Don't fear the filter (part 1)

Posted by Michael Bakich
on Monday, July 16, 2007

With Mars beginning to brighten to its best appearance of the year (which will happen Christmas Eve), I thought I'd blog about filters. Lots of articles will explain the best way to observe Mars — you can catch mine in December's Astronomy — and they'll all have something to say about color filters. It's a big subject, so I'm breaking it down into three parts.

First, a small point about filters. No filter makes any part of any astronomical object brighter. Because all filters subtract light of certain wavelengths, filters make celestial objects fainter. So, when you hear someone exclaim, "Wow! This filter makes the nebula so much brighter." Remember what they actually mean is, "Wow! This filter makes the nebula so much easier to see."

From my experience, beginning amateur astronomers fear color filters. They don't fully understand filter use, they don't know exactly what filters should show, and the views seem unappealing. This probably stems from the natural desire to appreciate the object aesthetically. Well, color filters have their uses, but making objects "pretty" isn't one of them. A filter's purpose is to exaggerate contrast. In fact, nearly all criticisms of color filters indicate the observer was looking for changes in color rather than changes in brightness.

Manufacturers label color filters along their circumferences. To use one, just screw it into your eyepiece's barrel. All eyepiece filters have threads that match eyepiece barrels' threads. In some cases, observers prefer to hand-hold the filter and move it back and forth between the eyepiece and their eye. This is a quick way to compare filtered and unfiltered views. I don't do this, however. I'm too afraid of dropping a filter, especially if I'm wearing gloves. Some manufacturers make filter holders. Such units allow up to four filters to be pre-loaded. Most observers who use such a setup leave one slot empty so they can study the unfiltered view.

You can tell which part of the spectrum a color filter transmits by looking at the filter's edge. Manufacturers label color filters by their Wratten numbers. Kodak developed the Wratten system in 1909, and it's been the standard for photographers, astronomers, and others ever since.

Be aware that all color filters work better in conjunction with large telescopes. It's a simple rule of light throughput. For example, on nights of excellent seeing, I've tried to use a violet filter with my 4-inch refractor to observe cloud features in Venus' atmosphere. It just doesn't work. However, when I switch to my 12-inch Schmidt-Cassegrain telescope, voilà! The features are easy to see.

Next week, I'll yak about how specific filters can help you view Mars and the other planets.

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