4.4 Bright Object Artifacts

4.4.1 Blooming

Blooming up and down a CCD column occurs when more than about 90,000e^- (the full well capacity) are collected in any pixel. When the pixel is full, the charge will flow into the next pixels along the column, and so on. The orientation of the bloomed column(s) on the sky depends on the readout direction of the particular CCD (see Figure 1.1 or Figure 3.12) and the roll angle of the spacecraft. This effect is visible in Figure 4.3 which shows a logarithmic stretch of the image resulting from a 100s exposure on a star of V magnitude 2.6 through filter F502N in the PC.

Extreme overexposure of the Loral CCDs is not believed to cause any permanent effects, and therefore the WFPC2 does not have a bright object limit.

The WFPC2 CCDs can be operated in a non-standard mode during the integration phase of an exposure, in order to limit the blooming to only those columns containing the bright sources. This is accomplished by operating the serial transfer register clocks during the integration (using the optional parameter CLOCKS as specified in the Proposal Instructions). See 2.6 Serial Clocks for details.