4.4 Near Infrared Camera and Multi-Object
Spectrometer (NICMOS)
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).
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. The short wavelength response cutoff at 0.8 micron (see Section 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. The successful installation of the NICMOS Cooling System (NCS) during servicing mission SM3B in March 2002 has fully restored NICMOS functionality, albeit at a higher operating temperature (~77.1K, about 15K higher than with solid nitrogen). Following its on-orbit installation, a series of tests to determine the stability and repeatability of the NCS control law verified that - barring any unforeseen performance degradation - the NCS is capable of maintaining the NICMOS detectors to within 0.1 K of their target temperature under all orbital and seasonal conditions. Because the NICMOS detectors react sensitively to temperature variations, this is extremely positive news for the scientific performance of NICMOS. At their new operating temperature, NICMOS detector characteristics such as quantum efficiency and dark current are different compared to Cycle 7/7N. The performance changes have been measured during the SM3B Orbital Verification program, and are reflected in the current NICMOS Exposure Time Calculator. The bottom line is that for most science programs, the renewed NICMOS is slightly more sensitive than during its earlier life, because of higher DQE and the absence of any anomalously high dark current levels.
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 (the "bump") in the dark current of all three detectors was observed. The elevated dark current could have compromised NICMOS sensitivity, and hence the question of whether or not the bump would be present after the cooldown was an important one. The NICMOS calibration program following the cooldown has shown that the dark current levels of all three NICMOS cameras are nominal. Therefore, the "bump" option in the NICMOS ETC has been removed.
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. Beginning in Cycle 12, STScI automatically schedules 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. STScI is currently investigating the possibility of including this part of the processing in the calibration pipeline.
