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Technical Frequently Asked Questions (FAQ)
Technical FAQ on a variety of mission issues, aspects and capabilities. Targeted at the science/technical community.
Webb and its Science Programs
- What is Webb?
Webb is the James Webb Space Telescope, sometimes called Webb, a facility-class space observatory operating in the visible, near and mid infrared. Webb's 6.6-meter diameter primary mirror has a 25-square-meter collecting area formed from eighteen hexagonal segments, and will be diffraction limited at 2 microns. It is a joint project of NASA, the European Space Agency (ESA) and the Canadian Space Agency (CSA). The telescope will be an infrared-optimized general-observer facility with four science instruments: a near-IR camera (0.6-5 microns) from the University of Arizona; a near-IR spectrograph (1-5 microns) from ESA; a near-IR imager and slitless spectrograph (0.7 – 4.8 microns) from CSA; and a mid-IR camera/spectrograph (5-28.5 microns) provided by the Jet Propulsion Laboratory, ESA and a nationally funded consortium of European institutes. In addition, CSA is providing the Fine Guidance Sensor. Webb is projected to launch in 2018 on an Ariane 5 ECA rocket to an orbit around the second Sun-Earth Lagrange point.
- How can I get more information about Webb?
For more information, see the Project website. A detailed description of the science and implementation for Webb has been published (Gardner et al. 2006, Space Science Reviews, 123, 485; available without subscription springerlink.com. Additional discussions of Webb science are in a series of science white papers. A conference on Webb science, entitled "Frontier Science Opportunities with the James Webb Space Telescope"was held at the Space Telescope Science Institute in June 2011; a webcast and the charts for the talks.
We send out a Webb email newsletter several times a year (sign upon our For Scientists page) and hold Webb Town Hall meetings at the winter meetings of the American Astronomical Society.
There is a lot of information about Webb on the STScI website, and articles about Webb in the STScI newsletter.
- How about a Webb talk?
If your institution would like to have a colloquium talk about Webb, or if you would like a talk about Webb at a conference you are organizing, please contact the Webb science team at: jwst-science@lists.nasa.gov. We are also available for public talks.
- Who was James E. Webb?
Webb is named after James E. Webb (1906 – 1992), NASA's second administrator. Webb is best known for leading Apollo, a series of lunar exploration programs that landed the first humans on the Moon. However, he also initiated a vigorous space science program that was responsible for more than 75 launches during his tenure, including America's first interplanetary explorers. For more information, see: http://jwst.gsfc.nasa.gov/whois.html, or Webb's official NASA biography at: http://www.hq.nasa.gov/office/pao/History/Biographies/webb.html.
Webb Science
- What science will Webb accomplish?
Topics in four areas of modern astronomy are being used to craft the engineering design of Webb: First Light and Reionization; The Assembly of Galaxies; The Birth of Stars and Protoplanetary Systems; and Planetary Systems and the Origin of Life. In addition, Webb's instrument suite will have wide applicability across a broad range of scientific issues. For a detailed description of Webb science see Gardner et al. 2006, SSRv, 123, 485:
(http://www.springerlink.com/content/h2374012xk30qpw5/). Additional discussions of Webb science are in a series of science white papers, available from:
(http://www.stsci.edu/jwst/science/whitepapers/.) Most of Webb's time will be allocated through a full peer-reviewed general observing program, similar to those of the Hubble and Spitzer Space Telescopes.
- How will Webb observe First Light and Reionization?
Theorists predict that the first stars formed through molecular hydrogen cooling at redshifts 20 < z < 30. They polluted their environment with metals and dissociated the nearby molecular hydrogen so that they did not form in large clusters. The first galaxies formed at z ~15 when dark matter haloes had built up to 108 solar masses and cooling by atomic hydrogen became efficient. These first galaxies had about 106 solar luminosities in stars, which corresponds to 1.4 nJy, or AB=31. Webb can detect these first galaxies with an ultra-deep field of about 100 hours per filter. The first galaxies then reionized the local inter-galactic medium, in a process that concluded by z=6. Using near-infrared spectroscopy of the bright z>7 quasars, galaxies or gamma-ray bursts, Webb will detect the Gunn-Peterson trough and patchy absorption that indicates the process of reionization.
- How will Webb observe the Assembly of Galaxies?
