MIRI Detector Performance
JWST MIRI's detector performance characteristics, such as read noise, dark current, saturation level, and persistence, were measured during ground testing.
The MIRI detectors offer relatively low dark current, read noise, and persistence. These types of detectors were used very successfully on all 3 of the Spitzer instruments. However, compared with detectors used in the optical and near-infrared, the MIRI detectors have a number of additional calibration challenges (Ressler et al. 2015, Rieke et al. 2015).
Unless otherwise specified, the following values were measured during ground testing in a flight-like configuration. Because the instrument's operating temperature will not be known until on-orbit commissioning, some of these values are subject to change.
Table 1 provides a summary of measurements obtained in "CV3" cryogenic vacuum ground testing followed by more detailed descriptions of the various parameters and measurement methods.
Table 1. Average MIRI detector properties measured in ground testing
|Pixel gain (e–/ADU)||~5.5|
Dark current (e–/s)
|Read noise (e– rms)||~14|
|Latent images (%)||~0.5|
Full well (e–)
Conversion gain, gc (e–/ADU), is a fundamental parameter in detector characterization that is used to measure many detector properties, including quantum efficiency (QE), dark current, and read noise. Measuring these parameters to a higher precision is becoming more important as the demand for low signal observations increases and the scientific requirements evolve in complexity. The MIRI detectors have a gain of ~5.5 e–/ADU.
MIRI darks were obtained with the contamination control cover closed to make the instrument interior as dark as possible (although, as always, one can measure only upper limits to the true dark current, given the possibility of photon leaks). In processing the data, the first and last frames of an integration ramp were rejected, to circumvent the effects of the reset anomaly and last-frame effect. Dark currents were then determined by the slopes of the integration ramps over a 100 × 100 pixel region selected to avoid bad pixels. The slope calculations included all exposures in a test run; the effects of settling of the detector output artificially elevate the apparent dark current, again making the results upper limits.
The read noise was measured by setting the detector bias voltage to 0 V; in this way, any dark or photocurrent was eliminated and only the voltage noise of the readout output amplifiers were measured. Experiments leaving the detector bias at 2 V were also performed to look at noise in the dark frames. The results were identical until the temperature climbed to ∼7.5 K. At this point, shot noise from the dark current began to dominate the noise, and the total noise began to exceed the requirement.
Quantum efficiency (QE)
Spectral quantum efficiency in impurity band conduction (IBC) IR devices is often referred to as "quantum yield" since it's only possible to measure the quantum efficiency times the internal gain. The QE of the detectors cannot be measured trivially in detector array form, so it's necessary to rely on measurements of test structures included on each wafer from which the flight detectors are selected.
Bright sources leave latents on the MIRI arrays, typically at a level of about 1% immediately after the source has been removed. The decay of these images shows multiple time constants, suggesting that there are a number of mechanisms that contribute to the effect. Further characterization of this complex behavior is needed to determine ways to correct it in the MIRI pipeline.
The response of the arrays is uniform, with pixel-to-pixel variations of no more than 3% rms. The best arrays have a small proportion of inoperative pixels (either hot or dead), of order 0.1%. All measurements indicate a level close to 3% for the cross talk to the 4 adjacent pixels around one receiving signal. A plausible cause of this behavior is interpixel capacitance, although there may be secondary contributions from electron diffusion and optical effects. In addition, at 5.6 μm there is an additional cross-like imaging artifact.
The Mid-Infrared Instrument for the James Webb Space Telescope, VIII: The MIRI Focal Plane System
Rieke, G. H. et al. 2015, PASP, 127, 665
The Mid-Infrared Instrument for the James Webb Space Telescope, VII: The MIRI Detectors