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 until 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 was revived in March 2002 when the NICMOS Cooling System (NCS) was installed. NCS was activated and NICMOS was cooled down to the current operating temperature of 77.15 K.
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. It 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.
The current NICMOS operating configuration is nearly the same as
Cycle 7 and 7N: NIC1/NIC2 close to being parfocal and in focus, NIC3 is non-parfocal with the other two cameras with the optimal focus slightly out of the PAM range. NIC3 is perfectly usable with the best achievable focus. See Chapter 4
for NIC3 operations in Cycle 11 and beyond.