Light Pollution Filters
For those who want to take astro-photographs or observe faint night-sky objects in a light polluted area, where the light pollution is mainly from low pressure sodium lights, it is possible to use a filter on the camera or telescope. Although not ideal it may be the only way you have to improve visibility without travelling great distances to find a darker sky.
There are a few types of filter available. The one described here is a Lumicon low pressure sodium Filter. The filter works by blocking the narrow wavelengths of light produced by low pressure sodium lighting. High pressure sodium lighting has many more and broader bands which can't be so easily filtered out without stopping the light that we want to record from the stars or nebulae.
There are two problems with using such filters: they can be quite expensive and they are usually only available in small sizes, which means you can't place them over the objective of the telescope. You are restricted to using them between the eye/camera and telescope.
Below are two photographs showing the effect of using the filter. The difference is quite marked. These photographs are of a low pressure sodium light against the night sky.
Photographs from IDA slide set
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