.. index:: Absolute Photometric Standards (NISP-P) (PV phase); Calibration observations .. _calobs-block-pv-absolute-standards-imaging: **CALBLOCK-PV-002:** NISP-P and VIS Absolute Photometric Standards ================================================================== .. include:: definitions.rst Observation configuration file ------------------------------ | :download:`ocf_CALBLOCK-PV-002_PHOTO-Y.csv ` | :download:`ocf_CALBLOCK-PV-002_PHOTO-J.csv ` | :download:`ocf_CALBLOCK-PV-002_PHOTO-H.csv ` Scheduling ---------- Once, during PV phase. Ideally immediately before the thermal relaxation period of :ref:`calobs-block-pv-phase-diversity`, to define a maximum thermal impulse for :ref:`calobs-block-pv-vis-thermal-timescale`. In case of failure, repeat at next possible opportunity. Time-critical ------------- No. Dependencies ------------ None. Objective --------- Obtain absolute flux standard measurements as a function of position on the focal plane, characterizing the entire optical path. The observations serve as an independent check of the global system response, as well as the impact of persistence and trap filling in case of NISP-P. The dither pattern is driven by NISP-P. NISP is putting the standard on a 4x4 grid centered on each detector. Due to the different focal plane layout, the VIS detectors will be sampled in a more irregular fashion. On average, 7 positions are realised on each VIS CCD. VIS must use a short-science exposure to avoid saturation. Observations ------------ A spectro-photometric standard star is stepped across a grid approximately centered on each NISP detector. 4x4 grid per detector, 120 arcsec step size, 3 exposures per grid position. The grid is completed in Y-band (arbitrary) first, followed by two more scans in J and H. The starting position for each scan is offset by 10 arcseconds with respect to the previous scans. This is to ensure that the reference star does not fall on the same pixels in the J and H exposures as in Y, modifying the persistence behaviour compared to the initial pass in Y. The observing sequence is as follows: .. code-block:: none :caption: Spectro-photometric standard (imaging) for filter in [Y, J, H]: if filter == J || filter == H: "global offset by 10 arcsec in x and y" for detector in [1,...,16]: for position in [1,...,16]: "3 NISP photo-exposures without offsets" if filter == Y: "1 VIS short-science exposure, parallel to NISP" .. index:: Absolute Photometric Standards (NISP-P); Self-cal Fulfilling ---------- :ref:`calobs-nisp-phot-absolute-standards` :ref:`calobs-vis-absolute-standards` Targets and pointing constraints -------------------------------- :command:`Pointing`: Straylight suppression as per MOCDA-SYS-003. .. index:: Absolute Photometric Standards (NISP-P); Spectro-photometric standard .. csv-table:: **Euclid spectrophotometric standards; WDJ175318+644502 shall be used**. :download:`CSV file ` :widths: 40, 25, 25, 10, 15, 15, 15, 30 :file: tables/targets-nisp-phot-absolute-standards.dat Our preferred choice, WDJ175318+644502, is stable to better than 1% on time scales of 100s to several years. However, Gemini fast photometry and Gaia photometry indicates that a sub-percent level variability is likely present. While this is not a problem for absolute flux scaling, where the goal is 5\%, it could be an issue if very accurate comparison measurements are being made using the :math:`4x4` grid measurements per detector. To this end, we have a list of stars within 10' and :math:`\pm0.2` Gmag of WDJ175318+644502 where Gaia data indicate a very high photometric stability. However, these stars do not yet have their SEDs determined. .. csv-table:: **Photometrically stable comparison stars in the vicinity of WDJ175318+644502**. The smaller ``Var``, the better the photometric stability (this metric is uncalibrated). :download:`CSV file ` :widths: 27, 25, 25, 15, 15, 15, 10, 30 :file: tables/targets-photstable.dat Instrument configuration ------------------------ Instrument: Wide survey Methodology / Uplink procedure ------------------------------ :command:`Entity`: VIS & NISP :command:`Commanding Method`: NISP: Observation; VIS: short science :command:`Dither Sequence`: Specialized Inter-operability constraints ----------------------------- None. In-orbit duration ----------------- :command:`In-orbit Duration`: 83.9 h Assuming 60 s for each dither and 12 s FWA move ``(12s + 60s + (3*111s + 60s) * 16 dithers * 16 detectors) * 3 filters`` Data volume ----------- Total: 251648 MB NISP: ``3*16*16*3 * NP = 135936 MB`` VIS: ``256*VS = 115712 MB`` Processing input ---------------- :command:`Ground Processing Input Data`: All calibration products to run the standard science pipeline (BPM, darks, flat, nonlinearity, etc), reference measurement of standard star, RSR curves (relative spectral response of the system (mirrors, optics, instrument) :command:`Data Description`: NISP: ``3*16*16*3=2304`` photo-MACC images VIS: 256 images (one VIS SS image at every dither point) Processing output ----------------- * Mean and stdev of the photometric zeropoint (conversion factor) per detector * Plot of zeropoint as a function of position in the focal plane (with the detector grid overlaid) * Plot comparing the fluxes of the 1st, 2nd and 3rd exposures Required accuracy, constraints ------------------------------ .. index:: Absolute Photometric Standards (NISP-P; routine phase); Persistence * Photometric flux measurement / photometric conversion factor derived from each position are within 1.5% of the mean. * The flux measurements in the first, second and third exposure at a given dither must be the same within the combined uncertainties of the measurement and the persistence model. * Comparison with expectations from the total system throughput. The NISP ZP should be within 10% of the ZPs reported in `EC: Schirmer+ (2022) `_ . We note that there is an inherent uncertainty of 5% due to the error in the absolute QE measurement. The hard requirement is that the nominal depth of 24.0 AB mag for NISP :math:`5\,\sigma` point soures is met. Likewise for VIS. Processing tools ---------------- L1 science pipeline and absolute calibration pipeline, additional offline trend analysis of flux measurements Quality control parameters -------------------------- DataModel: ``_ Contact person -------------- NISP-P calibration scientist .. raw:: latex \clearpage