Crescent Mercury!

I took my own advice from last night's post, set up the 14" Celestron before sunset tonight, and attempted to image Mercury before it got too low. Well, too low is a relative term, and while it was above the horizon a good 5 or 10 degrees in a still-bright sky, it still showed considerable atmospheric dispersion. At angles far from the zenith, the atmosphere acts like a thin prism so at low elevations, the lower edge of a bright object like the moon or Venus will show a red edge, the upper edge a blue edge. The same with the view of a low star through a telescope, or in this case, Mercury.

With the Canon XSi camera mounted at the Cassegrainian focus, the C-14 acts like a nearly 4,000mm telephoto lens. This is enough to show the crescent shape of Mercury using the "live view" to focus on, but such a long focal length is difficult to take short exposures with the camera's shutter and SLR mirror slap. Using a shutter speed of 1/250 second and using mirror lockup, image motion was minimized, and I took 30 frames to stack together. The crescent shape is prominent, as is the atmospheric dispersion mentioned above. The only way to do better would be with a webcam, stacking multiple images (with no moving parts), and shooting while the sun was up (Mercury higher off the horizon), likely with an IR filter to darken the sky. Amateurs are doing amazing things with this combination, as shown here.


Note: I got the idea today to include what a single shot looked like. This one was representative of the 30 shots. With Mercury that low in the sky, the image had to go through 6 or more airmasses (6 times more than if Mercury were overhead). Observing that low has a huge affect on the image due to atmospheric turbulence, and is analogous to looking at something at the bottom of a swimming pool - the waves distort the image. Averaging many images smooths out the image and increases the signal-to-noise of the image. You can note how much smoother the above average is compared to this single...