.. _calobs-block-pv-vis-distortion: **CALBLOCK-PV-023:** VIS Astrometric Solution ============================================= .. include:: definitions.rst Observation configuration file ------------------------------ | :download:`ocf_CALBLOCK-PV-023.csv ` Scheduling ---------- Once during PV phase. In case of failure, repeat at next possible opportunity. Should ideally be executed prior to :ref:`calobs-block-pv-selfcal`, but the latter should not be delayed in case of failure. Time-critical ------------- Yes, must be executed before the first run of :ref:`calobs-block-pv-selfcal`. Objective --------- .. index:: Optical distortion (VIS) .. index:: Stitch blocks (VIS) .. index:: Relative detector positions (VIS) .. index:: Astrometric reference sources (VIS) These observations have three goals. 1. Determine the relative stitch block alignment for each CCD. The stitch blocks have sizes of :math:`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. 2. 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. 3. Determine the optical distortion with high angular resolution (on arcmin scales). 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 V=17.5 AB mag) reference sources per detector, i.e. a stellar density of 25 stars per square arcmin or more. Such densities are only reached in the galactic plane or the LMC (the self-cal field offers 1.2 such reference source per square arcmin), and is also sufficient for the stitch block measurement. The optical distortion is expected to be stable over time. The data are not specifically useful for the NISP *optical* distortion, as the NISP filters have optical power and move in and out of the beam. Observations ------------ Execute the ROS_D1 20 times with random dithers on a dense stellar field. The number of dithers increases the effective stellar number density, and / or provides independent data for verification. Fulfilling ---------- :ref:`calobs-vis-distortion` :ref:`calobs-nisp-phot-distortion` Targets and pointing constraints -------------------------------- :command:`Pointing`: Straylight suppression as per MOCDA-SYS-003. A high density stellar field is required for the optical distortion, yielding :math:`20-50\;{\rm arcmin^{-2}}` GAIA references .. csv-table:: **LMC high-density stellar field** :download:`CSV file ` :file: tables/targets-vis-distortion.dat .. image:: ../images/VIS_astrometric_field.png :scale: 100% Figure: Gaia-DR2 stellar density (down to 17 mag) for the VIS astrometric field near the LMC. The DR3 density down to 20 and 21 mag is 15 and 35 :math:`{\rm arcmin}^{-2}`, respectively. Instrument configuration ------------------------ Instrument: VIS Methodology / Uplink procedure ------------------------------ :command:`Entity`: VIS :command:`Commanding Method`: VIS science :command:`Dither Sequence`: ROS_D1, random dither offsets in a 3' box Inter-operability constraints ----------------------------- None In-orbit duration ----------------- :command:`In-orbit Duration`: 6.0 h (5 executions of the ROS) Data volume ----------- ``5*ROS = 24905 MB`` Processing input ---------------- :command:`Ground Processing Input Data`: All calibration products to run the standard science pipeline (BPM, darks, flat, nonlinearity etc), astrometric reference catalog (Gaia), offline analysis :command:`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 positions and orientations * Monthly evaluation of the absolute detector position and rotation in the VIS FPA coordinate system. * Vector plots showing how the detectors drift and rotate over time Required accuracy, constraints ------------------------------ Accurate (TBD) source position measurements Processing tools ---------------- Pipeline: VIS_AstrometryCalibration PE: VIS_Astrometry Quality control parameters -------------------------- DataModel: ``_ Contact person -------------- VIS instrument scientist .. raw:: latex \clearpage