CALPRODUCT-VS-012: VIS Dichroic Ghost Model¶
Parent requirement¶
Scheduling¶
Once
Time-critical¶
No.
Objective¶
Ghosts above a certain intensity level must be masked, as they bias the shape and photometry measurements. The on-ground ghost models is based on Zemax simulations and has been verified by the PLM end-to-end tests. The in-orbit validation must confirm (1) the location and (2) amplitude of the ghosts. Size, shape and wavelength dependence of the ghosts are expected to be correctly characterized by the simulations and are not a formal part of the in-flight validation. However, should the current model be inaccurate for these parameters, then this will be evident in the masked pixel data and the models can be updated.
The main ghost source is reflection off the back side of the dichroic, with an amplitude ratio requirement of less than 2e-6 in the PLM requirement document (SRD-PLM-34). Ghosts due to reflections off the focal plane also have an amplitude ratio requirement of less than 2e-6 in the PERD (R-VIS-P-017), but are stopped anyway. A further ghost source is a reflection from the NISP optics, but is ~4 orders of magnitude lower than the dichroic ghost and thus not considered further; the corresponding sources are so bright (AB~5) that scattering effects render data from the respective detectors useless. Ghosts need to be masked for sources brighter than 15 AB mag.
The dichroic ghosts can be well measured using stars with VIS magnitudes in the range of 10-12 AB mag. As a minimum, such a star would be placed at the center and in the four corners of the field of view. The self-cal field contains numerous stars in this magnitude range and observes 60 dithers, hence no separate observations are required.
Fulfilled by¶
Methodology / Uplink procedure¶
Data volume¶
75*VS = 33900 MB = 271 Gb (same data will be used for CALPRODUCT-VS-013: VIS Large Scale Flat)
Processing input¶
Ground Processing Input Data: 75 VIS images
Data Description:
Processing method¶
With the astrometric solution known, determine the exact position (reference) of the bright star (saturated) in the FPA. Determine the size, location and shape of the ghost images, as well as the ratio of the ghost irradiance with respect to the bright star irradiance, and compare them to the ghost model predictions. Update the masking parameters if necessary.
Processing output¶
Ghost image maps, comparison with model predictions, updated ghost model (if necessary)
Required accuracy, constraints¶
Images successfully acquired, achieving sufficient S/N in the ghosts
Processing tools¶
Single-frame processing including astrometric calibration (L1?)
Validation¶
The ghosts must fall within the predicted areas of the ghost model. The masks should cover all light from the ghosts while minimizing the overall masked area. The ghosts must not fall on top of other bright stars, otherwise the ghosts’ centroids cannot be measured accurately. The Self-cal observations will provide a large number of clean dichroic ghost images.
Recovery¶
Update the ghost model.
Quality control parameters¶
DataModel: https://gitlab.euclid-sgs.uk/
Contact person¶
VIS instrument scientist