Mercury Transit: Comparing Between H-alpha and White Light (Shallow Thoughts)

Mercury Transit: Comparing Between H-alpha and White Light

The Mercury transit is over. But we learned some interesting things.

I'd seen Mercury transits before, but this is the first time we had an H-alpha scope (a little 50mm Coronado PST) in addition to a white light filter (I had my 102mm refractor set up with the Orion white-light filter).

As egress approached, Dave was viewing in the H-alpha while I was on the white light scope. When I saw the black-drop effect at third contact, Mercury was still nowhere near the edge in the H-alpha: the H-alpha shows more of the solar atmosphere so the sun's image is noticably bigger. This was the point when we realized that we should have expected this and been timing and recording. Alas, it was too late.

Mercury was roughly 60% out in the white light filter -- just past the point where the "bite" it made in the limb of the sun -- by the time Dave called out third contact. We guessed it was roughly a minute, but that could be way off.

For fourth contact, Dave counted roughly 45 seconds between when I couldn't see Mercury any more and when he lost track of it. This is pretty rough, because it was windy, seeing was terrible and there was at least a 15-second slop when I wasn't sure if I could any indentation in the limb; I'm sure it was at least as hard in the Coronado, which was running at much lower magnification.

So we had a chance to do interesting science and we flubbed it. And the next chance isn't til 2032; who knows if we'll still be actively observing then.

I wanted to at least correlate those two numbers: 45 seconds and 60% of a Mercury radius.

Mercury is about 10" (arcseconds) right now. That was easy to find. But how fast does it move? I couldn't find anything about that, searching for terms like mercury transit angular speed OR velocity. I tried to calculate it with PyEphem but got a number that was orders of magnitude off. Maybe I'll figure it out for a later article, but I wanted to get this posted quickly.

I didn't spend much time trying photography. I got a couple afocal snaps with my pocket digital camera through the white-light scope that worked out pretty well. I wasn't sure that would work for the Coronado: the image is fairly dim. The snaps I did get show Mercury, though none of the interesting detail like faculae and the one tiny prominence that was visible. But the interesting thing is the color. To the eye, the H-alpha scope image is a slightly orangy red, but in the digital camera it came out a startling purplish pink. This may be due to the digital camera's filters passing some IR, confusing the algorithms that decide how to shift the color. Of course, I could have adjusted the color in GIMP back to the real color, but I thought it was more interesting to leave it the hue it came out of the camera. (I did boost contrast and run an unsharp mask filter, to make it easier to see Mercury.)

Anyway, fun and unexpectedly edifying! I wish we had another transit happening sooner than 2032.