The performance of an optical system is best characterized by either the point spread function (PSF) or the optical
transfer function (OTF). However, for system budgeting purposes, it is convenient to use a single scalar metric, or a
combination of a few scalar metrics to track performance. For the James Webb Space Telescope, the Observatory
level requirements were expressed in metrics of Strehl Ratio, and Encircled Energy. These in turn were converted to
the metrics of total rms WFE and rms WFE within spatial frequency domains. The 18 individual mirror segments
for the primary mirror segment assemblies (PMSA), the secondary mirror (SM), tertiary mirror (TM), and Fine
Steering Mirror have all been fabricated. They are polished beryllium mirrors with a protected gold reflective
coating. The statistical analysis of the resulting Surface Figure Error of these mirrors has been analyzed. The
average spatial frequency distribution and the mirror-to-mirror consistency of the spatial frequency distribution are
reported. The results provide insight to system budgeting processes for similar optical systems.
JWST optical component in-process optical testing and cryogenic requirement compliance certification, verification &
validation is probably the most difficult metrology job of our generation in astronomical optics. But, the challenge has
been met: by the hard work of dozens of optical metrologists; the development and qualification of multiple custom test
setups; and several new inventions, including 4D PhaseCam and Leica Absolute Distance Meter. This paper summarizes
the metrology tools, test setups and processes used to characterize the JWST optical components.
NASA's Technology Readiness Level (TRL)-6 is documented for the James Webb Space Telescope (JWST) Wavefront
Sensing and Control (WFSC) subsystem. The WFSC subsystem is needed to align the Optical Telescope Element
(OTE) after all deployments have occurred, and achieves that requirement through a robust commissioning sequence
consisting of unique commissioning algorithms, all of which are part of the WFSC algorithm suite. This paper identifies
the technology need, algorithm heritage, describes the finished TRL-6 design platform, and summarizes the TRL-6 test
results and compliance. Additionally, the performance requirements needed to satisfy JWST science goals as well as the
criterion that relate to the TRL-6 Testbed Telescope (TBT) performance requirements are discussed.
The one-meter Testbed Telescope (TBT) has been developed at Ball Aerospace to facilitate the
design and implementation of the wavefront sensing and control (WFSC) capabilities of the
James Webb Space Telescope (JWST). We have recently conducted an "end-to-end"
demonstration of the flight commissioning process on the TBT. This demonstration started with
the Primary Mirror (PM) segments and the Secondary Mirror (SM) in random positions,
traceable to the worst-case flight deployment conditions. The commissioning process detected
and corrected the deployment errors, resulting in diffraction-limited performance across the
entire science FOV. This paper will describe the commissioning demonstration and the WFSC
algorithms used at each step in the process.
The primary mirror of the James Webb Space Telescope (JWST) consists of 18 segments and is 6.6 meters in diameter.
A sequence of commissioning steps is carried out at a single field point to align the segments. At that single field point,
though, the segmented primary mirror can compensate for aberrations caused by misalignments of the remaining
mirrors. The misalignments can be detected in the wavefronts of off-axis field points. The Multifield (MF) step in the
commissioning process surveys five field points and uses a simple matrix multiplication to calculate corrected positions
for the secondary and primary mirrors. A demonstration of the Multifield process was carried out on the JWST Testbed
Telescope (TBT). The results show that the Multifield algorithm is capable of reducing the field dependency of the TBT
to about 20 nm RMS, relative to the TBT design nominal field dependency.
The one-meter Testbed Telescope (TBT) has been developed at Ball Aerospace to facilitate the
design and implementation of the wavefront sensing and control (WFS&C) capabilities of the
James Webb Space Telescope (JWST). The TBT is used to develop and verify the WFS&C
algorithms, check the communication interfaces, validate the WFS&C optical components and
actuators, and provide risk reduction opportunities for test approaches for later full-scale
cryogenic vacuum testing of the observatory. In addition, the TBT provides a vital opportunity
to demonstrate the entire WFS&C commissioning process. This paper describes recent WFS&C
commissioning experiments that have been performed on the TBT.
