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Change due to uncollimated light

Utilising the filter in uncollimated (i.e. convergent or divergent) light involves slightly more complex considerations. Here, light enters the filter at a range of angles, so that different rays undergo unequal wavelength shifts. This results in not only a central wavelength shift, but a broadening of the bandwidth and lower peak transmission.

As a rough approximation, relatively uniform beams (with full cone angles less than 20 tex2html_wrap_inline5509 ) will undergo peak shifts of approximately one half of that which would be predicted for a collimated beam at the maximum angle of incidence of the cone.


The filters were specified for a focal ratio of 4.5, having a maximum angle of incidence of the cone of tex2html_wrap_inline6461 , and of effective refractive index 1.6.

Substituting these values in the equation,


The focal ratio of the WHT prime focus is 2.8, which gives a maximum angle of incidence of the cone of tex2html_wrap_inline6465 , so, substituting again into the equation,


Therefore, at the WHT prime focus, the central wavelength (i.e. the wavelength of peak transmission) is shifted towards shorter wavelengths by 0.15% from the specified central wavelengths. For example, for an tex2html_wrap_inline6469 filter with specified central wavelength 6560Å, the effective central wavelength will be 6550Å.

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Wed Sep 17 12:36:20 BST 1997