Space Telescope Science Institute   4.3.2 CCD Shutter Effects on Exposure Times  4.3.4 Dark Current

4.3.3 Readnoise


WFC

We measured the read noise level in the active area and overscan regions for all the amplifiers at the default gain settings. In general the read noise has been constant with time. On June 29, 2003, just after a transit through the South Atlantic Anomaly (SAA) the readnoise in Amp A changed from ~4.9 to ~5.9 electrons rms. Although the telemetry did not show any anomaly in any component of the camera, it is likely that the read noise jump was due to some sort of radiation damage. Amp A is the only amplifier that showed this anomaly. The amplitude of the variation (~1 e-) was the same for GAIN 1 and 2. After the following anneal date the readnoise dropped to ~5.5 e- and it remained constant for 27 days. After the following two anneal cycles the read noise reached stability at ~5.6 e- rms, approximately 0.7 e- higher than before the change, and has remained constant since then. Figure 4.4 shows the read noise in the image area for amplifier A during the "instability" period. The readnoise of all the other amplifiers have been very stable since launch with post-launch figures almost unchanged from the pre-launch measurements made during ground testing.

Even with a slightly higher read noise in Amp A most of the WFC broadband science observations are sky limited, while narrowband observations are primarily read noise limited.

Figure 4.4: Readnoise jump in WFC Amp A (occurred on June 29, 2003).

 
The vertical dashed lines indicate the annealing dates. See Table 4.2 for CCD gain and readout noise values.
 

HRC

The read noise is monitored only for the default readout amplifier C at the default gain setting (2 e-/DN). No variations have been observed with time. The read noise measured in the image area (4.80 ± 0.12 e-) is in agreement with the read noise measured in the two overscan regions, and it is comparable to the pre-flight value of 4.74 e-.


 4.3.2 CCD Shutter Effects on Exposure Times  4.3.4 Dark Current
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