Galaxy assembly is a process of hierarchical merging, during which the dark matter, gas, stars, metals, morphological structures and active nuclei build up to form the Hubble Sequence that we see today. Webb will conduct deep-wide surveys of galaxies in the rest-frame optical and near infrared over the redshift range 1 < z < 6 with both imaging and spectroscopy. The microshutter array, which provides multi-object near-infrared spectroscopy of ~100 targets simultaneously, will allow large samples of galaxies to be divided into bins of redshift, metallicity and morphological structure. Targeted follow-up with integral field spectroscopy in the near- and mid-infrared will determine the origin of global scaling relations such as Tully-Fisher and Faber-Jackson, and diagnose the energy balance within starbursts and active galactic nuclei.
- How will Webb observe the Birth of Stars?
Star formation begins when molecular cloud cores cool and fragment to form dynamic clusters of protostars spanning the mass spectrum from O stars down to planetary-mass brown dwarfs. Within the clusters, individual young stars are often encircled by disks of warm gas and dust, where material aggregates to from protoplanetary systems. The stars interact strongly with their environment through jets, outflows and radiation. Young stars, brown dwarfs and circumstellar disks emit the bulk of their radiation in the near- and mid-infrared, and in the early stages, the shorter wavelengths are absorbed by dust. Webb's near- and mid-infrared imaging capabilities will enable surveys of molecular clouds and star-forming regions. Webb's integral field spectroscopy capabilities will allow detailed observations of individual targets.
- How will Webb observe Planetary Systems?
Webb has several capabilities that will enable the characterization of exoplanets. For exoplanets that transit bright stars, the defocussed R~700 grism in the NIRISS instrument will enable Webb to characterize the atmospheres, possibly even detecting the signature of liquid water on rocky planets. For more distant transiting planets, the capabilities of NIRCam, NIRSpec and MIRI will enable photometric and spectroscopic detection of both primary and secondary eclipses, measuring both atmospheric absorption and thermal emission from a wide variety of planets. Webb also has coronagraphic capability in NIRCam and MIRI and a non-redundant mask providing sparse aperture interferometry. These capabilities will enable direct detection of exoplanets and detailed study of circumstellar disks.
Within our own Solar System, Webb's moving-target capability will allow observations of all planets, comets, asteroids and moons at or beyond the orbit of Mars. Some of the science that will be possible includes imaging all known Kuiper-Belt Objects in the mid-infrared, time-resolved imaging of dwarf planets, spectroscopy of cometary comae and dust grains and determination of organic material on the surfaces of the icy moons.
- Can Webb observe objects in our Solar System?
Yes. Webb is designed to be able to observe solar system objects having an apparent angular rate of motion of 0.030 arc seconds per second or less. This rate capability includes Mars, Jupiter, Saturn, Uranus, Neptune, Pluto, their satellites, and comets, asteroids and minor planets at or beyond the orbit of Mars. Webb has the near-IR and mid-IR sensitivity to be able to observe and study virtually all known Kuiper Belt Objects. The very large infrared brightnesses of Mars, Jupiter, and Saturn may limit Webb observations of these planets to a subset of the instrument modes.
- Is there more information about Webb observations of objects in our Solar System?
Yes. We have a technical-level FAQ specifically on Solar System observations.
- Can Webb observe targets of opportunity?
Yes. Webb has the capability of interrupting its previously scheduled observing plan to view new targets within 48 hours of the decision to do so. This capability could be invoked for supernovae, gamma-ray bursts, collisions within our Solar System or other time-critical, unscheduled or unpredicted phenomena. Target of opportunity observations can be allocated through the time-allocation committee peer-review process or through Director's Discretionary Time.
Webb Observations
- What advantages will Webb provide over Hubble,
Spitzer, and other existing telescopes?
Webb possesses a combination of large aperture, diffraction-limited image quality, and infrared sensitivity over a broad wavelength range hitherto not available from ground- or space-based facilities. Webb has a larger collecting area than Hubble and its capabilities extend to longer wavelengths in the infrared. Webb is also much larger than Spitzer, providing greater sensitivity and finer angular resolution at wavelengths shorter than 28.5 microns. With multi-object and integral field spectroscopy, Webb's instruments provide capability that Hubble and Spitzer don't have. Its capabilities will let us understand the full population of galaxies at redshifts from 6 to 10 (for example to determine why we are finding early galaxies that are brighter and older than some theoretical predictions) and to detect the first galaxies to form as early as redshift 15. Webb is also needed to explore the assembly of galaxies and their nuclear black holes and how they are inter-related through processes such as feedback. It will trace the earliest stages of stellar evolution, penetrating the dense cold cloud cores where stars are born. It will obtain spectra to reveal the conditions in protoplanetary disks and to search for biologically important molecules, and will map the evolution of planetary systems by imaging debris disks and by studying exoplanets through coronagraphic imaging and transit spectroscopy.
