Hubble Space Telescope Primer for Cycle 11

4.4 Near Infrared Camera and Multi-Object
Spectrometer (NICMOS)

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NICMOS provides HST's only infrared capability. The three 256 x 256 pixel cameras of NICMOS are designed to provide, respectively:

• diffraction limited sampling to 1.0 micron (Camera 1);
• diffraction limited sampling to 1.75 micron (Camera 2);
• a relatively large field of view of 51 x 51 arcsec (Camera 3).

The short wavelength response cutoff at 0.8 micron (see Table 4.1) is a limitation of the HgCdTe detector material, while the long cutoff at 2.5 micron was selected as the longest scientifically useful wavelength given HST's warm optics. The original coolant of the NICMOS dewar, solid nitrogen, was exhausted in January 1999. For Cycle 11, the installation of the NICMOS Cooling System (NCS) during servicing mission SM3B is expected to restore NICMOS functionality, albeit at a higher operating temperature (~75-86 K, with a best estimate of 78.5 K, a full 15-25 K higher than during Cycles 7 and 7N operations). Consequently, NICMOS detector characteristics such as quantum efficiency and dark current will be different from previous cycles.

It should be noted that the exact value of the temperature at which NICMOS will operate after NCS installation is uncertain. That temperature depends critically on many factors: the actual NCS on-orbit performance, the amount of the 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. Within the expected operating range, however, detectors' characteristics such as DQE and depth of the full well are expected to vary by ~10% or less. The dark current value is somewhat more uncertain (see Section 4.4.2 and the NICMOS Instrument Handbook). After the 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.

Each NICMOS camera provides 19 independent optical elements, offering a wide range of filter options. Cameras 1 and 2 have polarimetric filters; Camera 2 has a 0.3 arcsec radius coronographic hole and an optimized cold mask to support coronographic observations; and Camera 3 has three separate grisms providing slitless spectroscopy over the full NICMOS wavelength range.

4.4.1 Camera Focusing

The NICMOS cameras were designed to be operated independently and simultaneously. However, due to an anomaly in the NICMOS dewar, the three cameras are no longer confocal. While Cameras 1 and 2 are close to being confocal, the use of Camera 3 requires repositioning of the Pupil Alignment Mechanism (PAM). The PAM will be automatically moved to the optimal position whenever Camera 3 is the prime instrument (causing Cameras 1 and 2 to be out of focus).

4.4.2 Dark Levels

During the warmup of the NICMOS instrument following cryogen exhaustion, an anomalous increase in the dark current of all three detectors was observed. At this time, it is unclear whether NICMOS will exhibit dark current levels elevated above the expected increase due to the higher detector temperature.

The NICMOS Exposure Time Calculator (ETC) allows the user to choose between two dark current levels, which reflect the best and worst case scenarios. We recommend that Phase I proposers use the worst case scenario with the elevated dark current to calculate their orbit request. This is the default setting of the NICMOS ETC.

We emphasize that orbit allocations derived from optimistic dark current estimates will not be adjusted if the dark current is indeed elevated above the assumed levels.

Therefore, we strongly recommend that you use the default settings of the ETC which reflect the worst case scenario for the dark current. (Note: for most proposals the difference between the two scenarios is negligible; only the longest exposures at wavelengths shortward of 1.7 microns are dark current limited.) The uncertainty in the dark current for the NCS/NICMOS performance is still rather uncertain, and it is best to be conservative in the number of orbits requested.

4.4.3 South Atlantic Anomaly (SAA) Cosmic Ray Persistence

NICMOS data obtained within ~40 minutes of passage through the SAA (see Section 2.3.2) exhibited a persistent signal that significantly degraded the quality of the data. This signal, caused by persistence of the cosmic ray hits, was similar to a slowly decaying, highly structured dark current and could not be removed by the standard calibration pipeline processing.

Because HST passes through the SAA several times a day, a large fraction of NICMOS images are affected by cosmic ray persistence. STScI plans to automatically schedule a pair of NICMOS ACCUM mode dark exposures after each SAA passage. This data will provide a map of the persistent cosmic ray afterglow when it is strongest. Analysis has shown that it is possible to scale and subtract such "post-SAA darks" from subsequent science exposures taken later in the same orbit, which significantly improves the quality of the science data.