Prior to launch, the Spitzer Space Telescope (SST) secondary focus mechanism was set to a predicted desired in-orbit focus value. This predicted setting, determined from double-pass cold chamber measurements and calculated ground-to-orbit corrections, had an uncertainty greater than the required in-orbit focus accuracy. Because of concern about the potential for failure in a cryogenic mechanism affecting all Spitzer instruments, it was required that any focus correction be made in a set of moves directly from the initial to the desired setting. The task of determining the required focus moves fell to IRAC (Infrared Array Camera), the instrument most affected by and sensitive to defocus. To determine the focus directly from examining images at a fixed focus, we developed two methods, "Simfit" and "Focus Diversity" (W. F. Hoffmann, et. al.1). Simfit finds the focus by obtaining the best match between observed images and families of simulated images at a range of focus settings. Focus Diversity utilizes the focal plane curvature to find the best fit of the varied image blur over the focal plane to a model defocus curve. Observations of a single star at many field locations in each of the four IRAC bands were analyzed before and during the refocus activity. The resulting refocus moves brought the focus close to the specified requirement of within 0.3 mm from the desired IRAC optimum focus. This is less than a "Diffraction Focus Unit" (λx(f/2)) of 0.52 mm at the SST focus at the shortest IRAC band (3.58 microns). The improvement in focus is apparent in both the appearance and the calculated noise-pixels of star images.
We describe the process by which the NASA Spitzer Space Telescope (SST) Cryogenic Telescope Assembly (CTA) was brought into focus after arrival of the spacecraft in orbit. The ground rules of the mission did not allow us to make a conventional focus sweep. A strategy was developed to determine the focus position through a program of passive imaging during the observatory cool-down time period. A number of analytical diagnostic tools were developed to facilitate evaluation of the state of the CTA focus. Initially, these tools were used to establish the in-orbit focus position. These tools were then used to evaluate the effects of an initial small exploratory move that verified the health and calibration of the secondary mirror focus mechanism. A second large move of the secondary mirror was then commanded to bring the telescope into focus. We present images that show the CTA Point Spread Function (PSF) at different channel wavelengths and demonstrate that the telescope achieved diffraction limited performance at a wavelength of 5.5 μm, somewhat better than the level-one requirement.
The Multiband Imaging Photometer for Spitzer (MIPS) provides long wavelength capability for the mission, in imaging bands at 24, 70, and 160 microns and measurements of spectral energy distributions between 52 and 100 microns at a spectral resolution of about 7%. By using true detector arrays in each band, it provides both critical sampling of the Spitzer point spread function and relatively large imaging fields of view, allowing for substantial advances in sensitivity, angular resolution, and efficiency of areal coverage compared with previous space far-infrared capabilities. The Si:As BIB 24 micron array has excellent photometric properties, and measurements with rms relative errors of 1% or better can be obtained. The two longer wavelength arrays use Ge:Ga detectors with poor photometric stability. However, the use of 1.) a scan mirror to modulate the signals rapidly on these arrays, 2.) a system of on-board stimulators used for a relative calibration approximately every two minutes, and 3.) specialized reduction software result in good photometry with these arrays also, with rms relative errors of less than 10%.
The James Webb Space Telescope (JWST) Secondary Mirror (SM) is a 738 mm edge-diameter convex hyperbola that will be operating at 30K. Due to JWST’s science and technical requirements, the requirements on the SM are relatively tight. Therefore highly accurate, rigorous cryogenic testing of the surface figure as well as the prescription is required. The optical testing of a convex mirror of this size has not been performed before at cryogenic temperatures. This paper discusses the testing approaches and configurations that are under consideration at Ball Aerospace & Technologies Corp. (BATC) for testing the JWST SM at cryogenic temperatures.
This paper describes the "End to End" optical test conducted on the Space InfraRed Telescope Facility (SIRTF) Cryogenic Telescope Assembly (CTA) in 2001. It was critical to verify SIRTF's optical functionality and quality under optical and thermal conditions that as much as possible simulated the flight environment. The Liquid Nitrogen cooled "Brutus" chamber at Ball Aerospace was the test facility. Flight-like self cooling, thermal blanketing, and auxiliary cooling loops allowed the assembly to reach temperatures close to orbital conditions. (25-5K) Introducing optical sources at the SIRTF focal plane allowed the telescope to perform as the collimating source. A motorized and cryogenically characterized reflection flat was used to direct the refocused images of test sources to visible and IR focal planes in SIRTF's Multi-Instrument Chamber. A sequence of tests was performed to gather data on system focus position, image stability, telescope wavefront and instrument assembly confocality.
