Marcia Rieke | Steward Observatory, University of Arizona, USA

Abstract: JWST’s early history is described with a focus on how the mission and its capabilities were shaped.

Introduction

The James Webb Space Telescope (JWST) grew out of a confluence of technology, other NASA missions, and astronomical discoveries that made a large, infrared telescope an imperative. Three strands – astronomical discoveries, development of a mission architecture including optics for a large telescope, and infrared detector development – wove together to make JWST a required mission for the astronomical community and to make JWST feasible. Gardner et al. 2023 also present an overview of JWST’s history while here we concentrate on the early history which shaped the mission and its capabilities. Wright et al. 2023 give more background on the genesis of MIRI. Doyon et al. 2023 provide more detail on the history of NIRISS. Jakobsen et al. 2022 present some of the background leading to the final design of NIRSpec.

The Astronomy Strand

On the astronomical discovery side of the story of JWST’s development, much progress was made in discovering galaxies at redshifts above 0.5 in the 1980s (e.g. Spinrad et al. 1981, Gunn et al. 1986, Djorgovski et al. 1988, Koo and Kron 1992) although it was not known just how far in distance galaxies would be found. There was also a question about the angular extent of early galaxies with some predictions positing galaxies of order 10 arc seconds in size (Partridge and Peebles 1967). The first infrared observations of z~0.5 galaxies were executed using aperture photometers equipped with a single detector (Lebofsky 1981, Lilly and Longair 1982). These first infrared efforts demonstrated the feasibility and utility of observing redshifted galaxies in the infrared. The launch of HST in 1990 and the subsequent repair of the HST optics with the installation of COSTAR in 1993 provided the next opportunity to make a jump in the ability to find and characterize distant galaxies (e.g. Windhorst et al. 1994). The success of the post-repair observations led Bob Williams, then director of the Space Telescope Science Institute, to use some of his director’s discretionary observing time to execute a deep imaging program of an “empty” field to see what could be found (Williams et al. 1996). This field was dubbed the Hubble Deep Field (HDF), and the imagery spawned many follow-up observations. Of particular significance was the development of the Lyman break technique by Steidel’s group (Steidel et al. 1996) which afforded a method of selecting very high redshift galaxies from imaging alone. Figure 1 illustrates this absorption. Also important for the development of JWST was the realization from the HDF that galaxies exist at very great distances and appear to be compact in size, which enhances their detectability. Another element of the astronomy strand was the discovery of the relationship between nuclear black hole mass and the host galaxy’s bulge mass (e.g., Ferrarese and Merritt 2000). This relationship, expressed as M_• versus s (bulge velocity dispersion), strongly suggests that the evolution of nuclear black holes and their host galaxies are connected.

Combining the effect of the expansion of the Universe and the absorption of light below the Lyman limit led to the realization that HST images could not detect the most distant galaxies, nor would such images provide sufficiently large galaxy samples to study evolution much beyond a redshift of 7. A problem with extending HST’s wavelength range is the simple fact that HST’s primary mirror is heated to match the temperature of the lab where it was polished so the thermal background from the mirror would overwhelm the signal from faint galaxies at wavelengths beyond ~1.6 microns. The only way to make progress would be to observe deeper into the infrared. Infrared data would allow galaxy evolution and galaxy-black hole relations to be investigated. And once the decision is made to observe in the infrared, a cooled telescope with cold instruments becomes a necessity, and of course, this telescope would need to be large to provide both spatial resolution at these longer wavelengths and to detect distant and therefore very faint galaxies. The “HST and Beyond Report” (Dressler 1996) presented these and other arguments in favor of an infrared telescope. This report also pointed out that a large, infrared telescope would enable a broad range of topics to be studied.

The Large Optics and Mission Concept Strand

Even before the launch of the Hubble Space Telescope (HST) in 1990, Pierre Bely had begun thinking about larger space telescopes (Bely 1986), and pointed out that NASA missions take 10-15 years from inception to launch so it was not too early to start consideration of a 10-meter successor to HST. Bely’s initial thinking focused largely on an ultraviolet to visible light telescope, but this line of thought was conditioned by the state of knowledge of the distant Universe and the lack of high-performance infrared detector arrays. Quasars out to z~4 were known in 1986 but not galaxies at that redshift. The significance of absorption by hydrogen gas was not yet fully appreciated. He also emphasized that new approaches would be needed to reduce costs. He suggested that the project should be international in character for cost-sharing and to take advantage of launch capabilities available in other countries. He considered various telescope architectures including a segmented primary whose disadvantage to him was the potential unreliability of a system with many actuators required for control of the mirror segments, so he preferred a monolithic mirror with the attendant cost of an expensive heavy-lift rocket.

