The Independent Science Review (ISR) is a means for providing NASA and the Space Telescope Science Institute independent scientific, technical, or managerial advice on high-level issues of importance to the Hubble Space Telescope Project. The present review was the fourth in a series that has addressed an anomaly which initially developed in the Near Infrared Camera and Multi Object Spectrometer (NICMOS) before launch. As originally viewed, it involved a mechanical deformation of the NICMOS dewar, which resulted in three different foci for the three different cameras. The first ISR met in July 1996. It was chaired by Malcolm Longair; other committee members were Robert Bless, Arthur Davidsen, Holland Ford, Robert Fosbury, John Trauger, Edwin Turner, and Michael Werner. The team concluded that the problem could be overcome by internal refocussing of the instrument for observing with the different cameras. In view of this assessment, NICMOS was installed on the Hubble Space Telescope (HST) during the second servicing mission (SM2), in February 1997. Shortly after installation a thermal short appeared between the cold baffles and the surrounding Vapor Cooled Shield (VCS). This significantly lowered the expected lifetime of the cryogen and thus the anticipated period of scientific operations. The ISR team was therefore reconvened in May 1997, and advised a "fast track" call for proposals and an increased allocation of HST time to NICMOS science to derive the greatest potential benefit from the instrument in its shortened life span. The panel also recommended a further high-level review to deal with whether a cryocooler should be installed on NICMOS during a future servicing mission.
In September 1997 a new ISR was convened, chaired by Martin Harwit; the other committee members were George R. Carruthers, Judith G. Cohen, Robert A. E. Fosbury, Fred C. Gillett, Richard J. Harms, Jeffrey L. Linsky, Stefan Price, and Richard J. Wainscoat. The committee was charged with recommending whether or not a Reverse Brayton-Cycle Cryocooler, a new high technology mechanical cooler, should be installed on NICMOS on a future servicing mission. Two unknowns, however, needed to be addressed. The first was the question of whether the Reverse Brayton-Cycle Cryocooler would reliably work in space. The second dealt with the question of whether ground-based adaptive-optics would soon match capabilities offered by NICMOS, making a prolonged life for this instrument unnecessary.
The ISR recommended that the cryocooler be flight tested. The committee judged testing a cryocooler in space to be important in its own right, since future infrared astronomical missions will require active cooling. However, the committee reserved for a later date a recommendation on whether the cooler should be installed on HST. Such a recommendation, the committee felt, should await the results of flight testing. The additional passage of months would also more clearly indicate the quality of the scientific observations obtained with NICMOS and the rate at which ground-based techniques were becoming competitive.
In November, 1998, the cryocooler was tested in a space shuttle flight as part of the Hubble Orbital Systems Test (HOST) and operated successfully over the 10-day mission. In December, 1998, science observations with NICMOS were terminated to allow close monitoring of the instrument’s performance through depletion of the cryogen and warm-up, which occurred in early January, 1999.
The ISR reconvened on March 4 - 5, 1999. The panel consisted of the same team that had met in September 1997, but with Claire E. Max replacing Stefan Price, who was unable to attend.
The charge to the committee remained unchanged from its earlier session:
NASA specifically directed the Committee to consider these issues independent of the possible development of a wide-field infrared camera in WFC3 for installation on Servicing Mission 4.
In preparation for the meeting, several members of the ISR attended a review dedicated to issues raised by the upcoming third servicing mission (SM3). The "Cooling System Hardware and Systems Capability Review" was held at the Goddard Space Flight Center (GSFC) on February 9 and 10, 1999. Richard Wainscoat and Martin Harwit participated in the entire review; Richard Harms and Jeffrey Linsky attended selected parts.
The Committee wishes to thank the staffs of both the STScI and the HST Project at the GSFC for their thorough preparations, which greatly facilitated our tasks. The reports mailed to the Committee in advance were sharply focussed on issues of especial significance to the Committee’s deliberations, and showed both care in selecting relevant material and frankness in discussing potential difficulties and trouble spots. The verbal presentations were crisp, informative and to the point. The thrust of the materials indicated great progress since the last ISR, in September 1997. All of the concerns voiced by the panel at that time had in the interim been satisfactorily addressed.
The scientific capabilities of NICMOS retain certain important advantages over adaptive optics systems on ground-based telescopes. For the period leading up to Servicing Mission SM4, NICMOS will continue to have dramatically higher sensitivity in the H and J bands. It is a mature, well-characterized instrument, whose point-spread-function, backgrounds, and peculiarities are stable and thoroughly understood. This will allow NICMOS to provide more accurate photometry than currently possible with adaptive optics on the ground. Finally, NICMOS has close to 100% sky coverage. This may be compared to the current generation of natural-guide-star adaptive optics systems, which are able to access less than 10% of potential positions in the sky while maintaining high spatial resolution.