This paper describes the principal optical results of the "End to End" test conducted on the SIRTF Cryogenic Telescope Assembly. Test system focus was located using images from the shortest wavelength science instrument, IRAC, much as it will be on-orbit. Deep out-of-focus images were used to determine the system wavefront by Phase Retrieval methods with heritage to Hubble Space Telescope work. This work has been used to update the SIRTF optical models and aid in predicting the on-orbit performance of the observatory. Images made with other assemblies able to observe in the test (IRS, PCRS) were used to verify their function and co-focus to the IRAC established position. Image jitter was analyzed warm and cold, with visible images captured by the PCRS instrument and cold, with images captured by the IRAC instrument.
We describe the test approaches and results for the Multiband Imaging Photometer for SIRTF. To verify the performance within a `faster, better, cheaper' budget required innovations in the test plan, such as heavy reliance on measurements with optical photons to determine instrument alignment, and use of an integrating sphere rather than a telescope to feed the completed instrument at its operating temperature. The tests of the completed instrument were conducted in a cryostat of unique design that allowed us to achieve the ultra-low background levels the instrument will encounter in space. We controlled the instrument through simulators of the mission operations control system and the SIRTF spacecraft electronics, and used cabling virtually identical to that which will be used in SIRTF. This realistic environment led to confidence in the ultimate operability of the instrument. The test philosophy allowed complete verification of the instrument performance and showed it to be similar to pre-integration predictions and to meet the instrument requirements.
The Multiband Imaging Photometer for SIRTF (MIPS) provides the space IR telescope facility (SIRTF) with imaging, photometry, and total power measurement capability in broad spectral bands centered at 24, 70, and 160 micrometers , and with low resolution spectroscopy between 50 and 95 micrometers . The optical train directs the light from three zones in the telescope focal plane to three detector arrays: 128 by 128 Si:As BIB, 32 by 32 Ge:Ga, and 2 by 20 stressed Ge:Ga. A single axis scan mirror is placed at a pupil to allows rapid motion of the field of view as required to modulate above the 1/f noise in the germanium detectors. The scan mirror also directs the light into the different optical paths of the instrument and makes possible an efficient mapping mode in which the telescope line of sight is scanned continuously while the scan mirror freezes the image motion on the detector arrays. The instrument is designed with pixel sizes that oversample the telescope Airy pattern to operate at the diffraction limit and, through image processing, to allow superresolution beyond the traditional Rayleigh criterion. The instrument performance and interface requirements, the design concept, and the mechanical, optical, thermal, electrical, software, and radiometric aspects of MIPS are discussed in this paper. Solutions are shown to the challenge of operating the instrument below 3K, with focal plane cooling requirements done to 1.5K. The optical concept allows the versatile operations described above with only a single mechanism and includes extensive self-test and on- board calibration capabilities. In addition, we discuss the approach to cryogenic end-to-end testing and calibration prior to delivery of the instrument for integration into SIRTF.
The anomalous motion of the near IR camera and multi-object spectrometer (NICMOS) detector arrays was originally discovered and characterized during ground optical testing, in a large, high fidelity Hubble Space Telescope (HST) simulator. To monitor the state of the cryo-mechanical system, as NICMOS traveled among several testing sties, a portable stimulus was needed. The cold-well displacement monitor (CDM) was quickly assembled from a very simple design. The 'cheaper, better, faster' approach proved to be a winner here. Off-the-shelf optics, a simplified interface to the instrument, and a limited set of requirements were used. After calibration against the large refractive aberration simulator/Hubble opto-mechanical simulator (RAS/HOMS), the CDM gave results of similar accuracy to RAS/HOMS. It became the primary tool for the difficult job of managing the NICMOS cryogen system up through launch.