The Space Studies Board of the U.S. National Research Council chartered a report entitled “Space Science in the 21^st Century: Imperatives for the Decades 1995-2015” which included a volume on astronomy and astrophysics. This report, released in 1988, emphasized the need for a large area, high-throughput telescope. The report stated that some of the most fundamental questions in astronomy would require a telescope of 8-16-meters collecting area which would operate from the Lyman limit at 912Å to 30mm with radiative cooling to 100°K. The report also stated that the telescope should not just be a scaled-up version of HST because of the need for cooling and a large field of view to enable use of large detector mosaics and multi-object spectroscopy. However, this report was written before the launch of HST and well before the launch of Spitzer, and so detailed design work would have been premature.

The Space Studies Board Report was followed by a 1989 workshop entitled “The Next Generation Space Telescope” with a report that began to sharpen ideas for the HST successor (Bely et al. 1990), and which started the use of the name Next Generation Space Telescope (NGST) for the mission. This report examined the entire mission concept. The report weighed a lunar-based telescope against one in high earth-orbit while noting that low-earth orbit suffers from too many disturbances. The report concluded that the time was ripe to establish mission design teams. The lack of rockets with large nose cones suggested that segmented mirrors might be required for a large telescope.

HST was launched in 1990, and the discovery of its optical flaw led to the development of phase retrieval algorithms that are key to JWST’s wavefront sensing procedures which are necessitated by the segmented primary mirror. As described later, the repair of HST led to an understanding that a large, infrared space telescope would be necessary for further progress. In late 1995, NASA chartered a study of the Next Generation Space Telescope at Goddard Space Flight Center with John Mather as a science leader. The telescope was assumed to be only 4-m in diameter based on the considerations in the HST and Beyond report (Dressler 1996), but was later increased to 8-meters. An orbit at the second Lagrange point (L2) was suggested on the basis of the need to be distant from the earth for radiative cooling but still close enough to downlink large volumes of data. Participation in studies by potential aerospace companies was initiated in 1996. The European Space Agency (ESA) chartered an NGST Task Force to consider science drivers and technologies. NASA opened discussions with ESA, the Canadian Space Agency (CSA), and the Japanese Space Agency (JAXA). The Space Telescope Science Institute was involved in these discussions and was designated as the NGST Operations Center. Figure 2 shows some of the concepts that grew out of this work.

Following the kick-off of industry work, NASA organized a group of astronomers to form the “Ad-Hoc Science Working Group” (ASWG) in 1997 whose charter was to refine the JWST’s science goals and to prioritize instrument capabilities. Table 1 lists the ASWG membership. All of the interested space agencies had representatives on the AWSG. The ASWG sharpened the science goals for the mission and considered a variety of instrument concepts. The importance of extending the mission’s observational capability beyond 5 microns was discussed at length by the ASWG. The ASWG’s work and that of the industrial partners culminated in the NGST Science and Technology Exposition (Smith and Long 2000). The report from this meeting presented a concept very similar to the JWST that we have today, and also described the need for multi-object spectroscopy as realized in NIRSpec. What we now know as MIRI was also recommended (Mather 2000). By this time JAXA had decided that they could not participate in the project so the final collaboration of NASA, ESA, and CSA resulted.

During the period that the ASWG existed, work was progressing on infrared detector arrays and on technologies for multi-slit spectroscopy. The detector work is described below and resulted in 2048x2048 arrays being available for the near-infrared range. These detectors were presumed to be cooled radiatively to ~35K, and the feasibility of radiative cooling was confirmed with the launch of Spitzer whose outer shell cooled radiatively to this temperature. These detectors as well as the 1024x1024 detectors for MIRI were made by U.S. companies so all of JWST’s instruments have some U.S.-based components. The ASWG concluded its work with a suite of recommendations for science priorities which can be summarized in four themes: “End of the Dark Ages: First Light and Reionization”; “Assembly of Galaxies”; “Birth of Stars and Protoplanetary Systems”; and “Planetary Systems and the Origins of Life”. The ASWG also defined instrument capabilities which roughly match NIRCam, NIRSpec, and MIRI, but did not negotiate the division of instrument work among the participating space agencies.

The ASWG was disbanded in late 2000 and an Interim Science Working Group (ISWG), membership listed in Table 2, was formed. This change was necessitated to prevent a conflict of interest as NASA personnel began work on the Announcement of Opportunity (AO) that would select instrument teams and interdisciplinary scientists. A key recommendation from the ISWG was the need for JWST to incorporate non-sidereal tracking which would enable spectroscopy of outer solar system objects.

The three space agencies finalized the distribution of instrument work with Canada slated to provide the Fine Guidance Sensor (FGS), NIRCam to be a joint contribution from NASA and CSA, NIRSpec to be developed largely by ESA but with detectors and a multi-slit mechanism from NASA, and MIRI to be developed by a consortium of European groups with detectors and cooling provided by NASA. The subdivision of work enabled finalizing the AO. The ISWG disbanded in anticipation of the release of the AO. During this same period the 2001 decadal survey ranked NGST as the top priority for space astronomy. The cost estimate in the decadal survey was unrealistically low which caused problems later when the true cost precipitated an overrun (Gardner et al. 2023). The size of the telescope primary mirror was descoped from 8 meters to 6.5 meters during the preparation of the AO.

