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).
WFC3 Data Handbook
- • Acknowledgments
- Chapter 1: WFC3 Instruments
- Chapter 2: WFC3 Data Structure
- Chapter 3: WFC3 Data Calibration
- Chapter 4: WFC3 Images: Distortion Correction and AstroDrizzle
- Chapter 5: WFC3-UVIS Sources of Error
- Chapter 6: WFC3 UVIS Charge Transfer Efficiency - CTE
Chapter 7: WFC3 IR Sources of Error
- • 7.1 WFC3 IR Error Source Overview
- • 7.2 Gain
- • 7.3 WFC3 IR Bias Correction
- • 7.4 WFC3 Dark Current and Banding
- • 7.5 Blobs
- • 7.6 Detector Nonlinearity Issues
- • 7.7 Count Rate Non-Linearity
- • 7.8 IR Flat Fields
- • 7.9 Pixel Defects and Bad Imaging Regions
- • 7.10 Time Variable Background Contamination
- • 7.11 IR Photometry Errors
- • 7.12 References
- Chapter 8: Persistence in WFC3 IR
- Chapter 9: WFC3 Data Analysis
- Chapter 10: WFC3 Spatial Scan Data