CALPRODUCT-VS-010: VIS Focal Plane Astrometric Solution¶
Parent requirement¶
Scheduling¶
Once during PV phase to get a very accurate distortion model using a dense stellar field.
Monthly during routine phase to monitor relative detector positions, using the Self-cal field.
Time-critical¶
no
Objective¶
These observations have three goals.
PV-phase only: Determine the relative stitch block alignment for each CCD. The stitch blocks have sizes of \(256*512\) pixel, i.e. there are 128 stitch blocks per detector. The CCD is formed by projecting stitch block matrices onto the waver using photolitography. The alignment is not perfect, and can be accurately determined using absolute GAIA reference sources. To this end we need at least 10 stars per stitch block (scale, shift, rotation). With a stellar density of 1000 stars per detector we would have about 8 stars per stitch block. Shot noise will make this number lower for some stitch blocks, hence dithered observations are required. The stitch blocks are part of the crystalline structure of the CCD and will not change their relative orientation during the mission. Their positions need to be measured only once. The stitch blocks can also be detected in flats, albeit with lesser accuracy. Stitch blocks cannot be corrected for during coaddition in SWarp; their impact on PSF and shape measurements should be evaluated using simulations.
Determine the relative detector positions and orientations, including any other terms that can be described as a linear mapping (e.g. shear, flip). Relative detector orientations are subject to change and must be monitored regularly; they are given in the absolute VIS FPA coordinate system.
Determine the optical distortion with high angular resolution (on arcmin scales, during PV-phase). The distortions manifest as a change of the FPA’s plate scale. To this end, during PV phase, VIS needs at least 1000-2000 GAIA unsaturated (fainter than 17.5 AB) reference sources per detector, i.e. a stellar density of 25-50 stars per square arcmin. Such densities are only reached in the galactic plane. During routine phase, the self-cal field (1.2 reference source per square arcmin) is sufficient to monitor the optical distortions.
While the optical distortion is expected to be stable, the relative detector positions may drift over time due to outgassing, mechanical relaxation processes in the SiC mounting structure, and e.g. thermal effects. The drifts can be determined using the monthly self-cal observations and do not need further separate observations.
Fulfilled by¶
CALBLOCK-PV-023: VIS Astrometric Solution (stitch blocks and optical distortion)
CALBLOCK-F-001: Self Calibration (monitoring detector drift)
Data volume¶
5*ROS = 24905 MB
Processing input¶
Ground Processing Input Data: All calibration products to run the standard science pipeline (BPM, darks, flat, nonlinearity etc), astrometric reference catalog (Gaia), offline analysis
Data Description:
Processing method¶
Match the centroids of isolated stellar sources against the GAIA reference catalog, correcting the latter for the proper motion offsets accumulated since the GAIA reference epoch to the time of observation.
Processing output¶
Distortion map of the VIS focal plane
Relative detector orientations of the VIS focal plane
Monthly evaluations of the relative detector positions, i.e. trending CRPIX1/2 and the four CD matrix elements (CDij).
Vector plots showing how the detectors drift and rotate over time
Required accuracy, constraints¶
Accurate (TBD) source position measurements
Processing tools¶
L1 science pipeline, source position measurements
Quality control parameters¶
DataModel: https://gitlab.euclid-sgs.uk/
Contact person¶
VIS instrument scientist