- What advantages will Webb provide over a future 30-meter telescope on the ground?
Webb is designed to be complementary to existing and future ground-based facilities, making observations that are not possible from the ground, regardless of ground-based telescope aperture. The Astronomy and Astrophysics Advisory Committee, which evaluates ground- and space-based programs by NASA, NSF, and the Department of Energy, commissioned an extensive report on the complementary natures of Webb and a 30-meter ground-based telescope (available at http://www.nsf.gov/mps/ast/aaac/reports/gsmt-jwst_synergy_combined.pdf). Between 1 and 2.5 microns, Webb's strengths are complementary to those of the next-generation 30-meter aperture telescopes. Webb's wavelength coverage extends to 28.5 microns; beyond about 2.5 microns, where ground-based sensitivity is severely limited by thermal emission from the atmosphere, Webb's sensitivity advantage will be immense.
- What are the policies and plans for observing with Webb?
The policies for Webb observers will be very similar to those of the other Great Observatories, with more than 80% of the observing time available to those submitting general observing (GO) proposals. An additional 10% of the time is Director's Discretionary Time, while the remaining 10% of the first five years is Guaranteed Time. The current expectation is that GO programs will start in the first year of the mission with a Cycle 1 call for proposals nominally being issued one year before launch. Cycle 1 is expected to include a mix of small, medium and large GO programs. Guaranteed time observers will complete their programs in the first three years of the mission. No specific decision has been taken regarding Legacy, Treasury or Key programs involving non-proprietary datasets. It is expected that the community will have the opportunity to comment on these ideas through a community workshop and/or a User Committee and that any initiative will be properly advertised and open to the astronomical community. NASA is planning for all Webb data to be public one year after the data are first available to the observer, similar to the policies for the Hubble Space Telescope, and that Education/Public Outreach efforts will also be similar to those for Hubble. Astronomers throughout the world will be able to request data from the Webb archive through the Internet. The public will also be able to view many of Webb's pictures through press release and image release archives on the Internet.
- What is Webb's lifetime?
Webb will have a mission lifetime of not less than 5-1/2 years after launch, with the goal of having a lifetime greater than 10 years. The lifetime is limited by the amount of fuel used for maintaining the orbit, and by the testing and redundancy that ensures that everything on the spacecraft will work (mission assurance). Webb will carry fuel for a 10-year lifetime (with margin); the project will do mission assurance testing to guarantee 5 years of scientific operations starting at the end of the commissioning period 6 months after launch.
Webb's Budget
- How much does Webb cost? When will it launch?
Webb is planned to launch in October 2018. The NASA cost to build, launch and commission the observatory is $7.998B. Five years of operations and 2 additional years of data analysis are an additional $837M, for a total lifecycle cost of $8.835B. It should be noted that the facility is not necessarily limited to 5 years of operations, but that is the top level mission requirement. The observatory will contain sufficient consumables to operate for approximately 10 years. As of September 2011, NASA has spent approximately $3.5B on Webb.
- Can Webb be "descoped" or reduced in complexity to save money or time?
Since the selection of the science instrument teams and the prime contractor in 2002, Webb has descoped or simplified the mission 22 different ways. These descopes have included reducing the primary mirror size from 7 meters in diameter to 6.5 meters in diameter, to relaxing the requirements how the system performs at its shortest wavelengths (those wavelengths govern how difficult the optical manufacturing and testing are). All of these descopes have saved time and money, but still permit Webb to achieve the science goals that made it the number one choice of the astronomical community in the 2000 National Academy of Sciences decadal survey for Astronomy and Astrophysics. As of December 2011, all the optics are complete and tested, the science instruments are in their final stages of testing meaning that most of the telescope hardware is finished and is ready for the integration and test phase. When systems are this far along in their manufacturing and assembly, descopes which save substantial amounts of money or time are more difficult to realize. In the future as assembled systems are tested, the Webb team can consider "scientific capability descopes" if tests indicate that something does not perform precisely as desired. Such "capability descopes" are really acceptances of hardware performance as built and would mean that NASA would not spend additional funds to fix all of the problems that were found in test.
- Who paid for the full-scale Webb model?
The full-scale model was built and is supported entirely with Northrop Grumman internal funds.
Webb's Instruments
- What are the capabilities of NIRCam?
