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WFC3 Data Handbook > Chapter 7: WFC3/IR Sources of Error > 7.4 Blobs

7.4
Small blemishes in the WFC3/IR images are visible on all WFC3/IR data and are most noticeable in observations with high background or observations of large, extended objects. We determined that these are regions of lower sensitivity and call them “IR Blobs”.While the exact cause for the blobs is uncertain and speculative, they have been monitored and studied extensively since WFC3’s installation on HST.
Reduced reflectivity of the Channel Select Mechansm (CSM) mirror causes the reduced sensitivity in these regions. Blobs are not an artifact of the IR detector itself, though they appear as if they reside on the detector. The left panel of Figure 7.5 shows these blobs as they appear on a uniform background.
IR blobs were first observed shortly after WFC3 was installed on HST and the number of IR blobs initially increased steadily. We found little correlation between the number of CSM moves (which is used to reflect incoming light away from the UVIS channel and into the IR channel) and the appearance of these artifacts. The number of blobs has stabilized and remained roughly constant since December 2009.
The size and absorption properties of the blobs vary. Blobs are circular and fuzzy, with a measured half-light radius of 10-15 pixels, and absorb as much as 10-15% of the incoming light at their centers. It should be noted that as of this writing, the physical properties (position, size, etc.) of a blob, once it appears, do not vary. Blobs do not move in the field of view, nor grow in size, nor eventually disappear. Approximately 1.2% of pixels are affected by blobs. Pixels affected by blobs are flagged using the DQ=512 bit and stored in the DQ extension of the pipeline calibrated FLT file. Figure 7.6 shows the current IR blob map. WFC3 ISR 2010-06 describes blobs in detail. Appropriate dithering strategies can mitigate the effect of these artifacts in combined images, as described in WFC3 ISR 2010-09.
As an alternative method of correcting for blobs, a “blob flat field” is available at:
http://www.stsci.edu/hst/wfc3/analysis/ir_flats.
WFC3 ISR 2014-21 describes its construction, its effectiveness for stellar photometry is presented in WFC3 ISR 2015-06, and WFC3 ISR 2014-21 describes blob evolution and time dependent flags for pixels associated with the blobs.
Figure 7.5: IR Flat and Blobs. Left: Blobs seen in an IR flat field. The black spots are blobs. Right: the IR Blob Mask in 2013.
Figure 7.6: Current blobs on the WFC3 IR detector.
Strong (red), medium (green) and weak (blue) blobs are indicated by circles. Only the strong and medium blobs (red and green circles) are flagged in the published BPIXTABs. Blob radii increase from the upper right corner to the lower left corner. Four weak, large blobs (cyan) were added by hand after visual inspection; as the automatic algorithm did not flag them. A few dozen blobs identified by the algorithm were rejected by visual inspection and are not shown; most of them were associated with defects near the detector’s corners, the “death star” circle in the lower left quadrant, and other unrelated features of the flat field. (see WFC3-ISR 2014-21 for further details)

WFC3 Data Handbook > Chapter 7: WFC3/IR Sources of Error > 7.4 Blobs

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