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.