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  • July 09, 2021:  WFC3 ISR 2021-10 describing new Blob Flats for six filters
  • Jan  11,  2021:  WFC3 ISR 2021-01 describing new Filter-Dependent Sky Flats
  • Dec 03, 2020: WFC3 ISR 2020-10 describing the new IR photometric calibration, which uses updated flat fields and CALSPEC models
  • Oct 15, 2020:  New IR calibration reference files delivered to CRDS for reprocessing MAST data products
    • PFLTFILE:   P-flats derived via sky flats
    • DFLTFILE:   D-flats correct for new blobs appearing over time
    • IMPHTTAB: 'Image Photometry Table' with updated IR zeropoints (inverse sensitivites)

 

A grayscale image of a starfield. Several dark blobs are visible throughout the image.
Left: MAST drizzled dataset ‘ibkdf2010_drz.fits’ with a combined background of 10,400 electrons calibrated with the 2010 P-flats.
Grayscale image of a starfield. The dark blobs in the picture on the left are not there anymore.
Right: The same dataset as on the left calibrated with the 2020 P-flat + D-flat, which correct for filter-dependent residuals and IR blobs

Current Flat Calibration

P-flats

Filter-dependent sky flats were computed from in-flight science observations acquired over ~10 years. The new P-flat reference files ('4*pfl.fits') correct for 'pixel-to-pixel' sensitivity residuals of ±0.5% in the central 800x800 pixel region of the detector and up to 2% near the detector edges (WFC3 ISR 2021-01).

For the six most used filters (F098M, F105W, F110W, F125W, F140W, F160W), the combined sky image is now deep enough to fully replace the ground flat reference files, with errors less than or equal to 0.2%. For the remaining nine IR imaging filters, wavelength-dependent sensitivity residuals from the six most used filters were used to correct the P-flats acquired on the ground. Unlike the 2010 reference files, the updated P-flats no longer include the blobs, which are now corrected by a separate set of D-flat reference files.

D-flats

Delta-flats correct for IR ‘blobs’ in the six most-used filters (see WFC3 ISR 2021-10). Blobs are regions of slightly lower sensitivity caused by de-focused images of particulates on the channel select mechanism which accumulate over time. The new D-flat reference files ('4*dfl.fits') were computed by stacking the same archival data used for sky flats, but only after the appearance of each blob. Conversely, P-flats in blob-affected regions were computed by stacking images prior to each blob's appearance. The new D-flats should be used with the new P-flats described above. During pipeline processing, calwf3 multiplies the P-flat and the D-flat reference files together to form a combined flat field image. 

Prior to October 2020, only the strongest blobs were assigned a DQ flag (value=512, 'bad in flat') via the IR bad pixel tables. Now, the delta-flats provide DQ flags for all cataloged blobs, regardless of their strength. This increases the total number of flagged flat field pixels from 1.1% to 2.3% in the DQ array of calibrated WFC3/IR (*flt.fits) images.

Prior Flat Calibration

  • Dec 10, 2008: Delivery of P-flat reference files ('s*pfl.fits') from ground testing

During the spring of 2008, flat field images for the IR channel were produced in the laboratory by simulating the sky illumination using the CASTLE optical stimulus providing a mean signal-to-noise ratio of ~500, yielding corrections down to a level of 0.2%/pixel (WFC3 ISR 2008-28).

  • Dec 7, 2010: Delivery of P-flat reference files  ('u*pfl.fits') which provide an inflight flat field correction

Corrections to the ground flats were derived by combining a large number of on-orbit images after masking out objects, creating the equivalent of a sky image. Sky images were produced in multiple filters but showed no significant deviation between filters, with variations less than 1%. Therefore a single smoothed ‘gray’ correction of ~4% peak-to-peak was applied to the ground flat for each filter. (WFC3 ISR 2011-11).

Blob DQ Flagging

  • Mar 19, 2013:  Time-dependent IR bad pixel tables provide DQ flags for the 46 strongest blobs of 127 total blobs catalogued (WFC3 ISR 2014-21).
     
  • Apr  17, 2015:  Blob D-Flats from dark Earth flat calibration images provided for experimental use. Learn more. 
     
  • Oct 15, 2020: DQ flags are now assigned for all blobs via the D-flats in six filters (WFC3 ISR 2021-10).  Previously only 'strong' blobs were flagged via the BPIXTABs.
     
  • Aug 03, 2021:  'Master Blob Catalog' last updated to include 152 known blobs through June 2021  (WFC3 ISR 2018-06).
Last Updated: 01/31/2024

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