Saturday, June 30, 2012

Rings of Saturn

My ongoing adventures with astrophotography at the observatories at Chabot Space and Science Center bring me this time to Saturn. Amongst other things, we've been viewing the ringed planet during our night-time public viewings, and one Saturday I managed to get the telescope for myself for an hour to do my thing with the camera. I'm getting practiced with photographing through the 20" refractor (Rachael). Here's the image:
Saturn 2012-06-16, CSSC 20" refractor (rachael)
As with all my shots, this is taken with a Nikon D80, prime focus. And like my other shots, this was not a single exposure but a collection of exposures (about a dozen) that were stacked to brighten the image and reduce ISO noise.

This particular day, the seeing wasn't perfect. That is, although it was perfectly clear and the image nice and bright, the image jumped around and distorted a lot, so that leads to motion blur. I tried to keep the exposures short; I tried 1/6 second and 1/15 second speed, and stacked the best images; but even so, this is what I get. That's my excuse for the image above looking just a little bit out of focus. But that said, the results are pretty good, and there are some interesting details.

First of all, you can see some bands across the main disk of the planet. They run around the planet, and follow the "trade winds" that encircle the planet at high altitudes. Saturn's colors are not as dramatic as Jupiter's, so the bands are not as dramatic, but they are there, and the causes are pretty much the same.

Of course, everybody thinks about those rings when thinking about Saturn. The most obvious feature is a gap called the Cassini Division, so named because it was discovered by Giovanni Cassini in 1675. The division, and other fainter divisions, look like grooves on an (old fashioned) vinyl record and separate the disk that would otherwise be a single ring into several rings. We know from spacecraft that have visited Saturn that the gaps are created by tiny moons that clear out the space between the rings and shepherd the rings into bands the that we see.

If you look carefully, you can also see to the left and behind the planet a gap across the rings. That is the shadow of the main planet cast across the rings. (The Sun is behind our back and to the right.) And if you look real close, along the bottom edge of the ring, where the ring passes in front of the planet, you can see a narrow dark line that is a shadow cast by the rings onto the body of the planet. All that gives you a sense of perspective, doesn't it?

This is just about the same view you would have seen if you were with us looking directly through the telescope. In fact, with your eyes, you would have been able to see some of Saturn's moons, too, but the moons didn't make it into this picture. The camera doesn't have the dynamic range that your eyes have, so the moons were either too faint to be seen by the sensor, or washed out by Saturn's glare. Oh well. In another session, I might try to get a shot of the moons. I'll have to think about how to do that.

Saturday, June 9, 2012

The Black Drop

The recent Venus transit is a great opportunity to take a look at a phenomenon that has plagued astronomers for hundreds of years. In fact, there is a bit of history to the black drop effect; there are reports of this effect that go back to the early days of transit observations, including drawings of the stages of a Venus transit by Captain James Cook and Charles Green from observations they made in 1769. The black drop effect spoiled attempts to very accurately time the ingress of Venus, and set back efforts to improve measurements of the Earth's distance to the sun.

This year we got to look for the effect ourselves with the June 2012 transit, and among the huge stack of images I collected during the Chabot Space and Science Center event were these images. Notice the sun spots scattered around the disk of the sun, and the perfectly round nibble near the top edge.
Venus Transit, 5 June 2012

Venus Transit, 5 June 2012
These are zoomed out lots. So lets look closely at the first image. These next images are cropped from the previous images where Venus is entering the disk of the sun, then zoomed in a bunch.

Do you see it? Notice how the points where the edge of the disk of the sun and the edge of the disk of Venus meet. There should be sharp points there, but instead, it is rounded. Here is the same zoom from the later image.

This time Venus is far enough in that it should be surrounded by the yellow of the sun, but there is still that rounded opening. It is as if Venus is a drop of black fluid (oil?) falling into the fluid of the sun. Of course that's not the case, we know that Venus is a planet and the sun is the sun, so this black drop is an optical effect of some sort.

There have been a variety of explanations of this effect offered up, including:
  • Venus has an atmosphere that must be distorting the image and causing the effect, or
  • The Earth's atmosphere is distorting the image, or
  • Limb darkening combine with instrument effects.
Ultimately, the explanation that I'm going with is that it is caused by a combination of sensor limitations and limb darkening [here]. The experiment done in the linked paper looked for (and found) the black drop effect while looking at a Mercury transit (Mercury has no atmosphere) using a space telescope (outside the Earth's atmosphere). That pretty much ruled out any causes that involve atmospheric effects, and we are left with the limb darkening. So there it is. That's the story, and I'm sticking with it. As for actually observing it, there are the pictures. It is not as dramatic as I was expecting, but it sure is there.