The nearly final configuration of JWST was reached in 2002 with the selection of Northrop Grumman as the prime contractor with significant portions of the telescope work subcontracted to Ball Aerospace. The University of Arizona, teamed with Lockheed Martin’s Advanced Technology Center, was selected to lead the development of NIRCam. A project science working group (SWG) was constituted from the instrument team leaders and interdisciplinary scientists chosen through the AO process. Other members represented major subsections of the project (see full list in Gardner et al. 2023). The project was renamed the James Webb Space Telescope after announcing the selection of Northrop as the prime contractor.

One of the first decisions made by the SWG was endorsing a change from a primary comprised of 36 segments to one comprised of 18 segments. This change reduced the collecting area slightly but enabled a more favorable production schedule. Larger segments were not selected initially as larger segments were deemed too risky. The production of the 85-cm Spitzer primary mirror demonstrated that larger segments were feasible. Another change endorsed by the SWG was splitting off the Canadian tunable filter contribution to NIRCam, and the tunable filter became a separate instrument sharing an optical bench with FGS.

The Infrared Array Strand

While Bely was considering a successor to HST, infrared arrays based on indium bump bonding were just beginning to be used in the ground-based astronomical community (Vural et al. 1983, Forrest et al. 1985). One array type used InSb which had been used extensively in single detector photometers and a second array type used HgCdTe which had been used originally in earth remote sensing applications. A key next step in the development of these arrays came with work aimed at InSb arrays for the IRAC instrument on Spitzer (then called Space Infrared Telescope Facility, SIRTF) (Lum et al. 1992, Forrest et al. 1993) and HgCdTe arrays for the Near-infrared Camera and Multi-object Spectrometer (NICMOS) for HST (Rieke et al. 1989). NASA funding expedited the development of these arrays. The 256x256 HgCdTe devices fabricated for NICMOS were operated at 58K with NICMOS installed onto HST in 1997. This was the first use of HgCdTe arrays on an astronomy space mission.

By 2000 NASA was funding competing near-infrared detector development with sizes as large as 2048x2048 becoming available (Hall et al. 2000, Hoffman et al. 2003). These devices were aimed at meeting the demanding performance requirements that had been outlined for JWST, and these arrays came very close to meeting the requirements if they were operated at an optimal temperature. The final choice of detector type was made in 2003 by the NIRCam Instrument Team who selected HgCdTe arrays based on two factors. First, HgCdTe arrays perform well as warm as 40K which alleviated cooling concerns, and second, HgCdTe can be produced with a variety of cut-off wavelengths which reduced the production risk for NIRCam which uses more short wavelength, 2.5-micron cut-off, arrays than long wavelength, 5-micron cut-off arrays. Short wavelength arrays are somewhat easier to produce than the long wavelength arrays. NIRSpec, NIRISS, and FGS all use long wavelength HgCdTe arrays but only two for NIRSpec and only a total of three for FGS and NIRISS.

The array type used by MIRI, an Si:As IBC (impurity band conduction) detector, also benefitted from early work on arrays for IRAC (McMurray et al. 2000). Just as for the near-infrared detectors, further work was needed to achieve the desired 1024x1024 format and to achieve the level of performance needed (Ressler et al. 2008). A drawback to the Si:As IBC arrays is the need to cool them to ~6K. The initial cooling choice for JWST was a dewar using solid hydrogen. This arrangement was judged too heavy and was replaced by a pulse-tube cryocooler in 2005. The final MIRI design uses one of these arrays in an imager and two arrays in a medium resolution integral field spectrometer.

JWST in as We Know It

A few more decisions were made in the pre-2010 timeframe, including the choice of launcher which became the major ESA contribution to JWST. The Ariane V was selected since it is a very reliable rocket with the needed lift capability. The launcher selection was the last major project element to be chosen. A number of problems cropped up during the design and construction phase. The sun shield was redesigned several times to ensure that the five layers would deploy reliably. A major change to the instrument suite was the redesign of the Canadian contribution NIRISS to eliminate the tunable filter. NIRCam acquired grisms which were to be a backup for wavefront sensing in case the mirror segments were too far apart in focus after deployment to be phased using the dispersed Hartmann sensors. The replacement of degrading near-infrared detectors was another major hurdle, but did not result in any change to the mission design.

The JWST telescope and instruments were tested together in flight-like conditions in a large test chamber at Johnson Space Flight Center in 2016 (Acton et al. 2018). This test was very successful and demonstrated that all of the sensing and control required to align the 18 mirror segments worked properly. After this test, the telescope and instruments were mated to the spacecraft and sunshield at Northrop’s facility in California followed by packing into a shipping container for the trip to French Guiana. A very successful launch on December 25, 2021, was followed by six months of commissioning (Rigby et al. 2023) which demonstrated a telescope working nearly twice as well as planned. The launch was so successful that the astronomical community will enjoy using JWST for as long as twenty-five years because so little rocket fuel was used during launch for insertion into the L2 orbit.

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