CALBLOCK-PV-021: VIS Bias and Dark¶
Observation configuration file¶
Scheduling¶
Once, during PV phase. Should be taken early-on. (DENSE variant)
In case of failure, repeat at next possible opportunity.
Once, after a full decontamination. (STRAYLIGHT variant)
The STRAYLIGHT variant can also be run to reconstruct the closed-shutter straylight at high fidelity (usually at a fixed SAA/AA, as the straylight is attitude dependent). A specific use case is foreseen for the generation of a straylight model as a function of spacecraft attitude.
Time-critical¶
Must be done early during PV phase, or as needed during the survey.
After decontamination, it must be done after the VI-FPA temperature achieves operational temperature range (157±5 K) and stability within 0.1 K/h.
Dependencies¶
None.
Objective¶
BIAS:
The bias level of the VIS detectors may not be stable over time, and it also exhibits spatial variations at the beginning of the readout (bias stabilization). The spatial pattern is expected to be stable within each quadrant. The bias level can be accurately computed from a series of individual bias exposures.
Straylight can integrate into the bias image over the 72 second full-frame readout, for all exposures. The straylight depends on the spacecraft attitude relative to the Sun. Master Biases (along with Master Darks) at specific spacecraft attitudes are required for straylight modeling.
DARK:
The dark current of the VIS detectors is expected to be extremely low, \(\leq\,0.01 e^{-}\,s^{-1}\,pix^{-1}\). It could not be accurately characterized (overall amplitude, tempertaure dependence) on ground, neither by the manufacturer nor during vacuum tests (background signal). We assume that all variations in the dark current, both spatially and temporally, will introduce an error into the relative photometric error that is lower than the budget allocation.
While the initial plan was to use dark observations to determine the true VIS dark current at operational temperatures, the presence of stray sunlight makes dark current measurements difficult, if not impossible. At operational attitudes (AA<-3.0 degrees), the straylight is suppressed to a fraction of the Zodi sky background, but never completely disappears.
The dark exposures will instead be used to measure (and remove) the stray sunlight, which varies across the FoV and with the spacecraft attitude, but otherwise stays the same irrespective of the shutter open/close position (TBC). There is a straylight component that is only associated with the open shutter position, but it is significantly smaller compared to the straylight that is independent of the shutter open/close state.
The exposures are also used to check the cosmic ray flux, the cosmic ray removal routine, and to update the pixel cosmetic map.
Dark frames are to be obtained for 325 second integration time, to match those in the ROS observation.
Observations¶
Early in PV phase, take 60 bias frames (B) interleaved with 30 nominal (ND, 566s) and 30 short (SD, 95s) dark exposures in the following way:
Repeat 15x:
BIAS, SD, BIAS, ND, BIAS, SD, BIAS, ND (about 0.8 h)
dither (for HGA repointing)
After decontamination, or if a new Master Bias / Master Darks are required, take 32 to 60 bias frames each of (B) interleaved with the ROS dark exposures (RD, 325s) in the following way:
Repeat 8~15x [depending on the schedule]:
BIAS, RD, BIAS, RD, BIAS, RD, BIAS, RD (about 0.8 h)
dither (for HGA repointing)
Fulfilling¶
Targets and pointing constraints¶
Pointing: Straylight suppression as per MOCDA-SYS-003.
Instrument configuration¶
Instrument: VS-CU off; RSU closed; VIS Bias sequence VI-FCP-504 (EUCL-MSS-MA-6-013, v1.3), VIS Dark sequence (VI-FCP-503);
Methodology / Uplink procedure¶
Entity: VIS
Commanding Method: VIS
Dither Sequence: None
Inter-operability constraints¶
None. PLM TVac tests have shown no parasitic light paths from NISP (EUCL-MSS-TR-6-135 v1.0 §5.5.2)
In-orbit duration¶
In-orbit Duration: 11.5h + dither(x15) [or: 6.2h + dither(x8)]
A single block of [B-SD-B-ND–B-SD-B-ND] takes:
5s (Flush) + 0s (B) + 72s (Readout) x 8 +215s (CDPU for Bias) x 4 +225s (CDPU for Dark) x 2 +(5s (Flush) + 566s (ND)) x 2 = 3033sA single block of [B-RD-B-RD-B-RD-B-RD] takes:
5s (Flush) + 0s (B) + 72s (Readout) x 8 +215s (CDPU for Bias) x 4 +(5s (Flush) + 325s (RD)) x 4 = 2761sThese is to be repeated 15 times to obtain 60 exposures each of bias and darks.
Note that 66s of the CDPU processing time can be accommodated into the dither duration.
Data volume¶
60*VB = 15000 MB
60*VD = 19380 MB
Processing input¶
Ground Processing Input Data: BPM
Data Description: 60 VIS biases, 30 nominal and 30 short darks
Processing method¶
The master bias is computed from 60 individual exposures. Images are median combined (or averaged) after identification and masking of cosmics.
Likewise for the 30 nominal and short dark frames.
Processing output¶
Combined bias frames. Combined dark frames. Estimated dark current, updated cosmetics map.
Required accuracy, constraints¶
Expected levels within TBD%, stable within TBD%, spatial uniformity within TBD%
Processing tools¶
Pipeline: VIS_MasterBias_Pipeline
PE: VIS_BiasCalibration
Pipeline: VIS_MasterDark_Pipeline
PEs: VIS_BiasCorrection, VIS_Dark_Calibration
Validation¶
Single bias exposures:
Within a single quadrant the bias must be stable within 1 ADU (after row averaging, to beat down the readout noise). Within a 24 h period, the bias must not vary by more than 2 ADUs (after row averaging in a difference image). The bias stabilization is not parameterized and not checked in single exposures.
Monthly master bias:
Compute the difference image between the current master bias and previous master bias (taken the previous month, and during PV phase). Sudden changes by more than 1 ADU (TBD) in the mean bias level (quadrant) require further investigation. (This description applies to the routine phase and should be moved elsewhere as it has nothing to do with PV).
Single dark exposures:
Number of cosmics below some TBD threshold.
Recovery¶
Slow bias variations over timescales of a few months are tolerable for science. The root cause should be investigated. For example, single bias exposures might be affected by too many cosmics and should be rejected. In case of a hardware cause, further investigation by the instrument operation team is required.
Quality control parameters¶
DataModel: https://gitlab.euclid-sgs.uk/
Contact person¶
VIS instrument scientist