The INT has a radial distortion in image position of the form
r_true = k1*r + k3*r**3
where r is the measured radial position (ie. from optical axis) and r_true is the correct position on the sky. The INT handbook gives k1 = 24.7 "/mm and k3 = -9.202e-05 "/mm**3. In angular units this translates to
r_true = r*(1 - 6.106e-09*r**2)
where r_true and r are in arcsec
or in radians the above reads
r_true = r*(1 - 259.8*r**2)
The distortion amounts to some 10 arcsec near the edges of the CCD mosaic and has to be taken into account for accurate astrometric work.
Using the above default values (more precise values are being determined) gives astrometric solution residuals with respect to Schmidt plates of 0.25 arcsec for the several hundred Schmidt objects per CCD. Much of this error comes from the Schmidt astrometry so the likely accuracy is significantly better.
An example of a FITS WFC header for the INT is given below:
CTYPE1 = 'RA---ZPX' / Type of coordinate on axis 1 CTYPE2 = 'DEC--ZPX' / Type of coordinate on axis 2 CRPIX1 = 3901.25426827418 / Reference pixel on axis 1 CRPIX2 = 3004.30115854872 / Reference pixel on axis 2 CRVAL1 = 187.8708 / Value at ref. pixel on axis 1 CRVAL2 = 12.73336 / Value at ref. pixel on axis 2 CD1_1 = -2.1687941821018E-6 / Transformation matrix CD1_2 = -9.2590757277327E-5 / Transformation matrix CD2_1 = -9.2788255682060E-5 / Transformation matrix CD2_2 = 2.06981678285159E-6 / Transformation matrix WAT0_001= 'system=image' WAT1_001= 'wtype=zpx axtype=ra projp1=1.0 projp3=259.8' WAT2_001= 'wtype=zpx axtype=dec projp1=1.0 projp3=259.8'