Thursday, October 23, 2014

Solar Eclipse Sports a Great Sunspot

To see the eclipse today, Danielle and I set up on the roof of the parking garage of the North Austin office building at my place of work. All afternoon we had watched the weather forecast, hoping the the overcast skies would clear up before the eclipse began. Luckily we got a break in the clouds just in time for the show! Having seen a post on Bad Astronomy about the current huge sunspot AR-2192, we opted to bring a full set of equipment, not just the simple camera I had originally intended to use. In addition, we packed several "eclipse glasses" for my colleagues, who I encouraged to join us.

The equipment was very similar to that we had used on Haleakala for the Venus Transit. For white-light imaging, we setup a Canon 60Da with a Canon EF 100-400mm f/4L 4.5-5.6 IS lens. With a Kenko 1.4x tele-extender, this gave an effective focal length of 530mm. A Kendrick Visual Solar Filter mounted to the objective produced good images at ISO-100, f/8, and 1/250 sec. exposure. We used Astro Photography Tool to control the camera from the laptop and obtain better focus than we could manage by hand or autofocus. This setup was mounted on a tripod and had to be adjusted every few minutes.

For H-alpha imaging, we set up a Lunt 60T/PS pressure-tuned H-alpha telescope and captured short 250 frame AVI movies with an Imaging Systems DMK 41AU02 camera and an Antares 0.5x focal reducer. The latter required a shortened nose-piece to accommodate the limited back-focus of the telescope. These movie frames were later stacked using RegiStax 6. All of the H-alpha equipment was mounted on a GM-8 which we only roughly aligned to north. This was enough to prevent having to continuously find the sun in the small field of view of the telescope. We occasionally replaced the camera with a 15mm eyepiece to get the visual experience through the telescope.

The white light image below gives the best view of the photosphere showing the umbra and surrounding penumbra regions of the sun spot pairs. This view also shows some hints of granulation, evidence of surface convection cells. In contrast, the Hydrogen-alpha image gives a view of the hotter chromosphere. It shows lighter colored plages near the sun spots as well as filaments and prominences of plasma twisted by magnetic field lines. In addition to the AR 2192 region and several small spots, this image shows a large filament cutting across the top half of the solar disk. It looks like a Frankenstein scar. Though hard to see on the image below, several small solar prominences were also visible through the solar telescope.

Seeing the huge sunspot in conjunction with the eclipse was well worth the effort of setting up all of the equipment. I hope my colleagues were also impressed by the show that mother nature put on today, though I think they were more enthusiastic about the eclipse glasses than by the expensive imaging gear! There is certainly something cool about looking straight at the sun with these flimsy things on.

Update 10/24 - Phil Plait at Bad Astronomy was kind enough to link back to this blog page from his gallery of reader-contributed images of the eclipse. Take a look at some other creative shots he posted there. I have corrected the reversed time-stamps on my image above. The white-light image is the later shot. Since the moon has a faster retrograde motion, it should move across the solar disk from lower-right to upper-left as oriented in these images. My brother-in-law, Ross Cunniff, was clouded out part of the eclipse. See his YouTube video for a creative way of handling the disappointing conditions. As of 10/24, the AR 2192 region has thrown to X-class flares but no CME.

Update 10/26 - Had a look at AR 2192 this afternoon as it moved toward the edge of the solar limb. The sunspot is not as pronounced but there were a few more prominences.

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