Tuesday, March 10, 2015

Keeping Busy...

One of the joys of wide-angle astronomical imaging is that once the camera, tracking device and intervalometer start their work, they are quite happy running unattended. This leaves me to sit and watch the sky wheeling overhead, usually with mouth open in awe at the naked-eye view of a dark sky. Or if I had a van full of gear, I could set up a scope for viewing, but I'm normally not that organized. The other night, while imaging the "ghost comet" and current Comet Lovejoy, I had another impromptu experiment to try...

This is not an original idea - I most recently saw it on Optics Picture of the Day - an always interesting site on atmospheric phenomena, in a recent post on "Time Varying Lights". But I've seen this done even earlier. Imagine hand-holding a telephoto shot of a star for a several second exposure. The results can be interesting, as shown in the link and also here at left.  The star Canopus, only a few degrees off the southern horizon, was twinkling moderately, and in its long path through the atmosphere, different colors are refracted differently, resulting in a constantly-changing, almost psychedelic light show. The short time exposure, with the hand-held wiggles added record the changing intensity and colors. I braced myself against the open door of the van to provide a little extra "bounciness" to the image. I suspect that I could have done a better job of hand-holding otherwise. If you use an image-stabilized lens, make sure you turn that feature off! These shots were taken with the Canon 70-200 lens set to the maximum.

Another 30 degrees higher, Sirius, only a little brighter than Canopus, was visible, but the much-less air path to me showed much less scintillation and color effects. Generally astronomers avoid observing near the horizon for this very reason - the atmosphere is generally a lot steadier above 30 degrees or so off the horizon. The only exception are for objects that don't get that high, like comets that set shortly after sunset, or objects that skim the southern horizon and don't rise higher...


By this time, the planet Venus was getting low in the west, and was displaying a nice warm orange tint. I was wondering if the same color effects could be seen with the planet's disk. Venus was actually considerably lower than Canopus, and as a result, the atmosphere had already absorbed much of the blue part of the spectrum. The color ranges from greenish into red. In addition, in comparing it to Canopus, the scintillation, the variability of the brightness was lessened by the disk of Venus, even though small. Twinkling affects the point-sources of stars much more than a planetary disk.

So of course, I had to do another planet, perhaps higher in the sky. Jupiter was conveniently placed only about 15 degrees from the zenith. Even though the zoom setting and exposure didn't vary much between objects, Jupiter displays enough of a disk that it is easily seen in the widened trace of the image. So high in the sky you would expect a minimum of color variation, and also of brightness variation. Sure enough, the only place the trace dims is when the camera/lens was moving faster.

This was a fun little 10 minute project to do while my other camera was busy exposing. Would I do it again - absolutely! Perhaps even a little more focal length would show scintillation and color even better. I've got a little 80mm Meade scope that might be perfect! Go ahead and try it!

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