Filters and the Night Sky

I wanted to buy a light pollution filter. I saw there are 2 kinds of filters UHC (Narrow band filter) and the Broadband Filters (e.g. CLS). Which one should I get?

I think the easy way to answer the question is as follows.

For Narrowband substitute the word Red and for Broadband substitute the word Polarizer.

A Red filter will definitely block out light that is not red. A polarizer lets all colors through roughly equally, but a polarizer does diminish the total amount of light. If you use the red filter you’ll still see, but you’ll see only the red component of the light. If you use a polarizer things will look normal, but dimmer.

The same is generally true for Narrow and broadband filters. Of course a Narrowband filter doesn’t have to be red, it could be green, blue, ultraviolet, etc and a broadband filter also doesn’t have to be neutral. Indeed, the idea behind a broadband filter is to pass as much light as possible in as many wavelengths as possible EXCEPT for the predominant wavelengths of the light pollution.

The only broadband filter I’ve used, the Astronmik CLS, is a light blue color and definitely gives things a blue tint that requires correction.  The filter I have drops into my camera just ahead of the sensor. That setup makes is more versatile, but it only works on Canon cropped cameras, not on my 5D II, for example. I purchased my filter from OPT for about $130 and frankly haven’t used it much because it got stuck in my Canon 40D.

One problem with light pollution is that it comes in many different wavelengths (colors) depending on the light source: tungsten, fluorescent, halogen, low or high pressure sodium vapor, etc. So the effectiveness of any broadband filter changes significantly with the lighting conditions around your photography/observing location.

On a more practical matter visual astronomy is usually done with a telescope or binoculars and the filter must match (one of) the standard eyepiece sizes or simply be hand-held. Visual observing is also different from photography in that color is easily recorded by a camera, but the dim light of stars, galaxies and nebula is almost always perceived in grayscale by we human observers. For photography the filter size depends on where the filter is placed in the optical system – thus you may need differently sized filters for different uses. I’m much more of an astrophotographer than an astronomer so I can’t really comment on what works best for visual astronomers.  Fortunately Shawn Grant does a great job explaining this in his article.

One filter that MIGHT have some significant application in landscape astrophotography is a graduated neutral density filter.  But I’m not really a big fan of these filters as I describe in a previous article.

What do you think about this?