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Camera filters are useful tools for astronomers

Greetings, stargazers.

I was pleased when the telescope on the roof of Sitter Family Hall showed up in a front-page story July 30. As with many such projects, there are a steady stream of upgrades in the works. The most significant recent upgrade is to the filter wheel on the camera, which will soon let us select from up to 18 different filters.

To get the highest possible sensitivity, most astronomical cameras cover all their pixels with the same filter. This is unlike your phone, where red filters cover a fraction of the pixels, green filters cover others and blue filters cover the remainder. The ratios are chosen to produce a color image that closely matches what a human eye sees.

Early photographic filters were just colored glass, which filter light by absorbing the undesired wavelengths. Modern astronomical filters select which wavelengths get through by using multiple layers of dielectric coatings on clear glass. Clear, polished glass reflects light, and most of the time, these reflections are unwanted.

Eyeglass wearers are probably familiar with anti-reflective coatings that can be added for an extra cost. These coatings are very thin layers of a dielectric material that is evaporated onto the glass in an oven. Their anti-reflective properties come from the number, composition and thickness of the various layers. More layers at a more precise thickness improve performance, but at a proportionally higher price.

The same types of dielectric layers applied with different thicknesses can also be used to create hyper-reflective coatings that can reflect light much better than polished metal. To create modern astronomical filters, the number and thicknesses of multiple layers of these dielectric coatings can be adjusted to allow certain wavelengths of light to pass through easily, while causing other wavelengths to be reflected almost completely. As you might expect, the sharper the cutoff between wavelengths that are transmitted or reflected, the higher the quality of the filter. (And the greater the cost!)

The costliest filters are called narrow band filters. These only pass light from a narrow range of wavelengths corresponding to a specific spectral line to pass through while reflecting all other wavelengths. Using one of these filters allows you to detect the distribution of hydrogen or oxygen atoms within a cloud of interstellar gas. Looking at one of these filters is like looking at a mirror, since almost all of the light is reflected.

This month

The summer Milky Way is at its best. It is high in the sky right after sunset, with the brightest part toward Sagittarius near the southern horizon.

The planets are finally rising early enough to be seen in the evening. Saturn is the brightest object in the southeastern sky right after sunset. It reaches opposition, its closest approach to Earth, on Aug. 14. That evening it will cross the meridian and be at its highest point in the sky at “local midnight.” However, that highest point will be somewhat less than 40 degrees above the southern horizon, so you won’t need to strain your neck looking up. And with daylight saving time and Durango’s longitude, local midnight won’t be until 1:17 in the morning.

Jupiter will rise a couple of hours after Saturn, and will be the brightest thing in that part of the sky at that time. The even-brighter Venus continues to be the morning star. Elusive Mercury will be in the western evening sky, with its greatest angular distance from the sun coming on Aug. 27.

This year’s Perseid Meteor Shower peaked this morning (Aug. 13), but you should be able to see the debris from comet Swift-Tuttle for another week or so. It might be easier later this week, when the moon is less full.

Comet 2017 C/2017 K2 PanSTARRS, is now magnitude 8 and passing through Ophiuchus heading toward Scorpius. It should be visible through a small telescope for the rest of the year.

Charles Hakes teaches in the physics and engineering department at Fort Lewis College and is the director of the Fort Lewis Observatory. Reach him at hakes_c@fortlewis.edu.

Useful links

COMET 2017 C/2017 K2 PanSTARRS: https://bit.ly/3pcdmuY.

ASTRONOMY PICTURE OF THE DAY: http://apod.nasa.gov/apod.

OLD FORT LEWIS OBSERVATORY: www.fortlewis.edu/observatory.

AN ASTRONOMER’S FORECAST FOR DURANGO: https://bit.ly/2eXWa64.

FOUR CORNERS STARGAZERS:

https://bit.ly/2pKeKKa.

https://bit.ly/3hOMKwh.