The Near-Infrared Camera (NIRCam) provides filter imaging in the 0.6 to 5.0 micron range. With a dichroic splitting the light at 2.4 microns, NIRCam provides simultaneous imaging of a 2.2 by 4.4 arcmin2 field of view in two filters. The short wavelength channel contains eight 2048 by 2048 pixel detectors with 32 milliarcsec pixels, and the long wavelength channel contains two 2048 by 2048 pixel HgCdTe detectors with 65 milliarcsec pixels. NIRCam contains 7 broad-band filters, 12 medium-band filters, several narrow-band filters and long wavelength slitless grisms. It contains the weak lenses and other hardware that will be used for wavefront sensing for the telescope. NIRCam contains a coronagraphic capability. NIRCam broad-band imaging will reach 11.4 nJy (AB=28.8) point-source detection at 2.0 microns, 10 sigma in 10,000 seconds.
- What are the capabilities of NIRSpec?
The Near-Infrared spectrograph (NIRSpec) provides three different spectroscopic capabilities over the 0.6-5.0 micron domain. In its multi-object spectroscopy (MOS) mode, spectra of more than 100 sources can be obtained over a field of view of 9 square-arc-minutes. The selection of the spectroscopic targets for the MOS mode is performed using a state-of-the-art micro-electro-mechanical system (MEMS) that contains a total of more than 250,000 individually addressable micro-shutters. Micro-shutters will be opened at the location of the selected spectroscopic targets, each of them creating a small aperture of approximately 200 by 450 milli-arc-seconds on the sky.
NIRSpec also provides an Integral Field Unit (IFU) mode for spatially resolved spectroscopy of individual objects. Thanks to the use of an image-slicer, the IFU mode allows to obtain 900 spectra per exposure covering a 3 by 3 arc-seconds field of view with a spatial sampling of 100 milli-arc-seconds. Last but not least, a SLIT mode is available that will allow users to perform high-contrast spectroscopy using one of the 5 available slits. One of these slits is in fact a square 1.6-arc-second aperture introduced specially for the observation of extra-solar planet transits.
Three different spectral resolving power ranges will be available. Two series of 3 gratings will yield spectral resolving powers of R~1000 and ~2700 in 4 spectral bands covering the 0.7-5.0-micron domain. A prism will yield spectral resolving powers between 30 and 270 in a single spectral band covering the complete 0.6-5.0-micron domain.
The NIRSpec sensitivity in MOS mode will be 132 nJy (AB=26.1) in the continuum at 3 micron and at R~100. At R~1000, the line sensitivity will reach 1.8 10-18 erg s-1 cm-2 at 2 microns. These values are for a signal to noise of 10 and a series of 10 exposures of 1000s each.
- What are the capabilities of NIRISS?
The Near-Infrared Imager and Slitless Spectrograph provides three unique scientific capabilities over a field of view of 2.2 by 2.2 arcmin2. It conducts R ~ 150 slitless spectroscopy at 0.8 to 2.25 microns optimized for Lyman alpha emission-line galaxy surveys. It conducts defocused R ~ 700 slitless spectroscopy at 0.7 to 2.5 microns optimized for exoplanet transit spectroscopy of bright host stars. It uses a 7-aperture non-redundant mask to provide sparse-aperture interferometric imaging at 3.8, 4.3 and 4.8 microns, optimized for studying exoplanets. The NIRISS dectector is a single 2048 by 2048 pixel detector array with 65 milliarcsec pixels. NIRISS R ~ 150 slitless spectroscopy will reach 5.5 x 10-18 ergs s-1cm-2 line sensitivity at 1.4 microns, 10 sigma in 10,000 seconds.
- What are the capabilities of MIRI?
The Mid-Infrared Instrument (MIRI) provides imaging and spectroscopy over the wavelength range 5 to 28.5 micron. The imaging module provides broad-band imaging, coronagraphy and low-resolution slit spectroscopy. It has a 1024 by 1024 pixel detector array with 110 milliarcsec pixels. The coronagraphy is done with quarter-phase mask coronagraphs at 10.65, 11.4 and 15.5 microns, and a Lyot stop optimized for 23 microns. The low-resolution slit operates over 5 to 10 microns with R ~ 100. MIRI uses an image slicer and dichroics to provide imaging spectroscopy over four simultaneous concentric fields of view ranging from 3 to 7 arcsec on a side. The spectral resolution ranges from R ~ 2400 to 3600. MIRI spectroscopy uses two 1024 by 1024 Si:As arrays with plate scales between 200 to 470 milliarcsec. MIRI imaging sensitivity is 700 nJy (AB=24.3) at 10.0 microns and 8.7 μJy (AB=21.6) at 21.0 microns. MIRI spectroscopic line sensitivity is 1.0 × 10-17 erg s-1 cm-2 at 9.2 microns and 5.6 × 10-17 erg s-1 cm-2 at 22.5 microns. These are 10 sigma in 10,000 seconds.
