Frequently Asked Questions (FAQ)
This FAQ is of a technical nature. You can find a more general for the
public FAQ here.
JWST and its
JWST is the James Webb Space
Telescope, a facility-class space observatory operating in the visible,
near and mid infrared. JWST'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
tunable-filter imager (1.6-4.9 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. JWST is
projected to launch in 2014 on an Ariane 5 ECA rocket to an orbit around
the second Sun-Earth Lagrange point.
can I get more information about JWST?
For more information, see the
Project website (http://www.jwst.nasa.gov).
A detailed description of the science and implementation for JWST has been
published (Gardner et al. 2006, Space Science Reviews, 123, 485; available
without subscription at:
We send out a JWST email newsletter several times a year (sign up at: http://jwst.gsfc.nasa.gov/scientists.html)
and hold JWST Town Hall meetings at the winter meetings of the American
For technical information about
the status, design and plans for JWST, you can go to http://spie.org and search for
?webb" and "jwst?. On that site there are more than 500 technical
articles about JWST. (Note there is a fee per article on that site without
an institutional subscription.)
about a JWST talk?
If your institution would like to
have a colloquium talk about JWST, or if you would like a talk about JWST
at a conference you are organizing, please contact the JWST science team
We are also available for public talks.
was James E. Webb?
JWST 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.
science will JWST accomplish?
Topics in four areas of modern
astronomy are being used to craft the engineering design of JWST:
First Light and Reionization; The Birth of Galaxies; The Birth of Stars and
Protoplanetary Systems; and Planetary Systems and the Origin of Life.
In addition, JWST's instrument suite will have wide applicability across a
broad range of scientific issues. For a detailed description of JWST
science see Gardner et al. 2006, SSRv, 123, 485. (http://www.springerlink.com/content/h2374012xk30qpw5/).
Additional discussions of JWST science are in a series of science white
papers, available from http://www.stsci.edu/jwst/science/whitepapers/.)
advantages will JWST provide over Hubble, Spitzer, and other
JWST 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. 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. JWST 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.
advantages will JWST provide over a future 30-meter telescope on the
JWST is designed to make
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 JWST 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, JWST's strengths are complementary to those of
the next-generation 30-meter aperture telescopes. Beyond about 2.5
microns, where ground-based sensitivity is severely limited by thermal
emission from the atmosphere, JWST's advantage in sensitivity will be
immense, and JWST wavelength coverage extends to 28.5 microns.
are the policies and plans for observing with JWST?
The policies for JWST 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 from the astronomical community.?? 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
but 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 JWST data to be public one year after the data is first
available to the observer, similar to HST, and that E/PO efforts will also
be similar to those for HST.? Astronomers
throughout the world will be able to request data from the JWST archive through
is JWST's lifetime?
JWST 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). JWST 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.
much does JWST cost?
The total life-cycle cost to NASA
for the JWST mission is ~$5B in real-year dollars. In addition, there are
European and Canadian contributions.
does JWST plan to finish on time and within budget?
For this large complex space
project, much new technology was needed. To mitigate risk and
minimize overall costs, the JWST Project adopted the strategy of developing
all required technology early. Studies show that increased investment
in Phase A/B results in lower risk of overrun at completion. All critical
Project technologies reached Technical Readiness Level 6 (TRL-6) as
determined by a Technology Non-Advocate Review (T-NAR) in early 2007.
This achievement was well in advance of the Spring 2008 Project-wide NAR
and transition to Phase C/D (the time at which TRL-6 is required), and
fully half a decade prior to a 2014 launch. For TRL-6, a technology
must be demonstrated to provide the needed performance in a flight-like
environment; such demonstrations typically include thermal-vacuum testing,
vibration testing, or life-cycle testing, for example.
else is JWST doing to finish on time and within budget?
The Project is minimizing costs
and mitigating risk with a "tiered integration and testing
program" like other complex space missions. By testing at individual
instrument level, integrated-instruments level, instrument-package plus optics
level, and finally at full observatory level, anomalies can be caught
earlier in mission development, when mitigation strategies are simpler and
costs are lower. Substantial hardware is already in fabrication (e.g., the
near-infrared camera, mid-infrared instrument, and fine guidance sensor
have passed their Critical Design Reviews and are building engineering test
units; the mirror segments are well along). In other words, the JWST
project is much more front-loaded with technology-development costs than
space missions that use more conventional technologies: ~49% of the total
project cost was spent in Phase A/B. The price tag for these strategies
comes early in the project, as the community saw in 2005-2006. The benefits
will come later, when the Project budget profile begins to ramp down after
will JWST launch?
The JWST Program set its launch
readiness date as June 2014 as a result of the confirmation review process,
based upon considerations including its development progress to date,
estimates of the technical challenges remaining in the work from now to
launch, and the need to maintain an acceptable level of risk in its
upcoming integration and test program. This date was approved by the NASA
Agency Program Management Council in July 2008.
