NASA’s James Webb Space Telescope (JWST) is a 6.5m diameter, segmented, deployable telescope for cryogenic IR space astronomy. The JWST Observatory includes the Optical Telescope Element (OTE) and the Integrated Science Instrument Module (ISIM), that contains four science instruments (SI) and the Fine Guidance Sensor (FGS). The SIs are mounted to a composite metering structure. The SIs and FGS were integrated to the ISIM structure and optically tested at NASA's Goddard Space Flight Center using the Optical Telescope Element SIMulator (OSIM). OSIM is a full-field, cryogenic JWST telescope simulator. SI performance, including alignment and wavefront error, was evaluated using OSIM. We describe test and analysis methods for optical performance verification of the ISIM Element, with an emphasis on the processes used to plan and execute the test. The complexity of ISIM and OSIM drove us to develop a software tool for test planning that allows for configuration control of observations, implementation of associated scripts, and management of hardware and software limits and constraints, as well as tools for rapid data evaluation, and flexible re-planning in response to the unexpected. As examples of our test and analysis approach, we discuss how factors such as the ground test thermal environment are compensated in alignment. We describe how these innovative methods for test planning and execution and post-test analysis were instrumental in the verification program for the ISIM element, with enough information to allow the reader to consider these innovations and lessons learned in this successful effort in their future testing for other programs.
The James Webb Space Telescope (JWST) relies on several innovations to complete its five year mission. One vital
technology is microshutters, the programmable field selectors that enable the Near Infrared Spectrometer (NIRSpec) to
perform multi-object spectroscopy. Mission success depends on acquiring spectra from large numbers of galaxies by
positioning shutter slits over faint targets. Precise selection of faint targets requires field selectors that are both high in
contrast and stable in position. We have developed test facilities to evaluate microshutter contrast and alignment stability
at their 35K operating temperature. These facilities used a novel application of image registration algorithms to obtain
non-contact, sub-micron measurements in cryogenic conditions. The cryogenic motion of the shutters was successfully
characterized. Optical results also demonstrated that shutter contrast far exceeds the NIRSpec requirements. Our test
program has concluded with the delivery of a flight-qualified field selection subsystem to the NIRSpec bench.
The Detector Characterization Laboratory at NASA/GSFC has investigated the reciprocity failure characteristics of
1.7μm cut-off HgCdTe devices provided by Teledyne Imaging Sensors to the Hubble Space Telescope (HST) Wide
Field Camera 3 (WFC3) project. The reciprocity failure follows a power law behavior over the range of fluxes tested
(0.1-104 photons/second). The slope of the power law varies among detectors, ranging from ~0.3-1%/dex at 1.0μm,
which is much smaller than the ~6%/dex effect observed with the HST NICMOS 2.5μm cut-off detectors. In addition,
the reciprocity failure exhibits no wavelength dependence, although only a restricted range of wavelengths (0.85-1.0μm)
has been explored to date. Despite its relatively small magnitude, reciprocity failure is nevertheless an important effect in
the calibration of WFC3 data, as well as in other applications in which there is a large difference in flux between the
photometric standards and the scientific sources of interest.
In ground testing of the Hubble Space Telescope Wide Field Camera 3 (HST/WFC3), the CCDs of its UV/visible channel exhibited an unanticipated quantum efficiency hysteresis (QEH) behavior. The QEH first manifested itself as an occasionally observed contrast in response across the format of the CCDs, with an amplitude of typically 0.1-0.2% or less at the nominal -83°C operating temperature, but with contrasts of up to 3-5% observed at warmer temperatures. The behavior has been replicated in the laboratory using flight spare detectors and has been found to be related to an initial response deficiency of ~5% amplitude when the CCDs
are cooled with no illumination. A visible light flat-field (540nm) with a several times full-well signal level is found to pin the detector response at both optical (600nm) and near-UV (230nm) wavelengths, suppressing the QEH behavior. We have characterized the timescale for the detectors to become unpinned (days for significant
response loss at -83°C and have developed a protocol to stabilize the response in flight by flashing the WFC3 CCDs with the instrument's internal calibration system.
The Wide-field Camera 3 (WFC3) is a fourth-generation instrument planned for installation in Hubble Space Telescope
(HST). Designed as a panchromatic camera, WFC3's UVIS and IR channels will complement the other instruments onboard
HST and enhance the observatory's scientific performance. UVIS images are obtained via two 4096×2051 pixel
e2v CCDs while the IR images are taken with a 1024×1024 pixel HgCdTe focal plane array from Teledyne Imaging
Sensors. Based upon characterization tests performed at NASA/GSFC, the final flight detectors have been chosen and
installed in the instrument. This paper summarizes the performance characteristics of the WFC3 flight detectors based
upon component and instrument-level testing in ambient and thermal vacuum environments.
