.. index:: Photo-z calibration fields .. _calobs-block-f-photoz-calibration-fields: **CALBLOCK-F-005:** Photo-z Calibration Fields ============================================== .. include:: definitions.rst Observation configuration file ------------------------------ Not applicable Scheduling ---------- Scheduled by EC-SURV. In case of failure, repeat as soon as possible. It is critical for science to have these data available early-on in the mission, and some of the targets have limited visibility. Replanning by EC-SURV. **Blue-grism angle separations** must not be introduced at the cost of thermal transients. It is permissible to split the observations to provide angle separation for the blue grism, provided that the survey is not fragmentated too much, at the discretion of EC-SURV. This may delay or advance the completion of the data product by up to 6 months and is permissible. **Grism order:** At the end of all observations, an approximate exposure time ratio of 5:3 should be achieved for all fields, to maintain homogeneity. If the field is split up in two block visits, then both grisms should be used, but there is no strong constraint on the exposure time ratio. For example, a field that requires two block visits and a total of 18 passes, can be observed with a 6:3 ratio and a 5:4 ratio in the first and second pass, respectively. This is preferred over two 6:3 passes, which would skew the exposure-time ratio. Within a block visit the order of grisms does not matter. Time-critical ------------- There is a trade-off between depth and area, both of which are important to have early on in the mission. The 2x2 COSMOS field must be started in the first year, and completed in the following fashion: Observe the full area to 3x the depth for DR1, using passes over the full area. This will grow the depth uniformly in the presence of Solar activity. Then go to 5x the depth. Spread the observations as even as possible between the two yearly visibility windows, to get an even coverage of opposite dispersion directions for the blue grism. At least one of the other photo-z calibration fields shall be completed in each of the following years (starting with the second year). COSMOS and CDFS have priority in the first two years, since these data also fulfil :ref:`calobs-block-f-color-gradient-fields`. Objective --------- The purpose of these observations is to obtain NISP-P photometry in YJH bands for a sample of 20,000 galaxies with existing visible and near-infrared ground-based spectroscopy. The S/N must be 5 times better than that of the wide survey in the EDF-N area. Based on the non-Euclid spectroscopic reference data, the Euclid photo-z methods can be calibrated. VIS data are needed to perform the target detection and selection in an identical manner as in the wide survey, and to help with deblending. This significantly improves the photometric redshifts. In addition, OU-MER needs VIS data to provide consistent catalogs. The minimum depth for VIS is 2x the wide survey depth. For practial and legacy purposes, all observations are conducted with the ROS, so VIS will also go to 5x the survey depth, and we obtain Euclid spectrocopy. The ROS observations should employ blue- and red-grism observations to approximate the 25:15 (5:3) ratio as for the :ref:`calobs-block-f-deep-fields` and :ref:`calobs-block-f-cpc-fields`. Due to the number of pointings required, it might not always be possible to achieve exactly a 5:3 ratio for the blue:red grism exposures. Observations ------------ Observe the photo-z calibration fields with the ROS to 5 times the mean S/N of the wide survey. **Blue grism:** Both the blue and red grisms should be used in the ROS, with an exposure-time ratio approximating 5:3 for blue:red. The blue grism should ideally be scheduled at least at two different times, so that different position angles can be used for spectra-order decontamination. However, this may not be attainable for all fields due to scheduling constraints. Deliberate AA offsets are not allowed; the thermal stability of the telescope takes precedence. We note that the CDFS is contained in the EDF-Fornax field and hence would come for free by the end of the mission. However, the data are needed early on, and with a different tiling than the EDF-Fornax coverage, hence the CDFS must be scheduled independently. **Dithering:** At each dither point, one full ROS is executed. To avoid clustering of random dithers, we use points along a spiral. These produce a non-regular, yet fairly uniform distribution. In polar coordinates :math:`(\rho, \theta)`, and for a total of :math:`n` points, the position of point :math:`k` is given by: :math:`(\rho, \theta) = (scale * \sqrt{k} / 60, k* GoldenAngle)`, with :math:`scale = width / (2 \sqrt{n})` :math:`GoldenAngle = (3 - \sqrt{5}) * 180\;{\rm deg}` :math:`3' < width < 6'` (chosen by EC-SURV according to their constraints) This produces :math:`n` points inside a circle of diameter :math:`width` (in arcmin). The division by 60 converts arcmin to degree. The circle size increases with the number of points. **VIS Cycling:** The VIS ROS-inline calibrations follow the nominal wide-survey cycling, as described in :ref:`ros-inline-vis-calibrations-and-cadence`. Fulfilling ---------- :ref:`calobs-nisp-phot-photoz-calibration-fields` Targets and pointing constraints -------------------------------- :command:`Pointing`: Straylight suppression as per MOCDA-SYS-003. Blue-grism spectra must not introduce deliberate AA offsets. .. csv-table:: **Photo-z calibration fields.** N_ROS refers to the number of times the ROS needs to be repeated for each pointing to reach the desired S/N, compensating for extinction and background. **N_ROS is determined by EC-SURV and not imposed by calibration.** :download:`CSV file ` :widths: 15, 10, 10, 8, 9, 25 :file: tables/targets-photoz-calibration-fields.dat Instrument configuration ------------------------ Instrument: Wide survey Methodology / Uplink procedure ------------------------------ :command:`Entity`: VIS and NISP :command:`Commanding Method`: :command:`Dither Sequence`: Special. Full ROS executed at a number of computed offsets. Inter-operability constraints ----------------------------- None In-orbit duration ----------------- :command:`In-orbit Duration`: 745.3 h Data volume ----------- ``624*ROS = 3108144 MB`` Processing input ---------------- :command:`Ground Processing Input Data`: :command:`Data Description`: Processing output ----------------- Updated photo-z procedures Required accuracy, constraints ------------------------------ Must reach 5 times the average S/N of the wide survey in the EDF-N area. Quality control parameters -------------------------- DataModel: ``_ Contact person -------------- Calibration Lead .. raw:: latex \clearpage