CALPRODUCT-NI-006: NISP Dark Current¶
Parent requirement¶
Scheduling¶
Once, during PV phase.
Time-critical¶
No
Objective¶
Measure the thermal dark current for the photo- and spectro-MACC modes. During gound-testing, in-flight conditions cannot be established and hence this must be done from space. The dark current in the NISP detectors is very low (~0.02 e-/s) and thus a measurement with high S/N is time-consuming and will be carried out just once, during PV phase.
The number of dark exposures necessary to meet the CalCD-B requirement depends on the amplitude of the dark current. We must accuractely measure in particular the pixels with high dark current, since these appear as hot pixels if not corrected for, and might have to be masked if their Poisson noise violates the photometric error requirement.
For a dark current of \(d = 0.07\) e-/s, a photo-MACC readout noise \(RN=7\) e- (median 6 e-, 7e- is 95 percentile; R. Barbier), and wide-survey observation modes, 480 dark exposures are necessary to meet the requirement. The number depends weakly on the dark current itself, i.e. for the typical value of 0.02 e-/s we would need 440 exposures, for 0.2 e-/s we need 580. It can be estimated with \(750\,d + 8.8\,RN^2\).
The non-Gaussian distribution of the dark current determines how many pixels with high dark current are still problematic with 500 dark exposures. The combined budget for hot and dead pixels is 5%.
Note that in-flight the dark frames are contaminated by persistence from the zodiacal background and astrophysical sources, unless very long waiting times are obeyed prior to the measurement to allow for persistence decay.
Slew darks are described separately.
Fulfilled by¶
Data volume¶
500*NP + 100*NS = 58800 MB
Processing input¶
Ground Processing Input Data: BPM, persistence masks
Data Description: 600 NISP science images
Processing method¶
TBD
Processing output¶
Combined master darks for the photo- and spectro-MACC modes
BPM update
Histograms of dark current values for each detector
Notable changes with respect to ground-characterization data not explained by temperature variation
Required accuracy, constraints¶
Expected dark current levels within 0.02 e-/s
Spatial variations within TBD %
Spatial appearance as expected from ground-based measurements (possible dependence on detector temperature)
Absence of light leaks
Number of hot / bad pixels within expectations
Processing tools¶
L1 dark pipeline, trending statistics, histogram
Validation¶
Pure thermal dark conditions as for these characterizations are not encountered during routine operations. The dark current will always have contributions from persistence. For pure thermal darks, the prior illumination history must be respected and a waiting period calculated such that contamination from persistence is negligible. This period will be on the order of 10 h, and potentially longer. For example, the measurement could be done after orbital maintenance or other operations that prohibit NISP on-sky observations.
Contamination by persistence hampers the validation of the master dark because we are lacking a ground-based reference taken at the same detector temperature. A way forward would be a comparison of pixels with high dark current in the master dark with those in the slew darks. However, at PV phase, a sufficient number of slew darks for a meaningful comparison will not (yet) be available, and hence the master dark must got without initial validation.
Recovery¶
If the validation of the master dark fails, then one must first exclude other causes that could change the high flux tail of hot pixels. This can be changes in e.g. the RTN and persistence. Radiation damage toards the end of the mission is expected to chnage the number of hot / dead pixels, and therefore at some point a repetition of the thermal dark current could be required.
Quality control parameters¶
DataModel: https://gitlab.euclid-sgs.uk/
Contact person¶
NISP-P calibration scientist