Emily Kan

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.

Mechanisms and Temperature Dependence of Single Event Latchup Observed in a CMOS Readout Integrated Circuit From 16–300 K

Heavy ion-induced single event latchup (SEL) is characterized in a commercially available CMOS readout integrated circuit operating at cryogenic temperatures. SEL observed at 24 K and below is believed to be possible when free carriers produced by an ion strike initiate an exponential increase in the free carrier density via shallow-level impact ionization (SLII). This results in a large current increase that proceeds to a sustained latched state, even though the classic condition for parasitic bipolar gain product is not met since it is much less than unity. The LET threshold for SEL is significantly lower at 20 K as compared to 300 K although the saturated cross section is 2-3 times higher at 300 K. The temperature dependence of the SEL cross section is characterized from 16-300 K. SEL behavior attributed to the classical cross-coupled parasitic bipolar model is observed from ~135-300 K, and the reduction in the SEL cross section is remarkably modest as the temperature is lowered from room temperature to ~200 K. Temperature dependent electrical latchup characterization of a 130 nm pnpn test structure also indicates a change in the latchup behavior at ~50 K consistent with the SLII mechanism.

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.

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.

Reciprocity failure in 1.7 µm cut-off HgCdTe detectors

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.

Characterization of the Detector Subsystem for the Near Infrared Spectrograph (NIRSpec) on the James Webb Space Telescope

We present interim results from the characterization test development for the Detector Subsystem of the Near-Infrared Spectrograph (NIRSpec). NIRSpec will be the primary near-infrared spectrograph on the James Webb Space Telescope (JWST). The Detector Subsystem consists of a Focal Plane Assembly containing two Teledyne HAWAII-2RG arrays, two Teledyne SIDECAR cryogenic application specific integrated circuits, and a warm Focal Plane Electronics box. The Detector Characterization Laboratory at NASAs Goddard Space Flight Center will perform the Detector Subsystem characterization tests. In this paper, we update the initial test results obtained with engineering grade components.

Initial laboratory and sky testing results for the second generation H4RG-10 4k x 4k, 10 micron visible CMOS-Hybrid detector

We present the initial performance test results for the H4RG-10 (A2), the second generation of the H4RG-10 visible CMOS-Hybrid Sensor Chip Assembly (SCA). The first science grade H4RG-10 (A2), delivered in 2009, is an evolution of the first generation A1, first delivered and tested in 2007. The H4RG-10 is primarily intended for ground- and space-based astronomical applications. Our evaluation focused on the performance parameters as they are related to astrometric applications. We find that the A2 SCA shows high pixel interconnect (99.6%), and low read noise (10-15 e- RMS) when operated at high speeds, consistent with A1 results. Most importantly, the H4RG-10 (A2) shows a dramatic improvement in dark current vs. the A1, with a two order of magnitude reduction in mean dark level and significantly reduced hot pixel population below 200 K.

Interpixel crosstalk in Teledyne Imaging Sensors H4RG-10 detectors.

Complementary metal-oxide semiconductor (CMOS)-hybrid arrays have become competitive optical detectors for use in ground- and space-based astronomy. Interpixel capacitance (IPC) is one source of error that appears in most CMOS arrays. In this paper, we use a single-pixel-reset method to model IPC. We combine this IPC model with a model for charge diffusion to estimate the total crosstalk on H4RG-10 arrays. Finally, we compare our model results to 55Fe data obtained using an astrometric camera built to test the H4RG-10 B0 generation detectors.

Snowballs in Euclid and WFIRST detectors

Snowballs are transient events observed in HgCdTe detectors with a sudden increase of charge in a few pixels. They appear between consecutive reads of the detector, after which the affected pixels return to their normal behavior. The origin of the snowballs is unknown, but it was speculated that they could be the result of alpha decay of naturally radioactive contaminants in the detectors, but a cosmic ray origin cannot be ruled out. Even though previous studies predicted a low rate of occurrence of these events, and consequently, a minimal impact on science, it is interesting to investigate the cause or causes that may generate snowballs and their impact in detectors designed for future missions. We searched for the presence of snowballs in the dark current data in Euclid and Wide Field Infrared Survey Telescope (WFIRST) detectors tested in the Detector Characterization Laboratory at Goddard Space Flight Center. Our investigation shows that for Euclid and WFIRST detectors, there are snowballs that appear only one time, and others than repeat in the same spatial localization. For Euclid detectors, there is a correlation between the snowballs that repeat and bad pixels in the operational masks (pixels that do not fulfill the requirements to pass spectroscopy, photometry noise, quantum efficiency, and/or linearity). The rate of occurrence for a snowball event is about 0.9 snowballs/hr. in Euclid detectors (for the ones that do not have associated bad pixels in the mask), and about 0.7 snowballs/hr. in PV3 Full Array Lot WFIRST detectors.