NASA save money by scaling JWST down to a 4-m?
No. Blanks for the eighteen
primary mirror segments for JWST?s 6.6-m mirror have already been
manufactured and are in various stages of polishing. Changing to a
smaller diameter mirror now would require a major mission redesign with a
concomitant delay, yielding a large net cost increase. Even simply removing
several mirror segments from the current design would create problems with
stray light in the optics and the balance of mass in the spacecraft.
paid for the full-scale JWST model?
The full-scale model was built and
is supported entirely with Northrop Grumman internal funds.
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. For more details see: http://ircamera.as.arizona.edu/nircam/.
are the capabilities of NIRSpec?
The Near-Infrared Spectrograph
(NIRSpec) uses a MEMS micro-shutter array to provide simultaneous
spectroscopy of more than 100 sources over a field of view larger than 3 by
3 arcmin2. In addition to the multi-object capability, it
includes fixed slits and an integral field unit (IFU) for imaging
spectroscopy. Six gratings will yield resolving powers of R ~ 1000 and R ~
3000 in three spectral bands over 1.0 to 5.0 microns. A single prism will
yield R ~ 100 over 0.6 to 5 microns. The shutters in the microshutter array
project to 203 by 463 milliarcsec on the sky on a 267 by 528 milliarcsec
pitch. The mosaic of 4 subunits produces a final array of 730 (spectral) by
342 (spatial) shutters. The IFU consists of 30 slices, each 100
milliarcsec, over a field of view of 3 by 3 arcsec2. The NIRSpec
detector array consists of two 2048 by 2048 pixel arrays sensitive over 0.6
to 5.0 microns. The NIRSpec prism sensitivity is 132 nJy (AB=26.1) in the
continuum at 3.0 microns. The R ~ 1000 line sensitivity is 1.64 ? 10?18
erg s?1 cm?2 at 2.0 microns. These are 10 sigma in
are the capabilities of TFI?
The Tunable Filter Imager (TFI)
uses a Fabry-Perot etalon to provide narrow-band near-IR imaging over a
field of view of 2.2 by 2.2 arcmin2 with a spectral resolution
of R ~ 100. The wavelength coverage is 1.6 to 4.9 micron with a gap between
2.6 and 3.1 micron. The TFI detector is a single 2048 by 2048 pixel
detector array with 65 milliarcsec pixels. TFI features four coronagraphic occulting spots and a
non-redundant mask for high-contrast imaging applications. TFI
imaging will reach 126 nJy (AB=26.1) point source sensitivity at 3.5
microns, 10 sigma in 10,000 seconds.
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 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
the technology ready for JWST?
All the basic technologies
necessary for the mission were demonstrated in early 2007. At that time an
independent review board confirmed that they were at Technology Readiness
Level 6 (TRL-6), a year prior to the Project transition to Phase C/D when
TRL-6 is required. For TRL-6, a technology must be demonstrated to provide
the needed performance in a flight-like environment; such demonstrations
typically include thermal-vacuum testing, vibration testing, or life-cycle
testing, for example. Substantial progress on the remaining secondary items
has also occurred since (e.g., microshutter production, crycooler power
requirements). Flight detector arrays and microshutter arrays have been
selected and are in test. All the technologies required for the mission are
therefore developed to the necessary level.
is the current status of ISIM?
(Status as of November 2008)
The Integrated Science Instrument Module (ISIM) project?s current priority
is preparing for its Critical Design Review (CDR) in March 2009. All 4
science instruments plus the guider have been through their own CDRs
already. The flight science instruments begin to arrive for integration
into the ISIM in early 2010. The ISIM technical status is nominal with only
normal development issues in-work. The MIRI flight detectors and the NIRCam
short-wave flight detectors are in hand. Long-wave detectors for NIRCam,
NIRSpec, and FGS are in fabrication with no significant issues.
is the current status of the telescope?
(Status as of December 2009) There are 18 flight
JWST primary mirror segments plus 3 spares including the pathfinder
Engineering Demonstration Unit (EDU). The segment blanks were all machined
out of beryllium and light-weighted. Most of the segments have completed
fine polishing and are ready for the measurement of their cryogenic figure
at Marshall Space Flight Center (MSFC), in preparation for the final stage,
known as cryo-polishing. Upon completion of the initial polishing, the
segments are built up into a primary mirror segment assembly, with the attachment
of the mirror actuators onto the back. The mirrors are tested cryogenically
at the X-ray & Cryogenic Facility (XRCF) that was built at MSFC to test
and calibrate Chandra. The EDU has just completed it final cryo-polish and
is about to begin cryo-testing at the XRCF to verify its final performance
at cryogenic temperatures. The EDU will then be ready for its gold coating.
is the performance of the JWST detectors?
