Asymmetry in the noise equivalent angle performance of the JWST fine guidance sensor

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Proceedings titleSPIE - International Society for Optical Engineering. Proceedings
ConferenceHigh Energy, Optical, and Infrared Detectors for Astronomy VI, 22 June 2014 through 25 June 2014, Montreal, QC
Article number91540L
SubjectInfrared detectors; Instrument testing; Space telescopes; Stars; FGS; Guidance sensors; H2RG; HgCdTe; JWST; SIDECAR; Luminance
AbstractThe James Webb Space Telescope Fine Guidance Sensor makes use of three 2048×2048 five micron cutoff H2RG HgCdTe detectors from Teledyne Imaging Systems. The FGS consists of two Guider channels and a Near-InfraRed Imager and Slitless Spectrograph (NIRISS) channel. We report here on detailed tests results from the Guider channels originating in both instrument level performance testing and from recent Guider performance testing with the FGS integrated into JWST's Integrated Science Instrument Module (ISIM). A key performance parameter is the noise equivalent angle (NEA) or centroiding precision. The JWST requirement flowed down to the Guiders is a NEA of 4 milli-arcseonds, equivalent to approximately 1/20th of a detector pixel. This performance has been achieved in the testing to date. We have noted a systematic asymmetry in the NEA depending on whether the NEA in the row or column direction is considered. This asymmetry depends on guide star brightness and reaches its maximum, where the row NEA is 15% to 20% larger than the column NEA, at the dim end of the Guide star brightness range. We evaluate the detector level characteristics of spatially correlated noise and asymmetric inter-pixel capacitance (IPC) as potential sources of this NEA asymmetry. Modelling is used to estimate the impact on NEA of these potential contributors. These model results are then compared to the Guider test results obtained to date in an effort to isolate the cause of this effect. While asymmetric IPC can induce asymmetric NEA, the required magnitude of IPC is far greater than observed in these detectors. Thus, spatially correlated noise was found to be the most likely cause of the asymmetric NEA.
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AffiliationNational Research Council Canada; National Science Infrastructure
Peer reviewedYes
NPARC number21272633
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Record identifierf6c34fe8-ed45-4434-9fbc-d119d4090769
Record created2014-12-03
Record modified2017-04-24
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