CCD Performance

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 on page 2 or Figure 3.11 on page 62) 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 section 2.6, "Serial Clocks", on page 27 for details.

4.4.2 Horizontal Smearing

During readout of a badly overexposed image, there is spurious charge detected by the readout electronics. The apparent brightness of the stellar halo is higher to the right of the saturated columns. This is particularly obvious at the bottom of the image in Figure 4.3 which is a region in the shadow of the pyramid edge.

The horizontal "smearing" seen in highly saturated images can be modeled as an exponential function which decays over a few rows after a saturated pixel is encountered. The effect itself temporarily saturates after about ten saturated pixels (subsequent saturated pixels have no effect). The effect is twice as bad with gain 7 e- DN-1 than with gain 14 e- DN-1. This model only works on very highly saturated stellar images.

In Figure 4.3, the image to the right side of the saturated columns is brighter than the left side; and the brightness increases as the number of saturated columns increases. This effect appears to be a signal which starts at a saturated pixel and decays over the next few rows, wrapping around as it does so. The signal is additive with each successive saturated pixel. Jumps are obvious when the number of saturated columns changes. The problem is a known characteristic of the amplifier electronics, and effort was made to minimize it during design. The increase in signal in rows with saturated pixels is also seen in the over-scan region (the over-scans are provided in ".x0d" files from the pipeline)

An exponential function fits the effect reasonably well. An appropriate algorithm creates an array to contain the signal model. It searches through the uncalibrated image (with the over-scan region included) in the sequence in which the pixels are read out. When it encounters a saturated pixel, it adds an exponential function to the model array, beginning at that pixel. The function has the form s(x)=Ae-x/h, where x is the offset from the saturated pixel and only positive x values are included. The halfwidth, h, and amplitude, A, appear to vary from frame to frame and must be determined on the image itself. As more saturated columns are encountered in a row, the signal intensity builds up in the model image. The image can then be "improved" by subtracting the model from the raw image.

The amplitude and halfwidth parameters can be obtained by trial and error. The typical parameters vary slightly for each chip. The amplitude per saturated pixel is typically 1.75 DN (gain 7) or 0.2 DN (gain 14). On the other hand the halfwidth at a gain of 14 is larger (h=1800) than at 7 (h=350). So the total integrated effect is about twice as bad at gain 7. A straightforward application of the above algorithm cleaned up most of the signal in rows which had a few saturated columns, but over-subtracted in rows with a large number. The algorithm can be modified to saturate by making the parameter A, which gives the peak contribution from a single saturated pixel, depend on the current level of the effect: A=A(0)*(1-C/C(max)). This implies that the correction is never larger than C(max) no matter how many saturated pixels are encountered. C(max) is approximately 14 DN for a gain of 7 and 10 DN for a gain of 14.

The algorithm gives improvement only on highly saturated stellar images (where the star is saturated to 3 or 4 columns at the edges of the chip). On less saturated data, it over-subtracts significantly. This indicates that the problem is nonlinear, and therefore a general algorithm applicable to all data will be difficult to develop.

4.4.3 Diffraction Effects and Ghost Images

Several other artifacts that are common in saturated stellar images are also obvious in Figure 4.3. The spider diffraction spikes caused by both the OTA spiders and internal WFPC2 spiders are at 45 degrees to the CCD columns in all cameras.

The halo around the stellar image is well above the diffraction limit in intensity. Also there are ghost images which result from internal reflections in the filters and in the field-flatteners. These topics are discussed fully in the next Chapter.

Figure 4.3: Saturated Stellar Image Showing Horizontal Smearing.



4.4.1 - Blooming
4.4.2 - Horizontal Smearing
4.4.3 - Diffraction Effects and Ghost Images