.. index:: Color gradient calibration fields .. _calobs-block-f-color-gradient-fields: **CALBLOCK-F-004:** Color Gradient 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 ration. 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 ------------- The AEGIS field should be completed in the first year as it has the largest HST coverage. At least one field must be observed every following year. Objective --------- Color gradients within galaxies need to be taken into account by the shear measurement due to the wavelength dependency of the PSF. To this end, VIS and NISP will observe 5 fields (with ample ancillary HST data) to 4 times the mean S/N of the wide survey. Observations ------------ Observe the 5 color calibration fields with the ROS to at least 4 times the mean imaging S/N of the wide survey area. Three of these fields are already observed to 5 times the imaging S/N as part of :ref:`calobs-block-f-photoz-calibration-fields`, and hence do not need to be observed a second time. **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. **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-vis-color-gradient-fields` :ref:`calobs-nisp-color-gradient-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:: **Color gradient 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: 25, 10, 10, 8, 9, 17 :file: tables/targets-color-gradient-fields.dat \* Note that the COSMOS, UDS and CDFS fields are already fully covered by :ref:`calobs-block-f-photoz-calibration-fields` observations. 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`: 63.8 h (not counting 125.4 h for COSMOS, UDS and CDFS, which are covered by :ref:`calobs-block-f-photoz-calibration-fields`) Data volume ----------- ``54*ROS = 268974 MB`` Processing input ---------------- :command:`Ground Processing Input Data`: :command:`Data Description`: Quality control parameters -------------------------- DataModel: ``_ Contact person -------------- Calibration Lead .. raw:: latex \clearpage