Webb's Technology
- What is the current status of ISIM?
(Status as of January 2015) The ISIM is on schedule for delivery during December 2015. It is currently preparing for vibration and acoustic testing which will be followed by the third of three planned cryogenic space simulation tests during summer of 2015. After delivery during December, it will be integrated with the telescope.
- What is the current status of the telescope?
(Status as of March 2012) There are 18 flight Webb primary mirror segments plus 3 spares including the pathfinder Engineering Demonstration Unit (EDU). The segment blanks were all machined out of beryllium billets and light-weighted. All of the primary mirror segments have completed polishing at Tinsley, and gold coating at QCI. All eighteen segments have completed the final acceptance measurements of their cryogenic figure at Marshall Space Flight Center (MSFC). The composite surface figure error (SFE) of the primary mirror is 25 nm, meeting the requirement of 26 nm SFE. The secondary mirror has also completed acceptance testing and with a SFE of 20.8 nm, also exceeds its requirement. The tertiary mirror and fine steering mirror have completed acceptance testing as well. The telescope optics are in environmentally controlled storage until the integration of the telescope optics with the primary mirror backplane commences.
- What is the performance of the Webb detectors?
Webb uses Teledyne HAWAII-2RG detector arrays for the NIRCam, NIRSpec, and FGS/NIRISS. Both NIRSpec and FGS use 5 micron cutoff detectors. NIRCam's short wavelength channels use 2.5 micron cutoff detectors, while NIRCam's long wavelength channels use 5 micron cutoff detectors identical to those in NIRSpec and FGS. For approximate calculations, the system-level read noise of all 3 near-infrared instruments is about 20 e- rms per correlated double sample, dark current is about 0.01 e-/s/pixel, and the QE is 70% from 0.6-1 microns and 80% from 1 micron to 5 microns. The different instruments use different readout modes with different numbers of non-destructive samples to meet their requirements. For a typical 1000 second long science integration, the total detector system noise is about 10 electrons rms for NIRCam and 6 electrons rms for NIRSpec and NIRISS. This performance is sufficient to achieve the sensitivity listed below.
The MIRI detectors build on the heritage from the Infrared Array Camera (IRAC) on Spitzer, but with significant performance enhancements such as the 1024 by 1024 pixel format, a lower read noise, and modifications in the detector prescription for better performance in the 5 to 10 micron range. Excellent detector arrays have been produced at Raytheon Vision Systems (RVS) which meet the instrument requirements for sensitivity. They have been further characterized at JPL and have been integrated into the MIRI Optical System; the flight instrument has completed performance tests at the Rutherford Appleton Laboratory and is being prepared for delivery to Goddard for integration into the ISIM. For approximate sensitivity calculations, see http://ircamera.as.arizona.edu/MIRI.
- What are the instrument sensitivities?
The instrument sensitivities are given in the following table:
Sensitivity is defined to be the brightness of a point source detected at 10 σ in 10,000 s. Longer or shorter exposures are expected to scale approximately as the square root of the exposure time. Targets at the North Ecliptic Pole are assumed. The sensitivities in this table are subject to change. Prototype exposure time calculators are available at the Space Telescope Science Institute website.
- Will this complex spacecraft really work?
Webb requires several new technologies, but these have been validated by testing and external review many years prior to launch. The program has a seven-year integration and test (I&T) plan to validate the flight hardware in an incremental and thorough way. The Project has purposely phased the contingency funding to ensure it fully covers this I&T phase, where most of the risk is, and to help resolve any problems that crop up during I&T. The deployments involved in Webb are being provided by Northrop Grumman (NG), the world's leader in satellite deployments. NG has built satellites with more difficult and complex deployments, including 640 deployments with more than 2000 elements, with no mission failures. The Project follows NASA standard management requirements in which passage through each development stage is gated by independent expert review. The Project is managed by Goddard Space Flight Center whose mission success record exceeds that of any civil space sector organization (government or private).
- Will the thermal design really work?
