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A Brief Discussion About Nebula Filters By Alan Figgatt
Nebula filters are useful accessories for the seasoned observer. But many beginners or even experienced observers confuse nebula filters with light pollution filters. Common questions are: what does a nebula filter do? Should I get one? If so, should it be a narrowband or an O-III? This article, which grew out of the Sky Tour I presented at the October monthly meeting, presents a very brief overview on nebula filters. When we refer to nebula filters, it is important to make clear that these filters are not the same as simple color filters which are used to bring out subtle features in planetary and lunar observing. Broadband and nebula filters are multilayer "interference" filters which instead filter out specific light wavelength regions as shown in the diagram. These filters can be broken down into 3 types:
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