Camera filters are useful tools for stargazers – The Durango Herald

Greetings, astronomers.

I was thrilled when the Sitter Family Hall rooftop telescope appeared in a front-page story on July 30. As with many such projects, there is a steady stream of upgrades going on. The biggest recent upgrade is to the Camera Filter Wheel, which will soon allow us to select up to 18 different filters.

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

The first photographic filters were just colored glass, which filters light by absorbing unwanted wavelengths. Modern astronomical filters select the wavelengths that pass by using multiple layers of dielectric coatings on transparent glass. Clear, polished glass reflects light, and most of the time, those reflections are unwanted.

Glasses wearers are likely familiar with anti-reflective coatings that can be added at an additional 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 different 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 thickness of several layers of these dielectric coatings can be adjusted to allow certain wavelengths of light to pass easily, while causing other wavelengths to be almost completely reflected. wave. As you might expect, the sharper the cutoff between the transmitted or reflected wavelengths, the higher the quality of the filter. (And the higher the cost!)

The most expensive filters are called narrow band filters. These only allow light of a narrow range of wavelengths corresponding to a specific spectral line to pass through while reflecting all other wavelengths. The use of one of these filters makes it possible to detect the distribution of hydrogen or oxygen atoms in a cloud of interstellar gas. Looking at one of these filters is like looking at a mirror, since almost all the light is reflected.

This month

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

The planets are finally rising early enough to be seen at night. Saturn is the brightest object in the southeastern sky just after sunset. It will reach the opposition, its closest approach to Earth, on August 14. That evening it will cross the meridian and be at its highest point in the sky at “local midnight”. However, this highest point will be just under 40 degrees above the southern horizon, so you won’t need to strain your neck to look up. And with Durango daylight saving time and longitude, local midnight won’t be until 1:17 a.m.

Jupiter will rise a few 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. The elusive Mercury will be in the western evening sky, at its greatest angular distance from the sun on August 27.

This year’s Perseid meteor shower peaked this morning (August 13), but you should be able to see debris from Comet Swift-Tuttle for another week or so. It may be easier later this week when the moon is less full.

The comet 2017 C/2017 K2 PanSTARRS, is now of magnitude 8 and passes through Ophiuchus in the direction of Scorpius. It should be visible through a small telescope for the rest of the year.

Charles Hakes teaches in the Department of Physics and Engineering at Fort Lewis College and is Director of the Fort Lewis Observatory. Contact him at [email protected]

COMET 2017 C/2017 K2 PanSTARRS:





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