The Webb thermal concept is rooted in the experience with Spitzer. The performance of that mission demonstrates the accuracy of the thermal models that were used to predict its operating temperature through the same kind of radiative cooling being used with Webb. The Spitzer outer shell – which is analogous to the cold part of Webb – is running at exactly the temperature (35 K) predicted by its thermal models. An extensive program of iterative testing and modeling with full-size components – "pathfinders" will verify the Webb models. Pathfinders of increasing fidelity are constructed, along with tests of smaller assemblies (electrical harness, multi-layer insulation, radiator coatings). The pathfinders are compared with detailed thermal models so that scientists and engineers can be confident that the "bootstrapping"process results in a good physical understanding of the hardware. This understanding is verified by thermal vacuum testing. Because no single thermal-vacuum test can simulate realistic operational conditions for a fully integrated Webb observatory, a series of tests will verify performance of individual assemblies (instrument module, sunshield, spacecraft bus). These are followed by a comprehensive thermal test that involves the complete telescope and instrument module. Temperature data gathered during these tests are used to fine-tune thermal models to make them more representative of flight hardware. Independent thermal models are developed by NASA and the prime contractor team to mitigate risk. Finally, the Project uses external reviewers with relevant experience to assess the Webb thermal design and testing approach.
- Will astronauts be able to service Webb like they did Hubble?
Because Webb, like virtually every satellite ever constructed, will not be serviceable it employs an extensive seven year integration and test program to exercise the system and uncover any issues prior to launch so they might be remedied. Unlike Hubble, which orbits roughly 350 miles above the surface of Earth and was therefore accessible by the Space Shuttle, Webb will orbit the second Lagrange point (L2), which is roughly 1,000,000 miles from Earth. There is currently no servicing capability that can be used for missions orbiting L2, and therefore the Webb mission design does not rely upon a servicing option.
- What is Webb doing to ensure that its gyros last the full mission?
The gyroscopes on HST and Chandra are mechanical devices dependent on bearings for their function, and they face problems typical of such designs. Webb has adopted a different gyroscope technology. The "Hemispherical Resonator Gyroscope" (HRG) uses a quartz hemisphere vibrating at its resonant frequency to sense the inertial rate. The hemisphere is made to resonate in a vacuum, and the hemisphere's rate of motion is sensed by the interaction between the hemisphere and separate sensing electrodes on the HRG housing. The result is an extremely reliable package with no flexible leads and no bearings. The internal HRG operating environment is a vacuum, thus once the gyroscope is in space any housing leaks would actually improve performance. The HRG eliminates the bearing wear-out failure mode, leaving only random failure and radiation susceptibility of the electronics (which all such devices share, and which can be mitigated by screening and shielding). Stress analyses of HRGs show this design has a "mean time before failure" of 10 million hours. As of June 2011, this type of device had accumulated more than 18 million hours of continuous operation in space on more than 125 spacecraft without a single failure.
- How big is the Webb mirror?
The Webb primary mirror is made of 18 segments and stretches 6.6 meters from tip to tip. Its area of slightly more than 25 square meters and its diffraction-limited resolution are approximately equivalent to a 6.0 meter conventional round mirror. At 2 microns, the FWHM of the image will be about 70 milli-arcsec.
The 18 hexagonal segments are arranged in a large hexagon, with the central segment removed to allow the light to reach the instruments. Each segment is 1.32 m, measured flat to flat. Beginning with a geometric area of 1.50 m2; after cryogenic shrinking and edge removal, the average projected segment area is 1.46 m2. With obscuration by the secondary mirror support system of no more than 0.86 m2, the total polished area equals 25.37 m2, and vignetting by the pupil stops is minimized so that it meets the >25 m2 requirement for the total unobscured collecting area for the telescope. The outer diameter, measured along the mirror, point to point on the larger hexagon, but flat to flat on the individual segments, is 5 times the 1.32 m segment size, or 6.6 m (see figure). The minimum diameter from inside point to inside point is 5.50 m. The maximum diameter from outside point to outside point is 6.64 m. The average distance between the segments is about 7 mm, a distance that is adjustable on-orbit. The 25 m2 is equivalent to a filled circle of diameter 5.64 m. The telescope has an effective f/# of 20 and an effective focal length of 131.4 m, corresponding to an effective diameter of 6.57 m. The secondary mirror is circular, 0.74 m in diameter and has a convex aspheric prescription. There are three different primary mirror segment prescriptions, with 6 flight segments and 1 spare segment of each prescription. The telescope is a three-mirror anastigmat, so it has primary, secondary and tertiary mirrors, a fine steering mirror, and each instrument has one or more pick-off mirrors.
The Webb primary mirror consists of 18 hexagonal segments with three different prescriptions.
- How does the collecting area of Webb compare to Hubble?
