.. index:: Distortion (NISP-P); Calibration observations .. _calobs-nisp-phot-distortion: **CALPRODUCT-NP-002:** NISP-P Focal Plane Astrometric Solution ============================================================== .. include:: definitions.rst Parent requirement ------------------ | `R-CAL-B-NP-2200 `_ Scheduling ---------- Automatically with every Self-cal visit Time-critical ------------- No Dependencies ------------ None Objective --------- .. index:: Optical distortion (NISP-P) .. index:: Relative detector positions (NISP-P) .. index:: Astrometric reference sources (NISP-P) These observations have two goals. 1. Determine the relative detector positions and orientations, including any other terms that can be described as a linear mapping (e.g. shear, flip). This yields the reference pixels (CRPIX1/2) and CD matrices (CDij) for each detector in the focal plane array (FPA). 2. Determine the optical distortion, which manifests as a change of the FPA's plate scale. While the optical distortion is expected to be stable (apart from small offsets from filter wheel movements), 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. Overall, the astrometric solution is derived by comparing source positions against GAIA astrometric positions in multiply dithered self-cal observations. The GAIA-DR2 stellar density at the self-cal field is :math:`1.8 {\rm\;arcmin^{-2}}` for stars in the range :math:`16.5 < G < 21` AB mag. Taking into account typical stellar colors and that NISP saturates around 16.5 AB mag, we have :math:`1.3 {\rm\;arcmin^{-2}}` unsaturated GAIA-DR2 references in J-band, or about 140 per detector. Including fainter NISP detections down to 23 AB mag, the stellar density at the NEP is approximately 540 per detector. This is more than sufficient to reach accurate distortion models for all NISP purposes. Widely dithered observations are essential, as *Scamp* calculates the distortion model per detector. A 3x3 grid will be sufficient for this purpose. .. _figure_NEP_stellardensity: .. figure:: ../images/NEP_stellardensity.png :scale: 100 :align: left The GAIA-DR2 stellar density for the NEP (Self-Cal field at RA=270.0 and DEC=66.5) Fulfilled by ------------ Higher-density stellar fields are sufficient. :ref:`calobs-block-pv-selfcal` :ref:`calobs-block-pv-vis-distortion` :ref:`calobs-block-f-selfcal` 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. The NISP filters have optical power, and their relative placement with respect to the optical axis will vary (1 degree steps FWA movement). This is reflected by ":FILTERPOSITION", a FITS keyword that encodes the offset of the filter from the optical axis (or any other reference point) along a circle. This can be accounted for in the *scamp* run if needed. The distortion models must be obtained per filter. A single exposure and Gaia astrometric references are sufficient. .. _figure_NISP_platescales: .. figure:: ../images/NISP_platescales.png :scale: 66 :align: left The top panel shows the NISP plate scale variation for the Y-band filter (on average, 0.297 arcsec / pixel). The optical distortion causes a source in the corner to be pushed away from the field center by about 29 pixels or 8.7 arcsec. The bottom two panels show the *difference* in plate scale between J and Y, and H and Y. Compared to the Y filter, sources are shifted by up to 4 (3) pixels for the J (H) filter. Data were provided by Gregor Seidel. Processing output ----------------- * Distortion map of the NISP-P focal plane for each filter * WCS solution for NISP photo images * Evaluation of impact of FWA position uncertainty * 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 ------------------------------ The astrometric distortion must be known to 0.2 pixel or better. Processing tools ---------------- L1 science pipeline, source position measurements Quality control parameters -------------------------- DataModel: ``_ Contact person -------------- NISP-P calibration scientist .. raw:: latex \clearpage