Tuesday, April 15, 2014

April Lunar Eclipse From Georgetown

Last night, I stayed up for the total lunar eclipse, the first of a series of four back-to-back total eclipses visible in North America. The skies in Georgetown were clear with a bit of haze.

For this imaging sequence, I set up a Canon 60Da camera and a Canon EF 100-400mm f/4.5-5.6L lens piggybacked on my observatory telescope. I set the ISO to 100 and stopped down the lens to f/8. I captured a set of seven exposures ranging from 1/1000 to 4 seconds and repeated this pattern every ten minutes over the duration of the eclipse from 12:30 to 05:30. Multiple exposures were necessary because of the huge difference in brightness between the full and eclipsed moon, nearly 12 stops, and I had no idea ahead of time which would be best. I had wanted to automate this in my copy of Astrophotography Tool, but the current version of the software does not allow mixing of exposures less than and greater than 1 second.

The next day, I chose a representative exposure for each time interval, looking for those that gave the best detail and representation of what I was seeing at the time. I combined these in Photoshop, tweaking the exposure levels at each frame to give a smoother transition, and creating the animated GIF as show below.

Total Lunar Eclipse, April 15 2014 - Animation of frames with different exposure times
As the animation progresses, you can see the full moon obscured progressively by the Earth's shadow until there is only a think crescent left. All of these initial frames were from the 1/250 sec exposures.  At this point, the next few frames are from progressively longer exposure up to 4 sec and full eclipse. This causes the scene to brighten to simulate the eye adjusting to the dimmer view.

As the last crescent of the sun's direct illumination disappears, the colors become apparent and some stars become visible. The brightest of these is 76 Vir, not Spica which is down to the right three moon-widths and out of the field of view. The motion of these stars from frame to frame shows the rapid progression of the moon in its orbit.

During the full eclipse, the lunar disk is bathed in dim orange-red glow. This color results from sunlight in which blue light is scattered away as it travels through the Earth's lower atmosphere. It has been poetically referred to in the media as being "illuminated by all of the sunsets and sun-rises in the world." Even more interesting, just after the bright crescent of direct sunlight disappears (02:13 CDT frame) and just before it reappears (03:20 CDT frame) the edge of the moon glows blue rather than red. This is caused due to absorption of red light by ozone in the upper stratosphere. The NOAA Earth Systems Research Laboratory has a nice set of slides explaining these phenomena. The still image below shows this phenomenon.

Despite having finally found some trial software to automate the capture process so I could get some sleep, I still stayed up until 3:00 in the morning, fascinated by the view. What these images do not capture is the three-dimensional effect of the fully eclipsed moon floating in a dark sky. The full moon is normally so bright that it looks like a harsh flat disk to me. The eclipsed moon had the texture of a crescent moon but showed the full disk. To make the show even better, the planet Mars shone brightly at opposition, only a hands-width away from the moon. The bright star Spica in the constellation of Virgo was just below and to the right of the moon. At one point in the evening, I even saw a bright meteor shoot down between the the Moon and Mars ... awesome.

You can see the moon at full eclipse along with Mars and Spica in the image below. This was taken from a fixed tripod using a Canon 450D at ISO-100, f/3.5, 2 sec and a Canon EF 100mm f/2.8 USM lens. A longer exposure from a tracking mount would have given me more stars in the image. Something to try at the next eclipse.

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