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.