Hubble has a 2.4 m diameter round primary mirror. For the Advanced Camera for Surveys (ACS) and the Space Telescope Imaging Spectrograph (STIS), the central obscuration by the secondary is 0.33r, where r is the 1.2 m radius. Wide Field - Planetary Camera 2 (WFPC2) had a larger internal obscuration, which was oversized to ensure alignment, ranging between 0.39r and 0.43r. Using the 0.33r obscuration, the area of Hubble's mirror is π (1.22) (1-0.332) = 4.0 m2. Therefore, the ratio between the 25.0 m2 Webb mirror and the Hubble mirror is 6.25.
- What kind of reaction wheels does Webb have?
JWST will employ Rockwell Collins Deutschland GBMH (Formerly Teldix) reaction wheels. These wheels have heritage traceable to the Teldix wheels flown on NASA's Chandra, EOS Aqua and Aura Missions.
Webb Integration and Testing
- How is Webb being tested?
Webb will be tested incrementally during its construction, starting with individual mirrors and instruments (including cameras and spectrometers) and building up to the full observatory. Webb's mirrors and the telescope structure are first each tested individually, including optical testing of the mirrors and alignment testing of the structure inside a cold thermal-vacuum chamber. The mirrors are then installed on the telescope structure in a clean room at Goddard Space Flight Center (GSFC). In parallel to the telescope assembly and alignment, the instruments are being built and tested, again first individually, and then as part of an integrated instrument assembly. The integrated instrument assembly will be tested in a thermal-vacuum chamber at GSFC using an optical simulator of the telescope. This testing makes sure the instruments are properly aligned relative to each other and also provides an independent check of the individual tests. After both the telescope and the integrated instrument module are successfully assembled, the integrated instrument module will be installed onto the telescope, and the combined system will be sent to Johnson Space Flight Center (JSC) where it will be optically tested in JSC's largest vacuum chamber, which is being retrofitted for deep cryogenic operation. The process includes testing the 18 primary mirror segments acting as a single primary mirror, and testing the end-to-end system. The final system test will assure that the combined telescope and instruments are focused and aligned properly, and that the alignment, once in space, will be within the range of the actively controlled optics. In general, the individual optical tests of instruments and mirrors are the most accurate. The final system tests provide a cost-effective check that no major problem has occurred during assembly. In addition, independent optical checks of earlier tests will be made as the full system is assembled, providing confidence that there are no major problems.
- Why is Webb being tested this way?
The most expensive tests of a large space telescope are the final system tests. The Hubble Space Telescope did not have a final system test – which could have caught the problem in the fabrication of the Hubble primary mirror – because it was deemed too complex and expensive. Unlike Hubble, Webb is not designed for servicing; thus Webb must be done right. The challenge has been to design a test strategy that assures success but is also affordable. The Webb test plan emphasizes incremental testing, accompanied by independent checks at each level of assembly to minimize the uncertainties left for the final system test. The plan does include a final system test, and this test makes use of the Webb active optics. This final test will assure that Webb can be aligned on-orbit, making the test cost effective yet retaining adequate redundancy and accuracy to detect any problems.
- Why is this testing strategy cost-effective?
The overall Webb Project testing strategy will test all individual components as early as possible in the project schedule after they have been manufactured, so that time is available to fix or replace them if needed without costly schedule impacts. More complex systems, such as science instruments and operational systems, will get tested later in the Project schedule: early enough that fixes can be implemented if needed without major schedule impacts, but late enough in the project that necessary design effort and analyses have adequate time to complete. All critical Webb components and systems will be independently verified at the lowest possible level of assembly. In this approach, subtle manufacturing errors or system performance flaws have the best chance of surfacing early and unambiguously, which will minimize the risk of large and costly schedule impacts later in the project.
- What lessons has Webb learned from past missions?
Many lessons were learned from building UV, optical, infrared and X-ray missions like Hubble, Spitzer and Chandra, including a key aspect of the Webb strategy: early independent tests of key optical parameters, with the highest performance tests performed at the lowest levels of assembly. The strategy also includes a full-up system test of the final assembly to catch significant errors anywhere in the optical chain. The lessons learned from earlier cryogenic telescopes have directly led to a more robust cold-testing strategy, including early testing. The Webb test program will also employ specific techniques that have been shown to be effective in earlier programs, such as the precision photogrammetry used in WMAP testing (already applied with excellent results to ISIM structure cryo-testing) and the auto-collimation optical testing approach utilized by Spitzer.
Webb and NASA Programmatic Issues
- Who are the partners in the Webb Project?
NASA is the lead partner in
Webb, with significant contributions from the European Space Agency (ESA) and the Canadian Space Agency (CSA). Northrop Grumman Aerospace Systems (NGAS) is the main NASA industrial
contractor, responsible for building the optical telescope, spacecraft bus,
and sunshield and preparing the observatory for launch. NGAS is leading a
team including three major sub-contractors: Ball Aerospace, ITT Exelis , and Alliant Techsystems (ATK).
