Brent Mott

Detectors for the James Webb Space Telescope Near‐Infrared Spectrograph. I. Readout Mode, Noise Model, and Calibration Considerations

We describe how the James Webb Space Telescope (JWST) Near-Infrared Spectrographs (NIRSpec) detectors will be read out, and present a model of how noise scales with the number of multiple nondestructive reads sampling up the ramp. We believe that this noise model, which is validated using real and simulated test data, is applicable to most astronomical near-infrared instruments. We describe some nonideal behaviors that have been observed in engineering-grade NIRSpec detectors, and demonstrate that they are unlikely to affect NIRSpec sensitivity, operations, or calibration. These include a HAWAII-2RG reset anomaly and random telegraph noise (RTN). Using real test data, we show that the reset anomaly is (1) very nearly noiseless and (2) can be easily calibrated out. Likewise, we show that large-amplitude RTN affects only a small and fixed population of pixels. It can therefore be tracked using standard pixel operability maps.

Detector arrays for the James Webb Space Telescope near-infrared spectrograph

The James Webb Space Telescopes (JWST) Near Infrared Spectrograph (NIRSpec) incorporates two 5 μm cutoff (λco =5 μm) 2048×2048 pixel Teledyne HgCdTe HAWAII-2RG sensor chip assemblies. These detector arrays, and the two Teledyne SIDECAR application specific integrated circuits that control them, are operated in space at T ~ 37 K. In this article, we provide a brief introduction to NIRSpec, its detector subsystem (DS), detector readout in the space radiation environment, and present a snapshot of the developmental status of the NIRSpec DS as integration and testing of the engineering test unit begins.

Microshutter array system for James Webb Space Telescope

We have developed microshutter array systems at NASA Goddard Space Flight Center for use as multi-object aperture arrays for a Near-Infrared Spectrometer (NIRSpec) instrument. The instrument will be carried on the James Webb Space Telescope (JWST), the next generation of space telescope, after the Hubble Space Telescope retires. The microshutter arrays (MSAs) are designed for the selective transmission of light from objected galaxies in space with high efficiency and high contrast. Arrays are close-packed silicon nitride membranes with a pixel size close to 100x200 μm. Individual shutters are patterned with a torsion flexure permitting shutters to open 90 degrees with minimized stress concentration. In order to enhance optical contrast, light shields are made on each shutter to prevent light leak. Shutters are actuated magnetically, latched and addressed electrostatically. The shutter arrays are fabricated using MEMS bulk-micromachining and packaged utilizing a novel single-sided indium flip-chip bonding technology. The MSA flight system consists of a mosaic of 2 x 2 format of four fully addressable 365 x 171 arrays. The system will be placed in the JWST optical path at the focal plane of NIRSpec detectors. MSAs that we fabricated passed a series of qualification tests for flight capabilities. We are in the process of making final flight-qualified MSA systems for the JWST mission.

Individual photon counting using e2v L3 CCDs for low background astronomical spectroscopy

Sensitive, photon counting array detectors have the potential to dramatically improve the sensitivity of space-based astronomical spectrographs. We present first results from a program evaluating e2v L3 electron-multiplying CCDs as photon counting arrays. We find that L3 CCDs function well as photon counters, and see no show stoppers for our target applications. These include both ground and space-based instruments. Although we do detect spurious charge exceeding the dark current floor of the CCD, we find that physical dark current in the multiplication register is a significant component. This finding is significant because dark current, unlike clock induced charge (another potential culprit), is a problem that CCD designers have solved before.

Spectral contrast enhancement techniques for extrasolar planet imaging

We use analytical models to characterize the optical response of observing systems and explore spectroscopic techniques that exploit the planetary spectral signature to enhance the planet-to-star contrast in imaging. Radiative transfer model calculations of gas-giant extrasolar planets are employed in selecting spectral regions where the planetary signal shows enhancement with respect to the stellar spectrum. We specifically discuss the use of frequency switching, in conjunction with pupil plane techniques (shaping, apodizing), to optimize the detection of the planetary signal of potential planetary systems for both ground-based and space-borne observations. Expected properties of known extrasolar planets motivate the use of the near-IR spectral region. This work was motivated by the focused technology development of a MEMS tunable filter, and the application of such devices to frequency-switched imaging spectroscopy.

