Focus offsets for a filter of thickness t can be calculated as follows:
Consider the light from a converging beam falling on a translucent plate of thickness t and refractive index n as shown in Figure 20.
Figure 20: The light path of
the WHT-PF near the filter
Now the change in focus 
For the glass broad-band filters (and also the Kitt Peak
filters), the refractive index is 1.6,
so the focus offset = 0.38 
.
A second
correction is needed, due to the corrector lens.
In practise you have to multiply
the correction calulated above by (f/f') 
, where f is the focal
length without corrector lens, and f' the focal length of the whole system,
resp. 2.5 and 2.8. The final
change in focus is now 
.
For the interference filters, the values for the refractive indices are
approximately 1.45 for the OII(3727Å), SII(4070Å), HeII, H 
, OIII,
HeI(5876Å), OI and NII filters, and 1.48 for the H 
, SII(6724Å),
OII(7325Å), SIII(9069Åand 9532Å) and HeI(10830Å) filters.
(N.B. Do not use the refractive index of the substrate,
which, although of large refractive index ( 
2.1), is thin and enclosed
in thick glass.
The telescope can be automatically compensated for the filter thickness using the ICL PSETFOC routine (see Section 4.5).