CALPRODUCT-NI-007: NISP Intrapixel Sensitivity Variation¶
Parent requirement¶
P-R-NI-CAL-PV-004 in EUCL-MPIA-RD-1-001 (CalCD-B)
Scheduling¶
Once, during PV phase. It may be repeated at a later time during the survey to check whether radiation damage altered the IPSV.
Time-critical¶
No
Objective¶
Intra-pixel sensitivity variation (IPSV) has not been characterized on ground for the NISP flight detectors. Sparse checks were performed on flight-like detectors using spot scans. IPSV enters directly into the photometry error budget, by systematically biasing the flux measurements. In particular, areas with cross-hatching are affected.
If significant IPSV exist and can be modeled (i.e. a representative model for all pixels), then the model can be used to remove the bias in the photometric measurements.
However, the physical pixel size in H2RGs is highly variable (unlike CCDs), which may alter the IPSV measurement. PRNU maps from flats fields give an indication of the local pixel size.
Fulfilled by¶
Self-cal data and LED flats are sufficient.
Data volume¶
TBD
Processing input¶
Ground Processing Input Data: TBD
Data Description: Self-cal data
Processing method¶
Use GAIA reference astrometric positions to determine the accurate centroids of stars at a sub-pixel level. Using the Self-cal random dithers, and photometrically non-variable sources, determine systematic magnitude changes as a function of sub-pixel position. Likely, we must take into account PRNU maps from the LED flats to correct for the intrinsically varying pixel size in H2RGs. These tasks are currently not covered by OU-NIR.
Processing output¶
Sub-pixel sensitivity maps, representative for single detectors and, if possible, sub-regions. The spatial resolution depends on the density of stars that can be used for this analysis and is not known before in-flight data are available.
Required accuracy, constraints¶
Depends on the available density of usable stars and will be known once we are in-flight. The current processing baseline does not correct for IPSV; this is taken into account in the NISP photometry error budget. However, if IPSV can be successfully measured in flight, a pixel-level correction scheme could be developed.
Processing tools¶
Offline tool, TBD.
Validation¶
IPSV is a physical effect at the pixel level. It is not known if (and how) IPSV changes during prolonged radiation exposure, and whether we can actually measure IPSV and correct for it. If measurements are successful then they shall be trended to check for any evolutionary effects. Sudden and significant changes are more likely to arise because of changes in the analysis software than due to a true change at the hardware level.
Recovery¶
IPSV is an inherent property of the detector pixels. Significant changes are not expected, and likely due to measurement errors or procedural changes.
Quality control parameters¶
DataModel: https://gitlab.euclid-sgs.uk/
Contact person¶
NISP-P calibration scientist