The HgCdTe detectors for JWST's
three short wavelength instruments were under development early in the
Project's history. The 2.5-micron cutoff detectors for the short wavelength
arm of NIRCam have always met the performance requirements while some tailoring
of Teledyne's processes was required to meet the dark current requirement
for the 5-micron cutoff detectors. The recent issues hindering completion
of the flight detector fabrication have not been the result of poor
performance of the detectors but rather several production problems such as
one which caused a number of dead pixels around the edges of an array. All
of these problems have now been solved.
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 built into the
flight focal plane modules by JPL, have undergone a full suite of
characterization tests, and are ready to be integrated with the rest of the
instrument. Engineering grade focal plane modules have already performed
well in the verification model MIRI instrument testing at the Rutherford
Appleton Laboratories. For approximate calculations, the measured
performance can be taken to be 50% quantum efficiency from 5 to 24 microns,
falling linearly to zero from 24 to 29 microns, 30 electron read noise
(single read), and 0.1 electrons per second dark current.
this complex spacecraft really work?
JWST requires several new
technologies, but these have been validated by testing and external review
fully six years prior to launch. The program has a three-year integration
and test (I&T) plan. 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 JWST 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).
the thermal design really work?
The JWST 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 JWST. The Spitzer outer shell ? which is analogous to the cold
part of JWST ? 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 JWST 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
JWST 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 JWST thermal
design and testing approach.
astronauts be able to service JWST like they did HST?
Unlike HST, which orbits roughly
350 miles above the surface of Earth and is therefore accessible by the
Space Shuttle, JWST 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
JWST mission design does not rely upon a servicing option.
is JWST 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. JWST 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. Stress
analyses of HRGs show this design has a "mean time before
failure" of 10 million hours. As of June 2006, this type of device had
accumulated more than 7 million hours of continuous operation in space
without a failure. This new technology 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).
big is the JWST mirror?
The JWST 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 JWST primary mirror consists
of 18 hexagonal segments with three different prescriptions.
does the collecting area of JWST compare to HST?
HST has a 2.4 m diameter round
primary mirror. For ACS and STIS, the central obscuration by the secondary
is 0.33r, where r is the 1.2 m radius. 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 HST?s mirror is π
(1.22) (1-0.332) = 4.0 m2. Therefore, the
ratio between the 25.0 m2 JWST mirror and the HST mirror is
JWST Integration and Testing
is JWST being tested?
JWST will be tested incrementally
during its construction, starting with individual mirrors and instruments
(including cameras and spectrometers) and building up to the full
observatory. JWST'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 one of the JSC chambers. 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.
is JWST 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, JWST is not easily
serviceable; thus JWST must be done right. The challenge has been to
design a test strategy that assures success but is also
affordable. The JWST 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 JWST active optics. This
final test will assure that JWST can be aligned on-orbit, making the test
cost effective yet retaining adequate redundancy and accuracy to detect any
is this testing strategy optimal for JWST?
JWST must operate at very cold
temperatures so that its mirrors' and instruments' self-generated heat will
not swamp its sensitive infrared sensors. At the same time, JWST is being
designed to have optical performance that is significantly more challenging
than that required by the Spitzer telescope. Lessons learned from Spitzer,
as well as COBE, WMAP, and other cryogenic and comparable optical
telescopes, led the JWST team to design a test flow that is optimized for
the cold performance. This approach uses testing at incremental levels of
assembly, in order to minimize the chance the telescope will have to be
disassembled to fix problems late in the development (when it would be
time-consuming and expensive). The lessons also led to a final test
configuration in which minimal hardware is inside of the helium shroud used
to cool the telescope. Also, the test configuration ? with the primary
mirror ?cup up? ? eliminates the need for a large, heavy tower and permits
easier transportation of the telescope in and out of the chamber (via a
large side door).
is this testing strategy cost-effective?
The overall JWST 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 JWST 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.
lessons has JWST learned from past missions?
Many lessons were learned from
building UV, optical, infrared and X-ray missions like HST, Spitzer and
Chandra, including a key aspect of the JWST 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 most robust cold-testing strategy,
including early testing, a ?cup-up? design, keeping as much of the testing
hardware as possible external to the helium shroud, and designing the
testing equipment to be relatively accessible during the test period.
JWST and NASA Programmatic Issues
it be better to fly a number of smaller missions instead of one big
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. JWST 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.
JWST be reviewed in the Decadal Survey?
The formal answer to this question
is ?No? as the guidelines for the survey remove from reprioritization those
missions in development. ?In development? means in phase C, having passed
PDR and been confirmed, which happened to JWST in 2008. However, JWST will
be a major component of the NASA program in the next decade, and we expect
that the Decadal Survey report will discuss JWST and its role in astronomy.
How the community plans to use JWST might also be discussed.
does the astronomical community provide feedback on JWST?
Community input to JWST 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 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 JWST to the NASA Advisory Council. Finally,
as the operations phase of the JWST mission approaches, the Space Telescope
Science Institute will convene a Users Committee to advise on operations
will be the successor to JWST?
NASA expects to follow the
priorities recommended by the Decadal Survey for future missions.