Distortion Model Product

$SetSchemaPath vis/euc-vis-DistortionModel.xsd

Data product name

$PrintDataProductName

Data product custodian

$PrintDataProductCustodian

Name of the Schema file

$PrintSchemaFilename

Last Edited for DPDD Version

1.1

Processing Element(s) creating/using the data product

Astrometry

Processing function using the data product

VIS

Proposed for inclusion in EAS/SAS

Yes

Data product elements

$PrintDataProductElements

Detailed description of the data product

Simple world coordinate system containing a basic linear transformation from detector location to world coordinates, but images often contain higher-level distortion which we ideally want to correct. Distortion in astronomical images is introduced by some combination of the atmosphere, the telescope, and the detector. The atmosphere won’t be a problem for Euclid unless things go badly wrong. This leaves distortion due to the telescope optics, and the detector. For ground-based instruments the detector is usually mounted to the telescope via a rotator, meaning distortion can be a function of both focal plane location and detector location. For Euclid the detector is fixed with respect to the focal plane, for this reason WCSFIT uses the SIP convention to model distortion in VIS images, as opposed to the alternative TPV convention. TPV models distortion as a function of equatorial tangent plane location, where SIP models distortion as a function of detector location

Distortion Model product references a special config file to describe the distortion model for all CCD’s including givin the coordinates. Example of an output config is given below.

Assumption The main assumption we have made is that distortion and detector layout (including position, rotation, shear, stretch, reflection) remain constant between observations. Or at least between observations since the previous calibration run. I.e. that the only thing that changes astrometrically between two observations is where the instrument is pointing and its position angle. In practice this assumption is likely to be true only for low level distortion. It is anticipated that higher order distortions will be introduced due to e.g. thermal variation. It is anticipated that the modelling of such higher order distortions will be undertaken in future versions of WCSFIT.

Example Config GLOBAL RA = 269.710271627694 DEC = 66.8190091714787 PA = 38.3908591085259 A_ORDER = 3 B_ORDER = 3 CCDID 1-1 NAXIS1 = 4096 NAXIS2 = 4136 CRPIX1 = 2048.5 CRPIX2 = 2068.5 CRDEL1 = -0.44211182972669283 CRDEL2 = -0.02094841692590282 CD1_1 = 1.698508708266553e-05 CD1_2 = 2.226456533452789e-05 CD2_1 = 2.2138821799286164e-05 CD2_2 = -1.6912484705978774e-05 A_0_2 = -1.521813445590885e-07 A_0_3 = -5.374069316007574e-11 A_1_1 = 2.3702064764614898e-07 A_1_2 = 1.4833429090986117e-11 A_2_0 = -2.640743443038868e-07 A_2_1 = -3.2533388623588725e-12 A_3_0 = -1.6067317924145662e-11 B_0_2 = 1.3434099407641932e-07 B_0_3 = -5.11201549163031e-11 B_1_1 = -8.589136797622529e-08 B_1_2 = 1.9654254332792485e-11 B_2_0 = 7.050712858618846e-08 B_2_1 = -1.8941041275333634e-11 B_3_0 = -4.893762998212127e-12 CCDID 1-2 …

Detailed TechNote can be found here: https://euclid.roe.ac.uk/attachments/download/75548/wcsfit_euclid_docs-1.pdf