At present, we have 31 narrow band filters, mostly of 50Å bandwidth, comprising a set of emission line filters and a set of redshifted H redshifted filters. Only one set exists and therefore the filters are shared between the WHT, INT and JKT. These filters were specified for a focal ratio of 4.5 which explains the difference in wavelength between the actual peak wavelength and the required peak wavelength. The original specification for the emission line filters is given below:
Unless otherwise specified the effects of an f/4.5 converging beam of light has been taken into account and the central wavelength duly reddened. A correction of +2 Angstroms has also been made to correct the central wavelength for operation at +10 degrees Celsius on the assumption that the filters will be made, tested and specified at +20 degrees Celsius.
In all cases the thickness of the filter including blocking filters should not exceed 9mm and should be the same for all filters wherever possible. The effective refractive index should be near 2.1. Blocking in all cases should be 3000-12000 Angstroms and the tolerance on the central wavelength should be 3 Angstroms. The tolerance on the dimensions of each filter should be 0.5mm. Filters should be free from pinholes greater than 0.03mm diameter in the central 25mm diameter circle and free from pinholes greater than 0.1mm diameter elsewhere.
See Figures 17, 18, and 19 for details of these filters.