NASA’s Roman Space Telescope’s Wide Field Instrument mosaic detector array of 18 H4RG-10 chips requires regular and uncommonly accurate calibrations to meet its science objectives. In addition to the quasi-Lambertian diffuser used for detector flat-fielding, a novel engineered diffuser is incorporated as part of seven cold stop masks on the science bandpass filters. These engineered diffusers are used to illuminate the focal plane concurrently with an exposure from the observatory, enabling signal-dependent nonlinearity corrections. This presentation demonstrates via experimental data how these diffusers can produce a spatially uniform and smooth illumination profile with increased flux compared to a Lambertian diffuser.
The NIRSpec instrument on the James Webb Space Telescope (JWST) brings the first multi-object spectrograph (MOS) into space, enabled by a programmable Micro Shutter Array (MSA) of ∼250,000 individual apertures. During the 6-month Commissioning period, the MSA performed admirably, completing ∼800 reconfigurations with an average success rate of ∼96% for commanding shutters open in science-like patterns. We show that 82.5% of the unvignetted shutter population is usable for science, with electrical short masking now the primary cause of inoperable apertures. In response, we propose a plan to recheck existing shorts during nominal operations, which is expected to reduce the number of affected shutters. We also present a full assessment of the Failed Open and Failed Closed shutter populations, which both show a marginal increase in line with predictions from ground testing. We suggest an amendment to the Failed Closed shutter flagging scheme to improve flexibility for MSA configuration planning. Overall, the NIRSpec MSA performed very well during Commissioning, and the MOS mode was declared ready for science operations on schedule.
Commissioning the Webb telescope to realize the observatory’s full capability necessitated the development of robust wavefront sensing and control processes. These processes rely on techniques that were adapted or newly innovated for the mission, and further adaptation of these techniques may be expected for future segmented telescopes. Over the course of mission development, these techniques were refined to form a baseline wavefront commissioning plan that assumes several conditions and performance requirements are met. Herein we present efforts carried out to define and develop contingency concepts of operation for Webb telescope commissioning, and the mission-level approach to managing the response to deviations from the baseline plan in the event of significant off-nominal or anomaly scenarios encountered by the wavefront team. An overview of selected contingencies is presented along with more detailed example model cases and instances of interest encountered in flight.
The James Webb Space Telescope (JWST) is a segmented deployable telescope, currently operating at L2. The telescope utilizes 6 degrees of freedom for adjustment of the Secondary Mirror (SM) and 7 degrees of freedom for adjustment of each of its 18 segments in the Primary Mirror (PM). After deployment, the PM segments and the SM arrived in their correct optical positions to within a ~1 mm, with accordingly large wavefront errors. A Wavefront Sensing and Controls (WFSC) process was executed to adjust each of these optical elements in order to correct the deployment errors and produce diffraction-limited images across the entire science field. This paper summarizes the application of the WFSC process.
JWST is a world class space observatory that will add profound knowledge of the universe that Hubble began. Within the observatory there are four scientific instruments and a precision guider. During the commissioning phase,1 the primary instrument that will be used to align the mirrors and measure pupil shear will be the Near InfraRed Camera (NIRCam). The NIRCam instrument has two modules each including a focal plane positioned at the final image plane of the instrument. NIRCam2 can also use its focal planes to image the OTE pupil by using a deployable lens known as the Pupil Imaging Lens (PIL). When the PIL is deployed, NIRCam can image the pupil with the focal plane and the data can be analyzed to determine the amount of pupil shear between the Optical Telescope Element (OTE) and NIRCam. At various points in the commissioning process, the PIL is deployed and the alignment between NIRCam and the OTE is measured. Other commissioning activities measure the relative photometry and infer whether the signal is vignetted. Like NIRCam, MIRI can measure pupil shear of the system. MIRI, the Mid-Infrared Instrument comprises both a wide-field imager (5-28μm) and a medium resolution spectrograph (R 1500 to 3500)3 Unlike NIRCam which uses a deployable lens to measure the pupil, the Imager channel uses a filter wheel located at the pupil plane between the MIRI pick-off mirror and the imager detector. It has 18 filters and mask elements which can be selectively commanded into the science path. One of these elements, called the ALignment Lens (AL) or Filterwheel Lens (FLENS) projects a shadow onto the MIRI imager focal plane. The OTE shadow through the FLENS allows MIRI to serve as a cross-check to NIRCAm on pupil shear. This paper details the MIRI pupil shear analysis process used by the JWST team throughout commissioning.