The scientific capabilities of NICMOS are complementary to the adaptive optics systems on ground-based 8 - 10 meter telescopes, which have higher-resolution spectrographs and are capable of higher spatial resolution. Ground-based 8-10 meter telescopes with wide field cameras, using 1024 x 1024 and, soon, 2048 x 2048 arrays, and operating with good intrinsic image quality will be competitive with NICMOS Camera 3 for imaging surveys of faint galaxies in the J and H bands and superior in the K-band. In some applications, the increased spatial coverage and larger ground-based collecting area partially compensate for the higher background at J and H. The increasing array sizes and mitigation of seeing effects by better control of telescope enclosures and the use of tip-tilt secondary mirrors is likely to further increase the ground-based advantage during the period before SM4.
The next five years will be a crucial window for NICMOS on HST. The development of ground-based adaptive-optics systems on 8 - 10 meter telescopes is still in its infancy. Their initial performance with natural guide stars demonstrates great promise. They have achieved diffraction limited imaging and reached an infrared spatial resolution superior to NICMOS. However, it will be at least five years, and after SM4, before such performance can be routinely achieved on the majority of 8-10 meter telescopes over most of the sky through the use of laser guide-star systems.
In the era between SM3 and SM4, roughly the years 2001 and 2004, NICMOS will be the scientific instrument uniquely able to address, at near infra-red wavelengths, a number of major astronomical questions dealing with the solar system, evolution of stars and galaxies, and cosmology.
The NICMOS coronagraph enables close-in study of faint objects near bright point sources. The instrument will be able to image brown dwarfs and proto-planets, as well as dust-disks, close to their parent stars. This capability will also permit the study of heavily obscured nuclei of active galaxies and circumnuclear material around pre-main sequence stars.
NICMOS can observe at wavelengths that do not penetrate the Earth’s atmosphere. This enables unique observations of the giant planets in our own solar system, via imaging in important molecular transitions to which our own atmosphere is opaque. It also allows study of high-excitation regions in active galactic nuclei, as in recent observations with SiVI lines at 1.96 microns in NGC 1068.
NICMOS has extremely low sky backgrounds in the J and H bands. This facilitates studies of surface-brightness fluctuations in distant galaxies, which provides a competitive method for determining distances to galaxies beyond the Virgo Cluster. It also permits slitless spectroscopy of extragalactic sources and provides the basis for reaching very faint limiting magnitudes for imaging galaxies in the distant universe.
At present it is extremely difficult to determine redshifts of galaxies between z = 1.3 and z = 2 from the ground, because the strong spectral features normally used in the optical are shifted into the near infra-red. The slitless spectroscopy mode of NICMOS can pick out emission lines in precisely this redshift range. This mode also has the important ability to efficiently identify star-forming galaxies over a large cosmological volume.
NICMOS can perform stable and accurate photometry of faint sources in relatively short integration times. In our own galaxy, NICMOS will permit study of micro-fluctuations in stellar intensity among the stars in the Galactic Center, to learn the characteristics of the central black hole.
The Committee is aware of a substantial number of technical factors that will need to be resolved before launch of the NICMOS cryocooler:
The STScI has assembled an impressive expertise in the characterization and operation of the NICMOS instrument. It is clear that the instrument scientists who have worked on NICMOS operations and the monitoring of the warm-up phase will be in demand for other functional tasks. The Committee was concerned that the need to maintain this expertise during the next two years before SM3-B might place a strain on the resources needed for other tasks. The director of STScI assured the Committee, however, that the opportunity to retain and build on this experienced staff would be an advantage in preparing for NGST and, possibly, WFC3-IR.
Until after SM4, when power and thermal management of the spacecraft will become an issue, the operation of NICMOS/NCS appears unlikely to make demands on science operations that are significantly greater than those of other instruments. However, the Committee wishes to make certain that installation and operation of the NCS will not unduly interfere with the full characterization of ACS and the continuing operation of the other instruments.
The Committee briefly discussed funding issues with the Project, and received reassurances but did not go into sufficient depths to reach an independent assessment. A considerably more detailed review of budgetary matters would have been required for which there was insufficient time.
We recommend that the Reverse Brayton-Cycle Cryocooler of the NICMOS Cryocooler System (NCS) be installed on NICMOS on the SM3 servicing mission provided that a number of important concerns listed in sections 6 and 7 are resolved to the satisfaction of the usual technical and flight-readiness reviews prior to flight. If the installation is successful and the astronomical findings continue to be of unsurpassed quality, a future review should determine whether routine operations should continue beyond SM4.
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