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New static DQ masks for NICMOS

New static DQ masks for NICMOS. Tomas Dahlen & Elizabeth Barker TIPS 3/19/2009. NICMOS Data Quality (DQ) extension of calibrated images (*_cal.fits or *_ima.fits) contains information about which pixels may be problematic and should/may be excluded when combining dithered images. DQ Meaning

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New static DQ masks for NICMOS

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  1. New static DQ masks for NICMOS Tomas Dahlen & Elizabeth Barker TIPS 3/19/2009 TIPS, Mar 19, 2009 - Tomas Dahlen

  2. NICMOS Data Quality (DQ) extension of calibrated images (*_cal.fits or *_ima.fits) contains information about which pixels may be problematic and should/may be excluded when combining dithered images. DQ Meaning 0 No known problems 1 Reed-Solomon decoding error in telemetry 2 Poor or uncertain Linearity correction 4 Poor or uncertain Dark correction 8 Poor or uncertain Flat Field correction 16 Pixel affected by "grot" on the detector 32 Defective (hot or cold) pixel 64 Saturated pixel 128 Missing data in telemetry 256 Bad pixel determined by calibration 512 Pixel contains Cosmic Ray 1024 Pixel contains source (see Section 3.4) 2048 Pixel has signal in 0th read 4096 CR detected by Multidrizzle 8192 User flag 16384 Curvature in detector response • Most flag values are dynamic, i.e., are set during observation/calibration. • Static flag values are DQ=16 (“grot” affected pixels) and DQ=32 (“bad” pixels). • Each NICMOS camera has its own static mask given in the header keyword MASKFILE. TIPS, Mar 19, 2009 - Tomas Dahlen

  3. New static DQ masks for NICMOS • The current “post-NCS” static masks were created 2002. • We now have a large set of calibration images obtained during NICMOS monitoring programs 2002-2008. • We have used this data to create new static masks for the three NICMOS cameras based on better statistics. • The temporal coverage also allows us to investigate any changes in the masks with time or detector temperature. TIPS, Mar 19, 2009 - Tomas Dahlen

  4. DQ=32 “Bad” pixels • We use long dark exposures (>1000s) to identify “Bad” pixels as pixels with a dark current deviating from what is expected. • Hot pixels are defined as pixels with excessive charge compared to surrounding pixels. • Cold pixels have extremely low(/zero) dark current (i.e., “dead” pixels). • To quantify the number of bad pixels we use the recipe in Sosey (2002, NICMOS ISR 2002-001 “ Updating the NICMOS Static Bad Pixel Masks”). A median dark image is made using available monitoring dark data after excluding CR affected pixels A smoothed image is made and subtracted from the median image The resulting image is rescaled in units of its rms Bad pixels are defined as pixels outside 5 Sosey (2002) TIPS, Mar 19, 2009 - Tomas Dahlen

  5. DQ=32 “Bad” pixels Since we have monitored the darks continuously from 2002 to 2008, we can look for changes with time/temperature. Number of bad pixels Old DQ=32: Number of DQ=32 pixels in existing mask. For each year the number of additional hot pixels is tabulated. All new: Total number of different pixels flagged as hot during at least one year. Black dots: number of DQ=32 pixels in existing mask. Red dots: number of DQ=32 after adding new bad pixels

  6. DQ=32 “Bad” pixels Plots show pixels that are not flagged in existing DQ mask, but pass the criteria for being bad in this investigation. Number of pixels that “suddenly” turn bad and can be “saved” by using multiple DQ masks is ~5 in each camera. (In addition a few bad pixels drop below the selection with time.) We find that having multiple DQ masks to “save” at the most a few pixels per camera is not “worthwhile” and only a single mask is created for each camera.

  7. DQ=16 “grot” pixels • The grot consists of flecks of anti-reflective paint on the detectors (that were scraped off the optical baffles between the dewars due to the expansion of the solid nitrogen in Cycle 7). • Grot leads to areas with reduced sensitivity. • Size of grot ranges from 25m to over 100m and since NICMOS pixels are 40m on a side, grot can affect regions of several pixels, as well as a fraction of a pixel. The largest example of a grot region is the "battleship" feature in NIC 1 which affects approximately 35 pixels. Flat-field image DQ mask

  8. DQ=16 “grot” pixels • The grot affects the incoming light onto the detectors. • Flats are ideal to estimate the effects of grot due to their high “uniform” counts over the whole detector. • To quantify the grot we use the recipe in NICMOS ISR 2003-003 (Schultz et al.). We use a well sampled non-inverted flat-field (F160W filter). A smoothed flat image is subtracted from this flat. Grot pixels are defined as a pixels deviating more than 4 in the subtracted image (excluding bad DQ=32 pixels).

  9. DQ=16 “grot” pixels Table: The number of grot pixels. The number found here is significantly lower than in the existing mask for nic2 and nic3. Q: what are the characteristics of the pixels not selected using the new selection? To investigate this we look at the relative DQE response of the flagged pixels - this is the pixel value of a ratio image created by dividing the flat image with a smoothed flat image. “Normal” pixel has response=1.

  10. DQ=16 “grot” pixels Relative DQE response The new selection suggest that grot pixels have response ~< 0.8. The pixels that are flagged in the existing mask and NOT flagged in the new selection have a fairly normal response in the range 0.8-1.2. Existing New Existing - New

  11. DQ=16 “grot” pixels Relative DQE response The new selection suggest that grot pixels have response ~< 0.8. The pixels that are flagged in the existing mask and NOT flagged in the new selection have a fairly normal response in the range 0.8-1.2. Furthermore, the rms of the previously flagged pixels look normal Black dots: rms of non-flagged normal pixels Green dots: rms of previously flagged pixels Rms look similar, green dots should be OK We should therefore be able to “de-flag” these Rms vs. response Previously flagged

  12. DQ=16 “grot” pixels Q: Does the number of grot pixels change with time? A: No. But there are somewhat more flagged pixels in NIC2-2008, but these have fairly normal response 0.7-0.9. We will not include a time dependence in the grot mask (i.e., not create multiple masks). FINAL SELECTION As a final selection, we include all “4 pixels” from above as grot pixels, except those that have a response >0.8 and a normal signal-to-noise. The new MASKFILEs for the post NCS era will be delivered by end of March -09.

  13. New static DQ masks for NICMOS Summary: DQ=16, grot pixels Existing: New: NIC 1 NIC 2 NIC 3

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