Launched December 25th 2021 to its final destination at the Sun-Earth Lagrange Point 2, JWST will revolutionize the way the universe is seen back to the early universe beginnings. During the six month commissioning phase of the mission, there were various activities performed which aligned the segmented telescope from eighteen unique Point Spread Functions (PSFs) to a single PSF. One of these steps is to globally align the Secondary Mirror (SM) Assembly (SMA) to the 18 Primary Mirror Segment Assemblies (PMSA) in the primary mirror. This alignment process happens in two phases, and they are named Global Alignment 1 (GA1) and Global Alignment 2 (GA2). A successful completion of GA1 entails coarsely aligning the SM and each PMSA to achieve an RMS Wavefront Error (WFE) less than 200nm. This is accomplished by generating PMSA wavefront maps which are used to determine a correction in the SM using Phase Retrieval and Decomposition. One of the requirements is that the Fine Guidance Sensor (FGS) can guide on a segment during the exposures, minimizing the impacts of motion blur on the images. This paper details a contingency method on how global alignment can be achieved using the observatories star trackers for guiding. The star trackers line of sight (LOS) stability and pointing stability performance are intended for coarse guiding whereas FGS provides fine guiding. It is expected that motion blur will increase should the observatory use coarse guiding during the GA1 exposures. Within the paper, we detail the maximum motion thresholds needed to achieve the desired placement of the SM, as well as exposure methods which can be used to handle motions that exceed the thresholds.
This paper shows that a simple convolutional neural network (CNN) can be used to build an object-agnostic wavefront sensor. Using the well-known Phase Diversity approach as a point of departure, Fourier-space metrics are computed from the conventional and diversity images and then fed to the CNN, which predicts values of the underlying Zernike coefficients. The methodology is shown to work in the presence of Gaussian noise. Prediction errors for defocus, astigmatism, and spherical are on the order of 1/100 of the wavelength.
KEYWORDS: James Webb Space Telescope, Optical components, Space telescopes, Optical testing, Sensors, Calibration, Data modeling, Human-machine interfaces, Error analysis, Analytical research
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.
Maurice Te Plate, Stephan Birkmann, Marco Sirianni, Peter Rumler, Peter Jensen, Ralf Ehrenwinkler, Peter Mosner, Hermann Karl, Robert Rapp, Ray Wright, Rai Wu
KEYWORDS: Spectrographs, James Webb Space Telescope, Camera shutters, Near infrared, Staring arrays, Sensors, Space telescopes, Acoustics, Cryogenics, Neodymium
The James Webb Space Telescope (JWST) Observatory is the follow-on mission to the Hubble Space Telescope (HST). JWST will be the biggest space telescope ever built and it will lead to astounding scientific breakthroughs. The mission will be launched in October 2018 from Kourou, French Guyana by an ESA provided Ariane 5 rocket. NIRSpec, one of the four instruments on board of the mission, recently underwent a major upgrade. New infrared detectors were installed and the Micro Shutter Assembly (MSA) was replaced as well. The rework was necessary because both systems were found to be degrading beyond a level that could be accepted. Now in its final flight configuration, NIRSpec underwent a final cryogenic performance test at NASA’s Goddard Space Flight Center (GSFC) as part of the Integrated Science Instrument Module (ISIM). This paper will present a status overview and results of the recent test campaigns.
KEYWORDS: Space telescopes, James Webb Space Telescope, Aerospace engineering, Camera shutters, Sensors, Staring arrays, Electroluminescent displays, Acoustics, Instrument modeling, Spectroscopy
The Near-Infrared Spectrograph (NIRSpec) is one of the four instruments on the James Webb Space Telescope (JWST) which is scheduled for launch in 2018. NIRSpec is developed by the European Space Agency (ESA) with Airbus Defense and Space Germany as prime contractor. The instrument offers seven dispersers covering the wavelength range from 0.6 to 5.3 micron with resolutions from R ∼ 100 to R ∼ 2700. NIRSpec will be capable of obtaining spectra for more than 100 objects simultaneously using an array of micro-shutters. It also features an integral field unit with 3” x 3” field of view and a range of slits for high contrast spectroscopy of individual objects and time series observations of e.g. transiting exoplanets. NIRSpec is in its final flight configuration and underwent cryogenic performance testing at the Goddard Space Flight Center in Winter 2015/16 as part of the Integrated Science Instrument Module (ISIM). We present the current status of the instrument and also provide an update on NIRSpec performances based on results from the ISIM level test campaign.
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