Posted by Brian Ventrudo
The idea of an optical filter is fairly simple. It’s a piece of colored or coated glass that passes light of certain colors while blocking others. A filter can’t make an object brighter, but it can improve contrast. For example, if you look at a red and green light through a red-colored filter, the red light would appear bright and the green light would look dark.
We’ve covered colored optical filters before: a set of three or four colored filters are great for observing the planets, especially Mars and Jupiter.
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Around the Moon in 28 Days takes you on a full tour of the Moon during the month. Learn the seas, highlands, craters and mountains of the Moon, and get a glimpse of the solar system in its earliest days. Coming soon at Stargazer University.
Comet filters work the same way. They let through green light at a wavelength of 501 nanometers which is emitted by ionized oxygen. And they let through light at 511 nm and 514 nm emitted by carbon molecules. The improved contrast helps reveal the delicate ionized tail of gaseous comets, and improves the contrast of the coma (or head) of the comet against the sky.
For comets that are more dusty than gaseous, a comet filter will not help as much, since dust reflects light from the sun, which is made up of a broad range of colors. But this can help you tell at a glance if a comet is primarily dusty or gassy, if you will.
Spectrum of Comet Hyakutake (in 1996) showing strong C2 emission at 511 and 514 nanometers (green light)
If you aren’t inclined to spend $100 on a specialty comet filter, you might try using an OIII light pollution filter on a comet. While this filter doesn’t pass the 511 nm and 514 nm bands of carbon, they do pass light from ionized oxygen. An OIII filter is extremely useful for many types of nebulae as well, and should be in your toolkit as you get more serious about deep-sky observing.
If you have an OIII filter, try it on Comet Hartley 2 once the Moon’s brightness fades later this week. Let us know if it works for you…