This webpage contains information of the vignetting at the various foci of the WHT.

Vignetting at WHT

Vignetting at the various foci.  The curves show the percentage transmission as a function of field radius.  For accurate photometry, this should be used only with caution (see text).  These curves refer to the area outside the constant central obstruction of 8.4 per cent (Table II) and supporting vanes obstructing 0.6 per cent.