December 10, 2008
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 (ISR 2008-28). The flat field reference files were delivered to CRDS (Calibration Reference Data System) in December 2008 with filenames ‘s*pfl.fits’ populated in the image header ‘PFLTFILE’ keyword.
Dithered star cluster images acquired during Servicing Mission Observatory Verification (program 11453) revealed that the ground flats did not fully correct for low-frequency variations in the spatial sensitivity, with photometric residuals of +/- 1.5% in all filters due differences in the ground-based and inflight optical paths (ISR 2009-39).
December 7, 2010
Additional star cluster observations were obtained in Cycles 17 and 18 (programs 11928 and 12340), and these confirmed the observed sensitivity residuals. However, the small number of bright stars in the cluster images were not adequate for deriving high quality corrections over the whole detector.
Instead, corrections to the ground flats were derived by combining a large number of long exposure science images after masking out objects, creating the equivalent of a sky image (ISR 2011-11). Sky images were produced in multiple filters, but these 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. The star cluster observations were then used to confirm the sensitivity residuals from the sky flat, both in terms of the spatial signature and the amplitude of the correction, as well as the lack of any significant dependence on wavelength.
The updated flat reference files were delivered in December 2010 with filenames ‘u*pfl.fits’.
Spatial Stability Tests
January 4, 2013
Calibration program 12708 “WFC3/IR Spatial Sensitivity” was designed to test the spatial uniformity of observations after applying the improved IR flat fields. To quantify the photometric repeatability, the standard star WD-1057+719 was stepped across the detector using a 21-point grid, where at each grid point, two observations, slightly offset by∼40 pixels, were obtained, sampling a total of 42 points on the detector in F098M, F125W, and F160W.
This study reported the photometric uncertainty due to the flat field in the central 800x800 pixels of the detector as ~0.007 mag, with no apparent spatial signature (ISR 2013-01). Larger systematic errors of up to 0.015 mag were measured near the edges of the detector.
April 17, 2015
Blobs are defocused images of particulates on the channel select mechanism corresponding to regions of slightly lower than nominal sensitivity. New blobs appear occasionally throughout the mission as the CSM mirror accumulates particulates. "Blob flats" were generated by combining many IR images taken of the dark side of the Earth using the F153M filter. The seven images described here are unity everywhere except within the blobs cataloged as of mid-2013 (ISR 2014-21).
Caveat emptor! The correction of blobs by dividing by one of the blob_maskNN.fits file is experimental and may produce poor results, potentially even worse than making no correction, depending on the result desired. First, we do not recommend dividing by these "blob flats" if your scientific goal is stellar photometry. On the other hand, if your goal is either an aesthetically pleasing image, or a scientifically more-accurate surface brightness, then dividing by one of the "blob flats" may improve results. Note that there is some color dependency of the shape and depth of the blobs with different filters (ISR 2012-15). User feedback is welcome and may help us improve the methods, data products, or documentation.
Some users may wish to divide each calibrated IR "flt.fits" image by an appropriate blob_mask file, in order to correct for the absorption of blobs that creates dips in the measured surface brightness of sky or other approximately uniform, extended IR emission at the locations of the blobs. (The user MUST use the "flt.fits" file, not the "raw.fits" file, to be consistent with the method by which the blob_mask files were constructed. Also, the blob_mask files are 1014 pixels by 1014 pixels, consistent with "flt.fits" image sizes and inconsistent with "raw.fits" or "ima.fits" image sizes, which are 1024x1024.)
Nominally, the appropriate blob_mask file will be "blob_mask00.fits" because that one was created from images taken when the CSM was in its nominal position. Because of the slight play in the CSM mechanism itself, the most appropriate blob_mask file may be different than the nominal. The user can experiment by dividing by any of the other files with other values of NN.
The filenames take the form "blob_maskNN.fits.gz" where NN = -04, 00, 04, 08, 12, 16, or 20. The value of NN corresponds to the rotation of the channel select mechanism (CSM) about its nominal position (NN=00) in units of millidegrees. For example, for NN=12, the blob_mask file was generated from a set of images in which the CSM position was 0.012 degrees from nominal. The blobs shift in unison to the upper right (larger values of both X and Y in pixels on the IR detector) with larger values of NN. (A CSM rotation of 0.024 degrees corresponds to a shift of 1 pixel at the center of the IR detector.)
The filenames "blob_avgNN.fits.gz" in the second link contain the flat fields from which the "blob_maskNN.fits.gz" files were derived. The only difference is that for the latter the pixels outside the blobs were set to unity. Therefore, dividing by the "mask" has limited risk of doing harm, because it has absolutely no effect on pixels outside the blobs. Dividing by the "avg" will adjust all of the pixels, typically by a very small amount; whether doing so may improve upon the pipeline flat field has not yet been determined and is outside the scope of this analysis.
Download all blob masks (530 K tar file)
Download all blob masks and the average F153M flat fields from which they were derived (21 MB tar file)
An up-to-date list of all known IR blobs, including their date of appearance, position, and strength, is available via ISR 2018-06. Note that only the strongest blobs are flagged in the data quality arrays of calibrated data products with a value of 512.