WHT f/11 Nasmyth focus direct

• Focal plane position: Nominal position is 585 mm behind Nasmyth instrument rotator flange. Range of movement possible is 1500 mm about this position, although a move away from the nominal focus casues image degradation due to spherical abberation (see La Palma Technical Note no. 9).

• Field size: Unvignetted 7 arcmin, partial vignetting 23 arcmin. Note that vignetting due to the size of the Nasmyth flat drops slowly to only 70%.

• Image quality : At the nominal focus on a flat focal plane, 0.5 arcsec over a 10 arcmin field and 1 arcsec over a 20 arcmin field.

• Image scale: At nominal focus, 4.51 arcsec mm, 222 m arcsec.