NICMOS Instrument Handbook for Cycle 11

NICMOS History in Brief

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In order to understand the list of recommendations for proposal preparation given below, a brief history of the Instrument is presented here. A more detailed description of the NICMOS chronology, from installation on HST until its present status, is given in Chapter 2.

During its first operational period, which went from February 1997 (date of installation on HST) to January 1999, NICMOS was passively cooled by sublimating N2 ice. Science observations were obtained from the beginning of June 1997 till mid-November 1998, during which period the cryogen kept the detectors' temperature around 60 K, with a slow upward trend, from 59.5 K to ~62 K, as the N2 was sublimating. On January 3, 1999, the cryogen was completely exhausted, marking the official end of NICMOS operations under this cooling regime.

NICMOS offers infrared capabilities in three cameras, NIC1, NIC2, and NIC3 characterized by three magnification factors (see Chapter 2). The three cameras had been built to be parfocal and to operate simultaneously. A few months before launch, however, the NICMOS dewar underwent thermal stresses, which made the three cameras no longer parfocal (although they still retain the capability to operate simultaneously). Even worse, shortly after installation on HST, the NICMOS dewar developed a deformation which had two consequences: 1. it pushed the NIC3 focus outside the range of the Pupil Alignment Mechanism (PAM); 2. created a "heat sink", which caused the Nitrogen ice to sublimate at a quicker pace, thus shortening the lifetime of the instrument (from the expected 4.5 years down to about 2 years). A couple of months after the start of the short, the instrument stabilized at the operating configuration which remained during the duration of its `cryogenic lifetime' with NIC1 and NIC2 in focus and practically parfocal, NIC3 out of focus relative to the other two cameras and with its best focus slightly outside the PAM range. During Cycle 7 and Cycle 7N, two observing campaigns were organized to obtain in-focus NIC3 observations by moving the HST secondary mirror. See the next section and Chapter 4 for expected NIC3 operations in Cycle 11.

The dewar deformation is most likely plastic. At the time of this writing we believe that when the NCS will be installed the NICMOS operating configuration will be the same as Cycle 7 and 7N: NIC1/NIC2 close to being parfocal and in focus, NIC3 non-parfocal with the other two cameras with the optimal focus slightly out of the PAM range but still perfectly usable with the best achievable focus.