WHT prime-focus field distortion

At all WHT foci, the scale in arcsec/mm varies slightly with radius in the field of view, but the distortion is expected to be radially symmetrical.

For WHT prime focus, this is confirmed by the map below, which shows the few-% variation of scale across the central ~ 40 arcmin x 40 arcmin of the field of view.

The map (click for enlargement) was created by comparing the positions of stars on 100 overlapping images taken with the PAUCam wide-field imager, mounted behind the WHT's standard (pre-WEAVE) prime-focus corrector. Reproduced by kind permission of the PAUCam team.