Abstract
We present the analysis of James Webb Space Telescope near-infrared H2RG detectors with a 5-μm cutoff, which shows that, at temperatures <60 K, there is no measurable dark current. Instead, the observed signal in dark exposures is almost entirely due to multiplexer glow that arises as each pixel is selected. We are able to separate the per-sample glow from the time-dependent dark current by comparing the observed signal in both continuous and sparsely sampled dark exposures. Such explicit tests are required to break the degeneracy between dark current and uniform amplifier glow. We show that the glow is lower within the regions of the detector that are missing the epoxy back fill (voids). We also find that the glow from each pixel extends out to a radius of several pixels. Because of the higher sampling frequency of subarray observations, the per-sample glow leads to a higher apparent dark current in subarray exposures. Finally, we show that the magnitude of the glow is affected by the pixel source follower current, the pixel clocking rate, and the number of outputs running in parallel. Our measurement of an insignificant dark current shows that the detector noise is no longer limited by the quality of the mercury cadmium telluride layer but instead by the multiplexer and readout electronics.
Highlights
The long-wavelength sensitivity of infrared detectors makes them susceptible to self-generated emission sources.[1]
The sources of glow have been primarily attributed to amplifiers on the perimeter of the detectors. We show this example in a Hubble Space Telescope Near-Infrared Camera and MultiObject Spectrometer (NICMOS) channel 3 dark exposure; see Fig. 1
Because NICMOS had a variety of readout modes with different intervals between the samples, it was possible to show that there was a linear relationship between the observed amplifier glow and the number of samples in the exposure rather than the exposure time
Summary
The long-wavelength sensitivity of infrared detectors makes them susceptible to self-generated emission sources.[1]. We present lab results where we separate the two contributors to the observed dark slope images: the dark current and a per-frame multiplexer glow. To separate the dark current per-time and the glow per-sample from a dark slope image, we used a flight batch JWST H2RG detector, 17166. The relationship between measured slope S, exposure time t, dark current D, number of frames in the exposure Nf, and per-frame glow F is. ÐFNfÞ∕t þ D: The problem is that all of the JWST readout patterns continuously clock the array leading to a degeneracy between a per-sample signal and a per-time signal To break this degeneracy, we created a sparse sampling readout pattern that completely pauses the clocking of the detector between successive frames of the array using the single-step mode in the flight SIDECAR software. It could be that the epoxy has a better match to the index of refraction of the multiplexer limiting the internal reflection at the surface
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