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Calculation of Focus Offsets

Focus offsets for a filter of thickness can be calculated as follows:

Consider the light from a converging beam falling on a translucent plate of thickness and refractive index as shown in Figure F.3.

Now the change in focus

For the glass broad-band filters, the refractive index is 1.6, so focus offset = 0.38

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(9069Å and 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.)