The Focal ratio at WHT Prime is f/2.8, which gives an image scale of 17.5 arcsec/mm. The corrector is specified to give images better than 0.3 arcsec total encircled energy (FWHM will be considerable better than this) longward of about 3800 Angstroms, and out to 20 arcmin radius. Because of the sheer thickness of glass in the corrector the transmission in the U band is 60%, although in B it is 80%. Thus its primary purpose will be imaging in broad and intermediated passbands longward of 3600 Angstroms.
The Atmospheric Dispersion Corrector (ADC) provides compensation for
atmospheric dispersion at wavelengths from 3200
to 1
m, and at
zenith distances from 0 to 73 degrees.
The unvignetted field of the corrector is 40 arcmin in diameter although some
of this is required for the off axis CCD autoguider, which moves within a
rectangular field near one edge of the unvignetted field. The field of
view of the autoguider at any one time is 0.2 sq arcmin; the total field
accessible to the autoguider is 210 sq arcmin, although this is
partially vignetted. The Prime Focus Instrument Platform is described in more
detail in section 
.
The filter wheel will accomodate filters up to 125mm in diameter, and there is a dedicated set of ``Harris'' broad-band filters of this diameter. The field size at the focal plane is limited by a shutter, 100mm in diameter, which lies about 20mm below the focal plane. The unvignetted focal plane is in excess of 90mm in diameter. This corresponds to 26 arcmin.
, and can be provided for other sizes.
.