The three principal beryllium mirror
subcontractors to Ball Aerospace are Tinsley Laboratories, Axsys
Technologies, and Brush Wellman Inc. The instrument complement is provided
as follows:
- The Mid-Infrared Instrument (MIRI) is provided by a consortium of European countries and the European Space Agency (ESA) and the NASA Jet Propulsion Laboratory (JPL) with detectors from Raytheon Vision Systems.
- The Near-Infrared Spectrograph (NIRSpec) is provided by ESA.
- The Near-Infrared Camera (NIRCam) is built by the University of Arizona working with Lockheed-Martin.
- The Near-Infrared Imager and Slitless Spectrograph (NIRISS) are provided by the Canadian Space Agency (CSA).
- All of the near-infrared detectors are supplied by Teledyne Technologies, Inc.
The launch vehicle and launch services are provided by ESA. The Science and
Operations Center will be at the Space Telescope Science Institute (STScI).
- Which states are involved?
The Webb project has partners or contractors in 27 states and the District of Columbia. In addition, in a program with the Girl Scouts of the USA, Webb has Education and Public Outreach activities in 41 states, the District of Columbia, Guam and a US Air Force Base in Japan.
- Which countries are involved?
Fourteen countries are providing hardware components to build the James Webb Space Telescope: Austria, Belgium, Canada, Denmark, France, Germany, Ireland, Italy, the Netherlands, Spain, Sweden, Switzerland, the United Kingdom and the United States of America. In addition: Finland, Greece, Luxembourg, Norway, Poland, Portugal, The Czech Republic and Romania are members of the European Space Agency and are also contributing to the success of Webb. The launch of Webb will take place in French Guiana, an overseas department of France located in South America. After launch, scientists from around the world will use the telescope for astronomical investigations.
- Wouldn't it be better to fly a number of smaller missions instead of one big mission?
Science goals and their associated measurement requirements ultimately define mission sizes. For some science questions the appropriate mission size is large, for others smaller missions will suffice. The 2000 decadal survey defined a number of scientific challenges some of which required technically ambitious missions. Webb was the top-ranked priority in the 2000 Decadal Survey. It addresses science that cannot be done by any other means. The balance between big and small missions is the result of prioritization in the Decadal Survey and NASA's implementation strategy. Historical publication and citation rates of the Great Observatories, as well as flagship Solar System missions like Cassini and Galileo, show that they are extremely productive facilities, enabling thousands of scientists to do forefront research with state-of-the-art instrumentation.
- Was Webb reviewed in the ASTRO2010 Decadal Survey?
The formal answer to this question is "No" as the guidelines for the survey removed from reprioritization those missions in development. "In development" means in phase C, having passed PDR and been confirmed, which happened to Webb in 2008. However, Webb is a major component of the NASA program, and the Decadal Survey report discussed Webb's role in astronomy. The report, "New Worlds, New Horizons in Astronomy and Astrophysics," identifies three science themes for the next decade: Cosmic Dawn, New Worlds, and Physics of the Universe. As is made clear in the survey, the James Webb Space Telescope (Webb) plays a critical scientific role in the two first themes, and a strong supporting role for the third theme. Many of the survey recommendations build on groundwork to be laid by Webb for the next decade of astronomical exploration. A more detailed description of the role of Webb in the Decadal Survey Report is given in a white paper by Hammel et al., available at: http://www.stsci.edu/jwst/doc-archive/white-papers/.
- How does the astronomical community provide feedback on Webb?
Community input to Webb comes through several paths. The Science Working Group provides regular input to the NASA Headquarters Program Scientist and the Goddard Space Flight Center Project office. The SWG includes the NASA Project scientists, the principal investigators of each science instrument team and interdisciplinary scientists who are expert in the broad range of science encompassed by the mission. Their contact information can be found at: http://www.jwst.nasa.gov/workinggroup.html. The NASA Advisory Council Science Committee and its Astrophysics Subcommittee, (http://science.hq.nasa.gov/strategy/NAC_sci_subcom/astrophysics.html) also represents the broad astrophysics community, and provides input on the astrophysics portfolio including Webb to the NASA Advisory Council. Finally, as the operations phase of the Webb mission approaches, the Space Telescope Science Institute will convene a Users Committee to advise on operations aspects.
- What will be the successor to Webb?
NASA expects to follow the priorities recommended by the Decadal Survey for future missions.
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