/your_directory/bpm.