Report of the Independent Science Review: NICMOS Cryocooler (March 4 - 5, 1999) 1. Background 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. 2. The ISR Meeting of March 4 - 5, 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: *Review the cooler "in the context of all aspects of the performance of the HST and particularly of NICMOS, as well as the implications for the servicing mission and future instruments." * Examine whether the proposed technical approach appears credible. *Determine whether the instrument will be able to perform the full range of astronomical observations originally expected of NICMOS; *Investigate the cost of the cryocooler system, and check how the work will be funded without significant consequences to other scientifically critical areas of the HST program; *Assess the implications of the cryocooler system for science operations and the performance of the spacecraft and its several instruments, and determine whether this impact is acceptable; and, finally, *Recommend whether it is in the best interests of HST science to proceed with the cryocooler implementation on SM3. 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. 3. Preparations for the March 4-5 Review 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. *On the technical side, the cryocooler system had been successfully flown and tested in space; and great care had been taken to characterize the thermal, mechanical, and electro-optical changes that NICMOS had undergone late in 1998 and during the rapid warm-up of January 1999. We now know a great deal more about the technical issues surrounding the feasibility of successfully prolonging the life of NICMOS through the installation of a cryocooler. We commend the GSFC and Creare teams for developing and successfully flight testing the cryocooler on such a short time scale. *On the scientific side, the Committee was impressed by the exciting astronomical results that NICMOS has obtained and could continue to obtain following installation of the cryocooler. 4. NICMOS and Ground-based Astronomy 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. 5. Scientific Capabilities of NICMOS 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. 6. Technical Factors The Committee is aware of a substantial number of technical factors that will need to be resolved before launch of the NICMOS cryocooler: *Focus: During final NICMOS warmup, the location of the focal plane was measured over the temperature range from 62 to 67 K. The camera has subsequently warmed up to 220 K. It is critically important that when NICMOS is cooled back down, NIC1 and NIC2 be capable of focussing through the use of the Pupil Adjust Mechanism (PAM), and that a near-optimal focus on NIC3 be achieved through PAM adjustment. An analysis of what may have happened during the warmup, and what is most likely to follow during a renewed cooldown, particularly as regards plastic deformation, will be important to assure that the cameras will be capable of focussing at their nominal operating temperature. *Gas-Conductive Heat Loads: We recommend that the results of the NICMOS warmup program, particularly the release of gas from the charcoal getter, be included in an updated thermal model to explore the range of possible cooldown and equilibrium temperatures for NICMOS detectors and filters. *Detector Dark Current: We support further laboratory measurements of detector dark currents in order to provide a more secure estimate of this source of noise at detector temperatures that the NICMOS Cryocooler System (NCS) is likely to provide. *Vibration, EMI, and EMC: While current assessments of vibration and EMI/EMC properties of the NCS are generally reassuring, we strongly support the project's plans to evaluate and test these properties in greater detail in order to assure that they will have no significant effect on HST instruments or the telescope itself. *Reliability of Cryocooler Hardware: Although the NCS had a highly successful test flight on Shuttle mission STS-95, demonstrating the capability of NCS to achieve the temperature levels required for an extended NICMOS mission, there is still concern about the long-term reliability of the system. The duration of the Shuttle flight was only ten days, whereas the NCS will have to operate reliably over a 3-year span between servicing missions SM-3 and SM-4. Unfortunately the NCS circulator failed shortly after its successful Shuttle flight, and a replacement subsequently also failed. While the causes of these failures may now be understood, and corrective action is being taken, we cannot be confident that the problem has been solved without an extended series of tests, whose total run-time should be considerably longer than the duration of the Shuttle test flight. Since the problems do not appear to be related to the microgravity or thermal environments of the space mission, these tests can be performed on the ground; but long-duration tests will be essential. *Water Contamination: Contamination of the circulating neon gas by water appears to be a recurring problem. An assessment of all possible sources of water contamination, and reliable means to deal with this problem, are essential prerequisites to a final decision to fly. *Do-No-Harm Policy: The Committee wholeheartedly endorses the Project's "First, do no harm" policy for installing the NCS. We strongly endorse the emphasis of the test plans, both prior to launch and during the servicing mission. It is important to assure through a series of EMI/EMC, contamination, thermal, jitter, and other measurements, that the NCS will not substantially degrade the performance of HST or its complement of other instruments. An essential philosophy that the Project has adopted dictates that the NCS shall be powered off, if that is needed to avoid degrading the performance of other instruments, and that it be removed from the telescope if necessary. The quantitative criteria for deciding whether to retain or remove the NCS must be detailed in advance by the HST Project, in consultation with the instrument Principal Investigators. Our primary concern is that the NCS pre-flight test plan not be compromised due to schedule pressure if the cryocooler development is further slowed down for any reason, or if correcting the circulator anomalies proves more difficult than anticipated. We also urge that NCS development and testing not compromise the pre-flight testing and characterization of the ACS. *Likelihood of Success: The Committee stresses that because the NCS is using novel technology, it should be regarded as an experiment undertaken with all of the usual flight hardware and software development procedures to ensure success but without the guarantee of a high probability of success. 7. Impact of NCS and NICMOS operations on STScI 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-upphase 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. 8. Funding 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. Recommendation 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.