Laboratory and sky testing results for the TIS H4RG-10 4k x 4k 10-micron visible CMOS-hybrid detector

We present both laboratory and telescope testing results describing the performance of the H4RG-10 CMOS-Hybrid detector. The H4RG-10 is the largest visible hybrid array currently in existence and shows great potential for use in future space missions. We report read noise, dark current, pixel connectivity, persistence, and inter-pixel capacitance measurements for the temperature range 110-240 K. We report on quantitative astrometric and qualitative photometric performance of the instrument based on observations made at USNOs Flagstaff Station observatory and establish an upper limit to the astrometric performance of the detector. We discuss additional testing and future work associated with improving detector performance.

Complex MEMS device: microshutter array system for space applications

A complex MEMS device, microshutter array system, is being developed at NASA Goddard Space Flight Center for use as an aperture array for a Near-Infrared Spectrometer (NirSpec). The instrument will be carried on the James Webb Space Telescope (JWST), the next generation of space telescope after Hubble Space Telescope retires. The microshutter arrays (MSAs) are designed for the selective transmission of light with high efficiency and high contrast. Arrays are close-packed silicon nitride membranes with a pixel size close to 100x200 &mgr;m. Individual shutters are patterned with a torsion flexure permitting shutters to open 90 degrees with a minimized mechanical stress concentration. Light shields are made on to each shutter for light leak prevention so to enhance optical contrast. Shutters are actuated magnetically, latched and addressed electrostatically. The shutter arrays are fabricated using MEMS bulk-micromachining technologies and packaged using single-sided indium flip-chip bonding technology. The MSA flight concept consists of a mosaic of 2 x 2 format of four fully addressable 365 x 171 arrays placed in the JWST optical path at the focal plane.

James Webb Space Telescope Near-Infrared Spectrograph : Dark Performance of the First Flight Candidate Detector Arrays

The James Webb Space Telescope (JWST) Near Infrared Spectrograph (NIRSpec) incorporates two 5 μm cutoff (λco =5 μm) 2048×2048 pixel Teledyne HgCdTe HAWAII-2RG sensor chip assemblies. These detector arrays, and the two Teledyne SIDECAR application specific integrated circuits that control them, are operated in space at Τ ~ 37 K. This article focuses on the measured performance of the first flight-candidate, and near-flight candidate, detector arrays. These are the first flight-packaged detector arrays that meet NIRSpecs challenging 6 e- rms total noise requirement. The current version of this paper has had a correction made to it at the request of the author. Please see the linked Errata for further details.

Detectors for the James Webb Space Telescope near infrared spectrograph (NIRSpec)

The Near Infrared Spectrograph (NIRSpec) will be the James Webb Space Telescopes (JWSTs) primary near-infrared spectrograph. NIRSpec is a multi-object spectrograph with fixed-slit and integral field modes. EADS/Astrium is building NIRSpec for the European Space Agency (ESA), with NASA is providing the detector subsystem and programmable multi-aperture mask. In this paper, we summarize recent progress on the detector subsystem including tests demonstrating that JWSTs Rockwell HAWAII-2RG sensor chip assemblies have achieved Technology Readiness Level 6 (TRL-6). Achieving TRL-6 is an important milestone because TRL-6 is required for flight.

Getting JWST’s NIRSpec back in shape

The James Webb Space Telescope (JWST) Observatory is the follow-on mission to the Hubble Space Telescope. 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. The techniques and procedures that were applied during this campaign will be elaborated in this paper. Some first cold test results of the upgraded instrument will be presented as well.