.. index:: Photometric large-scale flat (VIS) .. _calobs-vis-shuttermap: **CALPRODUCT-VS-028:** VIS Shuttermaps ====================================== .. include:: definitions.rst Parent requirement ------------------ | `R-CAL-B-VS-4244 `_ | `R-CAL-B-VS-4245 `_ Scheduling ---------- Monthly. Time-critical ------------- No Objective --------- **Uneven illumination:** The VIS shutter causes uneven effective exposure times across the VIS focal plane. The shutter map is an integral part of the :ref:`calobs-vis-large-scale-flats`, but it also needs to be known individually. **Impact on defocused PSF:** The shutter has a pronounced effect on the surface brightness distribution of the defocused images taken during :ref:`calobs-block-pv-phase-diversity`. This effect must be measured. Observations ------------ No dedicated observations are required for the in-focus shutter map. They can be obtained from the VIS nominal and short science exposures. For :ref:`calobs-block-pv-phase-diversity`, the impact of the shutter function on the PSF is needed, which requires a dedicated set of 5s and 20s integration pairs (not counting the shutter opening/closing time of 6s) to minimize Poisson noise. Fulfilled by ------------ :ref:`calobs-block-pv-selfcal` :ref:`calobs-block-pv-phase-diversity` (defocused shutter maps) :ref:`calobs-block-f-selfcal` Processing input ---------------- :command:`Ground Processing Input Data`: BPM :command:`Data Description`: Processing method ----------------- In general, the shutter map can be obtained as follows: Consider a single pixel of the focal plane. If the true flux on sky at this pixel position is "I0", then the observed flux "I1" for an exposure time t1 is given by :math:`I1 = I0 * (t1+S)` where "S" is an additive modification of the exposure time by the shutter. Now, consider a second exposure with different exposure time: :math:`I2 = I0 * (t2+S)` By dividing I1 through I2, and solving for S, we have :math:`S = (I1*t2 - I2*t1) / (I2 - I1)` Thus the shutter correction is obtained by taking a pair of images with different exposure times, without dither offsets. In case of pixels containing stars, the AOCS jitter is considered negligible, as the effective change of exposure time within a pixel is likely too small to be measured. The measurement can be done with any illuminated pixels (zodiacal background would suffice), likely involving suitable binning to get sufficient S/N. Processing output ----------------- The shutter map for VIS. In general, this would be a 2D pixel map. Required accuracy, constraints ------------------------------ TBD Processing tools ---------------- TBD Quality control parameters -------------------------- DataModel: ``_ Contact person -------------- VIS instrument scientist .. raw:: latex \clearpage