Yiting Wen

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.

Commentary: JWST near-infrared detector degradation— finding the problem, fixing the problem, and moving forward

The James Webb Space Telescope (JWST) is the successor to the Hubble Space Telescope. JWST will be an infrared-optimized telescope, with an approximately 6.5 m diameter primary mirror, that is located at the Sun-Earth L2 Lagrange point. Three of JWST’s four science instruments use Teledyne HgCdTe HAWAII-2RG (H2RG) near infrared detector arrays. During 2010, the JWST Project noticed that a few of its 5 μm cutoff H2RG detectors were degrading during room temperature storage, and NASA chartered a “Detector Degradation Failure Review Board” (DD-FRB) to investigate. The DD-FRB determined that the root cause was a design flaw that allowed indium to interdiffuse with the gold contacts and migrate into the HgCdTe detector layer. Fortunately, Teledyne already had an improved design that eliminated this degradation mechanism. During early 2012, the improved H2RG design was qualified for flight and JWST began making additional H2RGs. In this article, we present the two public DD-FRB “Executive Summaries” that: (1) determined the root cause of the detector degradation and (2) defined tests to determine whether the existing detectors are qualified for flight. We supplement these with a brief introduction to H2RG detector arrays, some recent measurements showing that the performance of the improved design meets JWST requirements, and a discussion of how the JWST Project is using cryogenic storage to retard the degradation rate of the existing flight spare H2RGs.

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.

Hot pixel behavior in WFC3 CCD detectors irradiated under operational conditions

A Hubble Space Telescope Wide Field Camera 3 (WFC3) CCD detector was tested for radiation effects while operating at -83°C. The detector has a format of 2048 x 2048 pixels with a 15 μm square pixel size, a supplemental buried channel, an MPP implant, and is back side illuminated. Detector response was tested for total radiation fluences ranging from 1x103 to 2.5x109 of 63.3 MeV protons/cm2 and for a range of beam intensities. Radiation damage was investigated and the annealing of damage was tested by warming up to +30°C. The introduction rate of hot pixels and their statistics, hot pixel annealing as a function of temperature and time, and radiation changes to the mean value of dark current were investigated. Results are compared with the experiences of other HST instruments.

A study of hot pixel annealing in the Hubble Space Telescope Wide Field Camera 3 CCDs

A Hubble Space Telescope Wide Field Camera 3 (WFC3) CCD detector was tested for radiation effects while operating at -83C. The goal of the experiment was to evaluate the introduction and annealing rates of hot pixels and to assess the dynamics of that process. The device was irradiated while cold and warmed to +30°C for a 4 hour soak, then cooled back down to -83°C. Hot pixel populations were tracked during warm up and cool down. The results showed that the hot pixels begin to anneal around -40°C and the anneal process was largely completed before the detector reached +30°C. It was also found that, although a large fraction of the hot pixels dropped below the threshold, they remained warmer than the remaining population.

The Wide-Field Camera 3 detectors

The Wide-field Camera 3 (WFC3) is a fourth-generation instrument planned for installation in Hubble Space Telescope (HST). Designed as a panchromatic camera, WFC3s UVIS and IR channels will complement the other instruments onboard HST and enhance the observatorys 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.

The 55 Fe X-Ray Energy Response of Mercury Cadmium Telluride Near-Infrared Detector Arrays

A technique involving 55 Fe X-rays provides a straightforward method to measure the response of a detector. The detectors response can lead directly to a calculation of the conversion gain (eADU � 1 ), as well as aid detector design and performance studies. We calibrate the 55 Fe X-ray energy response and pair production energy of HgCdTe using 8 HST WFC3 1.7 μm flight grade detectors. The results show that each Kα X-ray generates 2273 � 137 electrons, which corresponds to a pair-production energy of 2:61 � 0:16 eV. The uncertainties are dominated by our knowledge of the conversion gain. In future studies, we plan to eliminate this uncertainty by directly mea- suring conversion gain at very low light levels.

Wide Field Camera 3 CCD Quantum Efficiency Hysteresis: Characterization and Mitigation

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 instruments internal calibration system.

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.

Radiation effects in WFC3 IR detectors

A Hubble Space Telescope (HST) Wide Field Camera 3 (WFC3) flight-like IR detector was tested for radiation hardness by exposing it to high energy protons while operating at the nominal flight temperature of 150 K. The detector is a 1.7 μm cutoff HgCdTe detector with a CdZnTe substrate. The device is hybridized to a silicon multiplexer. The detector response was tested for gradually increasing fluence from less than 1x103 to a total of 5x109 63 MeV protons/cm2. Dark current changes were evaluated after each step. An increase in dark current and new hot pixels were observed after large steps of irradiation. The increased dark current was observed to partially anneal at 190K and fully anneal at room temperature. Radiation effects, hot pixel distribution, and results of annealing at different temperatures are presented here.