Small blemishes are present in the WFC3-IR images. These have been visible on
all WFC3-IR data and are most noticeable in observations with high background or observations of large, extended objects. We have determined that these are regions of effectively lowered sensitivity and we refer to them as “IR Blobs”.
The reduced sensitivity in these regions is caused by reduced reflectivity of the
Channel Select Mechanism mirror and are not an artifact of the IR detector itself. They however appear as if they reside on the detector for all practical purposes. The top panel of Figure 6.5
shows these blobs as they appear on top of a uniform background. While the exact cause for the existence of these blobs remains uncertain and speculative, they have been monitored and studied extensively since the installation of WFC3 on HST
The IR blobs were first observed shortly after WFC3 was installed on HST
and the total number of IR blobs was initially seen to increase steadily. We however 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. But while their number initially increased, it has now stabilized and remained constant since December 2009. The current number of known blobs is 19.
The size and absorption properties of the blobs vary, and have a somewhat
circular and fuzzy shape, 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 appeared, have remained fixed and do not vary. Blobs are not seen to move in the field of view, nor to grow in size, nor to eventually disappear. While extended, the total number of pixels affected by blobs is ~1.2%. Information about the pixels affected by these blobs is contained in pipeline calibrated FLT
files as the corresponding pixels are flagged using the DQ=512 bit in the DQ extension of the calibrated file. The bottom panel of Figure 6.5
shows the current IR blob map in the DQ array. Blobs are described in details in WFC3 ISR 2010-06
. Appropriate dithering strategies can be used to mitigate the effect of these artifact in combined images, as described in WFC3 ISR 2010-09