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

Abstract

The James Webb Space Telescope Fine Guidance Sensor makes use of three 2048x2048 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 1/20 th 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.