.. index:: Nonlinearity (VIS) .. _calobs-block-pv-vis-nonlinearity-onsky: **CALBLOCK-PV-024:** VIS On-sky Nonlinearity ============================================ .. include:: definitions.rst Observation configuration file ------------------------------ | :download:`ocf_CALBLOCK-PV-024_NL-NOMINAL.csv ` Scheduling ---------- Once, early during PV phase. In case of failure, repeat as soon as possible. Time-critical ------------- No. Objective --------- Radiation damage may slightly alter voltages in the VIS electronics, changing the :ref:`VIS nonlinearity ` as well as the PSF model. The VIS nonlinearity must be monitored continuously in flight. This ``CalBlock`` obtains the nonlinearity estimate from on-sky observations (stellar flux measurements), complementing :ref:`calobs-block-pv-vis-nonlinearity-flat`. Observations ------------ **Parallel schedule:** :ref:`calobs-block-pv-nisp-LED`; This ``CalBlock`` is driven by VIS. Nonlinearity is determined per pixel, folding in a PSF model to account for AOCS jitter. At least 1000 pixels are required per detector quadrant. For a field with stellar density of 2 arcmin\ :sup:`-2` in the range V=17.5-21 AB mag (such as at the Self-Cal field), and assuming that one star provides 5 usable pixels, 16 dithers yield about 1760 pixel per quadrant. Since the data are read out with the shutter open, trailing will occur, which must be fixed at software level. An exposure with 0 seconds integration time will provide information on the trails. The VIS 4-exposure NL sequence is executed on a stellar field, followed by a dither offset. This will take 4 exposures with exposure times of 0 (flush and readout trails only), 25, 155 and 540s integration time. Repeat 32 times, where every 8 NL sequence begins with a single BIAS acquisition. .. code-block:: python :caption: VIS on-sky nonlinearity sequence Slew to VIS NL field VIS: Repeat 4x: take BIAS Repeat 8x: Execute predefined VIS 4-exposure NL sequence Random dither offset in a 3' box Buffer for CDPU processing if necessary Fulfilling ---------- :ref:`calobs-vis-nonlinearity` Targets and pointing constraints -------------------------------- :command:`Pointing`: Straylight suppression as per MOCDA-SYS-003. .. csv-table:: **VIS nonlinearity field:** The area within the Self-cal field without brighter stars is suitable. :download:`CSV file ` :file: tables/targets-selfcal-faint.dat Instrument configuration ------------------------ Instrument: VIS 4-exposure NL sequence (VI-FCP-501) Methodology / Uplink procedure ------------------------------ :command:`Entity`: VIS :command:`Commanding Method`: :command:`Dither Sequence`: Random dithers in a 3' box Inter-operability constraints ----------------------------- NISP mechanisms must not move while VIS observes on sky. In-orbit duration ----------------- :command:`In-orbit Duration`: 15.1 h + dither (x32) Duration of a single 4-exposure NL sequence (repeated 32x): ``10s (RD+O+F) + 2*297s (1st-2rd exposures processing) + 545s (4th) + 4*72s (Readout) + 286s (4th processing) = 1723s`` Adding a single BIAS before the NL will add another 284 seconds; there will be 4 BIASes in total. 0.6 hours for 32 dither slews. Note that 66s of the CDPU processing time can be accommodated into the dither/slew duration. Data volume ----------- ``4*VB = 1000 MB`` (BIAS) ``32*4*VS = 57856 MB`` (NL) If the trailed images compress less efficiently, then the data volume would be higher. Processing input ---------------- :command:`Ground Processing Input Data`: TBD :command:`Data Description`: 128 VIS images Processing tools ---------------- TBD Quality control parameters -------------------------- DataModel: ``_ Contact person -------------- Calibration lead .. raw:: latex \clearpage