Monday, May 9, 2016

Spots on the Sun - the Mercury Transit!

Today, 9 May, was the latest transit of Mercury, where from the Earth's vantage point, the planet's disk is seen in silhouette against the disk of the Sun.  You would think that it would happen every inferior conjunction as it swings between Earth and Sun, but because of the inclination of Mercury's orbit, it only happens about 13 times per century.  It is always fun to see the inky black disk slide across the sun, much darker than the sunspots, magnetic storms on its surface, demonstrate. 

For us here in Tucson, the transit started about 1.5 hours before sunrise, so of course, we miss that part. I had envisioned shooting the sunrise against the Catalina Mountains so that it could be spotted as soon as it rose, perhaps a few saguaros seen in silhouette too. I spent nearly an hour scouting locations Saturday evening, finally settling on a spot near the east terminus of Roger Road where it meets up with the Rillito Wash near the UA Farm.

Of course, documenting the sunrise means getting up well before sunrise, and since I never use my alarm, actually had to try it to see if it worked! It does, but I ended up waking 10 minutes before it was to go off at 4:50. That gave me plenty of time to set up and be sure of focus, exposure, camera level - all those little details.  My setup consisted of the TEC 140 telescope with a Canon 1.4X converter used with the XSi camera.  The shot at left shows the view where I set up next to the van.  For a few minutes I was thinking it would rise very near Thimble Peak, but ended up rising off the right side of the indicated hill.  In the image at right Mercury just cleared the horizon - the little black round spot at lower left.  Note how much clearer the hillside is than the edge of the sun, only 10 miles away compared to the hundreds of miles of atmosphere the sun's image traversed.

I always find it interesting in the edges of low elevation objects. Most observers know about the atmosphere acting like a thin prism that disperses the light into colors - the upper edge of the setting sun has a green rim, the lower edge is red. Yet, on a black, inky spot, the LOWER edge is blue-green and upper is red! Of course, when you think about it the upper edge of the black disk is really a lower edge of the sun, so should be red like the lower limb of the sun. An enlargement of the effect is shown here at left. The effect holds for the dark sunspot group at top too...

After the sunrise, just as I was starting to shut down, some walkers strolled by and got to see Mercury on my camera's Live View. Finally I shut down and retired to home about 2 miles away for the rest of the transit. Fortunately I had set up the AP1200 equatorial mount in the back yard, and aligned it Sunday night, so was ready to go. Unfortunately, my sleep-addled state didn't realize I had hooked up the wires incorrectly, so spent about 30 minutes figuring why it wouldn't track! The trees in the east side of the yard block the low sun anyway, so didn't lose much important time. At left is shown the setup in the "jungle" part of the yard next to the not-yet-completed observatory...

I set up the timer to take images every 2 minutes for a possible future time-lapse movie - we'll see! In the meantime, I can pick-and-choose images that have the best seeing to show here. At right is a full-frame image that is taken a little after mid-transit. Even though the tracking mount had a "solar" rate, which worked perfectly, BTW in keeping the largest sunspot on one of the camera focus marks, it needed a bit of touchup every 15 minutes or so as the sun continued to track a little northward as we approach Summer Solstice in a month-and-a-half.

While the full-disk views are neat, the resolution of this system is capable of so much more! At left here is a full-resolution picture of the TEC and XSi, taken about 15 minutes after mid-transit. Adjusting the levels of the image you can spot details in Sunspot Group 2542, as well as start to see the granulated surface of the sun.

The transit was a slow-motion event, the entire event lasting over 7.5 hours from some parts of the world.  I was fine with letting the camera run mostly unattended, checking the northward drift every 15 minutes or so.  Finally, it drew to a close about 1140 local time (MST), but before it did, just before 3rd contact I collected upwards of 10 frames in quick succession to make sure I'd get a couple good sharp images.  At right, 2 are stacked just before Mercury meets the edge of the sun.

After putting gear away, I went in to work, and finally looked at some of the images tonight.  The easy ones to do something with are shown here.  Anything more complicated may take some time!

But talk about setting a high bar! Tom Polakis, observing from his back yard in Tempe, AZ used a 4" solar telescope and high-speed video system to record many frames of Mercury. Here he made 31 images of the last 10 minutes of Mercury's egress off the Sun's surface, only keeping about half of the data, using only the sharpest images for highest resolution. In making the time-lapse, the flames and prominences at the edge of the sun are real, recorded in Hydrogen-Alpha. Just an amazing amount of work to get this, and a great achievement too. If you go to his Pbase page on this image, you can poke around his other galleries and find other gems he has collected!

If you had a chance to observe the transit, I hope you had a good one!  The next one is in November of 2019, so not so long to wait!


Anonymous said...

nice images

Anonymous said...

why are the green/red edges of mercury and sunspots inverted from the sun's green red edges?

Dean said...

As I pointed out, perhaps not-so-clearly in the post, the upper edge of the dark silhouette of Mercury is actually a lower-edge of illuminated sun (with Mercury blocking light), so would have the same effect as the lower limb of the sun. It is sort of a "surprise" to see the effect reversed, but since only illuminated edges can show dispersion, the upper edge of Mercury looks the same as lower limb, and the lower edge of Mercury's disk matches the upper limb of sun. Not sure that is any clearer! -Dean

Anonymous said...

ahhh ok I'm staring to understand it a little better. thanks