7.5 Blobs

Small blemishes called “blobs” appear in all WFC3/IR images ( Figure 7.4 and Figure 7.5). They are most noticeable in observations with high background or in observations of large, extended objects. Regions of lower sensitivity, by as much as 15% in a few cases but often much less than that, the blobs are caused by particulates on the surface of the mirror mounted upon the Channel Select Mechansm (CSM). A number of reports describe the blobs and their effects on IR data (WFC3 ISRs 2010-06, 2012-15, 2014-21, 2015-06, 2017-16).

IR blobs were first observed shortly after WFC3 was installed on HST. The blobs have been increasing in number monotonically in time; no blob, once it appears, has disappeared. Hence, they have been increasing in number monotonically with time. The rate of appearance was higher in the first year after launch (2009) than it was later. WFC3 ISR 2014-21 describes the time-dependent flag used by calwf3 to mark pixels associated with the blobs (see also the discussion later in this Section).

Once a specific blob appears, it remains unchanged in size, cross section, or position relative to other blobs. However, due to slight non-repeatability in positioning of the CSM mechanism, the pattern of blobs on the IR detector can shift slightly from one image to the next by up to ~1 pixel.

Because the CSM mirror is near to the telescope’s Cassegrain focus, and because the flat CSM mirror is slightly tilted with respect to the focal plane, the blobs’ radii systematically increase from the detector’s upper right corner (where they are nearly in focus, with radii less than ~4 pixels) to the lower left corner (where they have radii of ~13 pixels). Given their number and sizes, as of 2017, the blobs affect less than 2% of all pixels. An up-to-date list of blobs (i.e., new blobs since 2014) is maintained in WFC3 ISR 2014-21 Table 4.

Each blob has a unique absorption cross section. Pixels associated with the “worst” blobs are flagged using the DQ=512 bit and stored in the DQ extension of the pipeline calibrated FLT file. Appropriate dithering can permit cleaning of blobs from combined images, as described in WFC3 ISR 2010-09. An alternative method of correcting for blobs using a blob flat field is described in WFC3 ISR 2014-21; its effectiveness for stellar photometry is presented in WFC3 ISR 2015-06.

We note for completeness that because the CSM mirror is moved out of the beam in order for light to enter the UVIS channel, no blobs occur in UVIS images. However, UVIS images do contain “droplets” (cf. Section 5.4.1).

Figure 7.4: IR Flat and Blobs. Left: IR flatfield. The black spots are the blobs. Right: the IR Blob Mask in 2016.

Figure 7.5: Blobs on the WFC3 IR detector as of Jan 2018 ( WFC3 ISR 2014-21).

A dark earth flat image ratio with blobs known through January 2018 highlighted. The blobs are color-coded by strength (total counts in a circular aperture encircling each blob) from weak to strong: cyan, blue, green, red. Only the green and red blobs are flagged in the IR DQ array. WFC3 ISR 2014-21 illustrates the method for finding the original list of blobs. These were found using a mixture of automated and visual methods. Since then all the new blobs have been visually identified by the WFC3 team.