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At present we have 24 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 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:
The quotations are invited for the groups of interference filters specified below and the quotations may be submitted either as a whole or in complete groups:-
Unless otherwise specified the effects of an f4.5 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.
Size: 50mm square
Bandwidth: 50Å 5Å at half peak transmission not more than 70Å at 1% transmission.
§ This filter should have a bandwidth of 100Å 10Å This filter should have a bandwidth of 20Å 5Å This filter should have a bandwidth of 15Å 5Å
Note:- (1) All the filters should be of image quality (ie that the surfaces are flat to about ). (2) All exterior surfaces of the filters should be treated with an anti-reflection coating. (3) The size of the filters is not to exceed 50.0 x 50.0 mm.