Many astronomers who are interested in the deep sky side of astronomy try to purchase the largest aperture which they can possibly afford in order to be able to view deeper into space. One of the greatest problems is the lack of contrast between the sky and the object being viewed, or the darkness of the sky itself. Even in a dark rural area out in the countryside the sky is not completely dark. Out in the countryside when there is no Moon the main problem is neutral oxygen sky glow and in an urban area it is light pollution such as high and low pressure sodium and mercury emissions from street and security lighting.
There are two main manufacturers of light pollution filters, Orion Optics and Lumicon Ltd. Orion has produced two very useful filters, the Orion Sky glow, which can be used visually and photographically, and the Orion ultra block U.H.C. Filters visual only. U.H.C. stands for ultra high contrast, a very useful all-round filter. Lumicon, however, have produced seven filters which are, Lumicon Deep Sky, Lumicon H alpha, Lumicon minus-violet, Lumicon U.H.C. Lumicon OIII, Lumicon H beta and a Lumicon Swann and filter.
Lumicon sky and Orion sky glow filters. These filters are visual and photographic filters with an increase in contrast factor of three. Bright objects, such as the Orion Nebula, can be captured from city sites without any orange sky glow polluting the image. From my back garden in Newport I have even photographed the Horsehead Nebula which is extremely faint. In fact, the photographs I have taken from Newport are better than the photographs taken from a much darker site such as the Usk area. Photographing with these filters you can over expose. Instead of an orange sky background, a dark magenta background will appear on over exposed photographs. Without the use of these filters you will find that you would have to drive for a hundred miles of so to completely escape the orange glow in the sky in the UK. Not many of these dark sites still exist. Visually these filters have the greatest contrast gain on galaxies and reflection nebulae. For the very first time I had seen the nebulosity in the Pleiades, the Triangulum galaxy M33, the Owl Nebula M7 and the Crab Nebula M1 from my back garden in Newport. From a dark site I have seen the Veil Nebula, the Trifid Nebula M20, the Lagoon Nebula M8 and the dust lane in the Sombrero Galaxy M104. The telescope I was using at the time was a 6" F8 reflector. These are broadband pass filters with a band pass 436 to 546 nanometers.
The Lumicon H alpha filter.
H alpha stands for hydrogen alpha, the first wavelength of hydrogen, 656 nanometers. The H alpha filter blocks out all light below 630 nanometers. This means that all forms of light pollution are blocked out except the H alpha line 656. This is a narrow band pass filter and is photographic only. Only red sensitive gas-hypered 2415 film can be used with this filter.
The Lumicon minus-Violet filter.
This filter again is photographic only and is designed to sharpen colour telephoto images of star fields. It blocks off the violet end of the spectrum giving you much sharper lighter images.