1.8 Changes Relative to Cycles 7 and 7N
During Cycles 7 and 7N, the temperature of the three NICMOS detectors was kept within the range 59-62 K by the solid N2 coolant. Now the NCS maintains NICMOS detectors to a temperature of ~ 77.1 K with an rms variation of 0.07 K. The higher operating temperature causes a number of changes in the detector's performance:
- For a temperature of 77.1 K, the linear dark current is about
0.1 - 0.2 e- /second/pixel.
- The average DQE has increased by ~20%-60% relative to the values at 63 K. The increase depends on wavelength and is largest for short wavelengths. The full well has decreased by ~ 3%, 7% and 9% for NIC1, NIC2 and NIC3 respectively. (see Figure 7.7 for details)
- The read-out noise is ~26 e-
- The filters, which thermally emit over the entire wavelength response of the detectors, are cooled to about 160 K. This is close to their original design temperature. At 160 K, the background of the filters remains negligible.
Detector performance is discussed in more detail in Chapter 7. The NICMOS Exposure Time Calculator ( Chapter 9) has been updated to reflect the observed changes.
From an operational point of view, a number of changes have been implemented in Cycle 11 which will considerably improve NICMOS efficiency, data quality and scientific use:
- A new syntax for dither patterns has been developed which now allows multiple exposures (e.g. in different filters) to be taken at each dither position (for details, refer to Chapter 11).
- In order to reduce the degrading impact of cosmic ray persistence (see Chapter 4 for details) after passage through the South-Atlantic Anomaly (SAA), a pair of ACCUM dark exposures will be obtained immediately after each HST orbit through the SAA. The scheduling of these dark exposures is automatic and transparent to the user. The darks yield a map of the persistence pattern, and can be used to subtract a significant amount of the unwanted persistence signal. Software for implementing this correction is being tested and will be distributed when we will have gained experience with the necessary procedures using on-orbit data.
- A new readout scheme to avoid electronic bands is implemented which reduces the likelihood that a detector reset occurs while another detector is being read out (see Chapter 7).
- As mentioned above, NIC3 is scientifically usable at the best achievable PAM focus (see also Chapter 4). Therefore, no dedicated NIC3 campaigns (involving moves of the HST secondary mirror) are planned.