Microshutter arrays are one of the novel technologies developed for the James Webb Space Telescope (JWST).
It will allow Near Infrared Spectrometer (NIRSpec) to acquire spectra of hundreds of objects simultaneously
therefore increasing its efficiency tremendously. We have developed these programmable arrays that are based
on Micro-Electro Mechanical Structures (MEMS) technology. The arrays are 2D addressable masks that can
operate in cryogenic environment of JWST. Since the primary JWST science requires acquisition of spectra
of extremely faint objects, it is important to provide very high contrast of the open to closed shutters. This
high contrast is necessary to eliminate any possible contamination and confusion in the acquired spectra by
unwanted objects. We have developed and built a test system for the microshutter array functional and optical
characterization. This system is capable of measuring the contrast of the mciroshutter array both in visible and
infrared light of the NIRSpec wavelength range while the arrays are in their working cryogenic environment. We
have measured contrast ratio of several microshutter arrays and demonstrated that they satisfy and in many
cases far exceed the NIRSpec contrast requirement value of 2000.
The Space Telescope Imaging Spectrograph (STIS) operates from the UV to near IR providing a general purpose, imaging spectroscopic capability. An internal, two mirror relay system corrects the spherical aberration and astigmatism present at the STIS field position. Low and medium resolution imaging spectroscopy is possible throughout the spectral range and over the 25 arcsecond UV and 52 arcsecond visible fields. High resolution echelle spectroscopy capability is also provided in the UV. Target acquisition is accomplished using the STIS cameras, either UV or visible; these cameras may also be used to provide broad band imaging over the complete spectral range or with the small selection of available bandpass filters. A wide selection of slits and apertures permit various combinations of spectral resolution and field size in all modes. On board calibration lamps provide wavelength calibration and flat fielding capability. We report here on the optical performance of STIS as determined during orbital verification.
The Space Telescope Imaging Spectrograph (STIS) is a second- generation instrument for the Hubble Space Telescope (HST), designed to cover the 115-1000 nm wavelength range in a versatile array of spectroscopic and imaging modes that take advantage of the angular resolution, unobstructed wavelength coverage, and dark sky offered by the HST. STIS was successfully installed into HST in 1997 February and has since completed a year of orbital checkout, capabilities that it brings to HST, illustrate those capabilities with examples drawn from the first year of STIS observing, and describe at a top level the on-orbit performance of the STIS hardware. We also point the reader to related papers that describe particular aspects of the STIS design, performance, or scientific usage in more detail.
The Space Telescope Imaging Spectrograph (STIS), installed into the Hubble Space Telescope (HST) during the second servicing mission (SSM) in February 1197, has undergone the required servicing mission orbital verification (SMOV). The overall sensitivity of STIS is summarized for direct imagery in the visible with the charge coupled device (CCD), the near UV multi-anode microchannel-array (NUV MAMA) and the far UV MAMA (FUV MAMA) detectors and likewise for the spectroscopic modes. The FUV MAMA has exceedingly low background. The NUV MAMA has a higher, temperature-dependent background due to window phosphorescence. The principle gains of the CCD over WFPCs for limiting imaging sensitivity are: high quantum efficiency, wide bandpass, low dark current and low readout-noise. The CCD, like the WFPC2 CCDs, must ge annealed periodically to heat the hot pixels generated by radiation hits. Throughput of all modes has been stable at the 1 percent level or better except for the far UV, where sensitivity is dropping slowly across the order, but more rapidly below the Lyman alpha, and beyond 150 nm. This loss in sensitivity may be due to contamination similar to that which affected the first generation HST instruments. The thermal environment for STIS is warmer than specified in the HST Interface Control Document with the result that the back end of the STIS optical bench is not under positive thermal control. Temperature swings occur due to the spacecraft solar orientation and also due to power cycling of the MAMA low voltage power supplies that are turned off during orbits that encounter the South Atlantic Anomaly. Some motion of spectral and direct image formats occurs on the detector that is correlatable with changing aft bulkhead temperature and changes in external heatloads. The MAMA detectors are capable of time-tagging photon events within 125 microsecond resolution. The Crab Pulsar was used as a time standard and demonstrates the desired performance.