Last Weekend - Part 2!

In the last post, I showed some of the first results with the new-to-me 6D. While taking the wide shots of the transiting Milky Way, with the lower edge of the field near the horizon, I noticed the strong green cast of an airglow display. The last image of that set, shown at left, shows the airglow at its highest extent that I caught. While it has the greenish glow of aurora (558nm), airglow is completely different from the northern lights they can resemble. First, they can appear in any direction - due south in this exposure. Airglow (the green kind) is caused by the recombination of oxygen atoms that were photo-ionized during the daytime.




But when airglow is visible as green in the camera or faintly visible to the eye as a glowing white cloud (too faint to trigger eye's color sensors), chances are there is a good display in the near infrared as well. I've got an IR-modified camera that replaces the IR blocking filter with an IR-pass for some cool landscape effectsI've also posted some airglow images and time-lapse clips here before. Of course, being duly prepared for anything, I had packed the camera for the outing Sunday nite, so set up the modified Canon 20D with the Nikon 16mm fisheye on a tripod to shoot a clip. There was indeed a nice display that filled the field of the fisheye lens. In addition, the structure showed good motion, so was great for a time-lapse clip. Interestingly, while the green airglow showed very faintly to the eye, this IR stuff, that looks so much like bands of cirrus moving through the field, was totally undetectable! Oh, and just to show the subtle changes that shooting in the near-IR (NIR) shows, at right is a close-up of the Milky Way center.  While the Teapot asterism of Sagittarius and the end of Scorpius are readily visible, there are some bright stars that seem out-of-place. Eta SAG and G SCO seem brighter - as bright as any of the other stars in the constellations (compare to color image above!). Sure enough, checking their Wikipedia entries, both are spectral class K or M supergiants that would appear brighter in the red...

The IR airglow emissions, interestingly enough, are known as OH Meinel Airglow, named for the scientist, Aden Meinel, who identified the source of the infrared glow from OH emissions in the upper atmosphere. And if you don't know Aden Meinel, shame on you! A Lick Observatory astronomer when he was hired by the NSF to establish a National Observatory (eventually located on Kitt Peak), he served as its first director before moving across the street to serve as director of Steward Observatory, and then went on to found the Optical Sciences Center! He is really the reason the Tucson area is a major center of astronomy and optics, even now 50 years after his efforts! I blogged about one of his last appearances in Tucson 6 years ago (he passed in October, 2011)...

In his first paper (1950) where he presents his evidence the spectral lines were due to OH+, he talked about the "short exposures" of 4 hours at Yerkes Observatory, compared to the 32 hour exposures required (over several nights!) with older instrumentation at Lick Observatory! Of course, this was due to the extremely slow IR photographic plates of the day. Imagine what he would think of the 60-second exposures shown here! So about 80 minutes of monitoring are included in the clip here, after some minor stretching. The banding structures are known as "gravity waves" - not the newly found electromagnetic waves from merging black holes, but bands in the upper atmosphere caused by upper winds and the restoring force of gravity...

Here is the clip - enjoy!