NICMOS Instrument Handbook for Cycle 11

Changes Relative to Cycles 7 and 7N

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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. During Cycles 11 and later, the NCS will cool the NICMOS detectors to a target temperature in the range of ~75-86 K, some 15-25 K higher than during previous Cycles. The higher operating temperature causes a number of changes in the detector's performance which are discussed in more detail in Chapter 7. The NICMOS ETC (Chapter 9) has been updated to reflect the expected changes.

At the time of this writing, the exact value of the temperature at which NICMOS will operate after NCS installation is uncertain, and it will not be known until the NCS is installed and the on-orbit performance of NCS/NICMOS is characterized. That temperature depends critically on many factors: the NCS on-orbit performance, the amount of parasitic heat loads on the NICMOS dewar and on the NCS, the temperature attained by the heat-rejecting external radiators, and the thermal gradient within the NICMOS cryostat. Current estimates suggest the detector operating temperature will be somewhere in the range ~75-82 K, with a best estimate of 78.5 K. Should the best attainable temperature be near the peak of the dark current bump at 82 K, then operation at the higher temperature of 86 K would be considered. Given this possible range of operating temperatures, predictions of the instrument performance are somewhat uncertain.

 

• A dark current with 2 e^-/s is expected to represent the worst case scenario for all operating temperatures in the ~75-86 K range during Cycle 11 (the best case scenario being a dark current of ~0.4 e-/s, see Chapter 7 for details). In fact, after NCS installation, we will attempt to operate NICMOS at the minimum stable detector temperature for which the dark current is ~2 e^-/s or less. However, only the longest exposures at wavelengths shorter than ~1.9 microns will be slightly affected by the increased dark current (see Appendix 1). The ETC allows the user to choose between scenarios.

  Orbit allocations for Phase I proposals which are based on optimistic dark current values will NOT be adjusted in the case of higher dark current. We therefore recommend that proposers use the default settings of the NICMOS ETC (78 K + dark current bump) which reflect the worst case scenario for the dark current.

  
   
  
• The DQE is expected to increase ~40-45% at 1 µm, ~30-35% at 1.6 µm, and ~20-25% at 2 µm relative to the values at 63 K. Within the temperature range ~75-86 K, the DQE and the depth of the full well are expected to vary by ~10% or less. These changes are small and the uncertainty in the final operating temperature can be neglected in this case for the purpose of designing a Phase I proposal.
• The read-out noise is expected to remain ~30 e^- per read-pair.
• The filters, which are viewed by the entire wavelength response of the detectors, are expected to be cooled to about 160 K. This is close to their original design temperature. During Cycles 7 and 7N, the filters reached ~100 K due to the thermal short. At 160 K, the background of the filters will remain negligible.

From an operational point of view, a number of changes have been implemented for 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 will be tested and distributed after the start of Cycle 11 observations, when we will have gained experience with the necessary procedures using on-orbit data.
• A new readout scheme to avoid electronic bands will be 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 for Cycle 11 and later.