Venus Dancing on the Sun

It's been almost two months since the last Venus transit this century. Fortunately, I collected a huge stack of imaging data. I posted a few key images in my "Black Drop" post last month, but all along I planned to make a time lapse video; and I finally uploaded that hand assembled video. Here it is, but I recommend following the link and viewing it at the YouTube site. The JPEG artifacts of this embedded version are not as bad there. More about this video below.

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.

Our Mother Star

There is going to be an annular eclipse of the sun this Sunday, 20 May 2012, and Venus is going to transit the sun on 5 June, 2012. Two very different, yet very similar events! In both cases, a nearer astronomical body (moon or Venus) is going to pass between Earth and the sun. The moon will cover a big piece of the sun, so we get an "eclipse." The moon is obviously not as big as the sun, but it is so much closer that it appears almost as big as the sun (we call that "angular size") and we get eclipses. This particular eclipse will be partial in the Oakland, CA area. Even though Venus is much bigger then the moon (it is about the size of the Earth) it is much farther away, so when Venus transits the sun it will cover only a small fraction of the sun from our point of view. It will not cover enough of the sun to make the sky darker, but we will be able to clearly see it with a filtered telescope, and we might be able to see it without the telescope. (But still filtered!)

I want to catch and photograph both events, so I have collected some equipment and have put some effort into dry runs. For the solar eclipse, I want to be able to watch the moon take a bite out of the sun, and for the Venus transit, I want to be able to watch Venus crawl across the face of the sun. For the Venus transit I'll need some magnification and filtration, and for the solar eclipse I'll need at least some filtration. So I've decided that I'll use the same setup for both events. This will be my 120mm refractor telescope and my Nikon-D80 camera. And of course, a filter.

Type-IIa glass on left, film on right

I have two possible filters. Both go on the front of the telescope (over the main lens) and both reflect most of the light away to leave only a small amount to go through the telescope. The light that gets through the filter and into the telescope is focused into a magnified image, and will still be bright enough behind the filters to use short exposures. Which filter looks better and which exposure settings work best are the point of my experimentation.

I have two types of solar filters: a Type-IIa glass filter and a typical aluminized mylar filter. They both were purchased to match my telescope, so the filter frame fits over the glare shield of my telescope. (It is important that the excess light is rejected before it enters a telescope. An eyepiece filter will burn.)

I took a bunch of images with each of the filters, and here are representative images, one using each filter type. The first image is taken through the film filter. This looks pretty good. You can see the sunspots clearly and it is bright even at ISO500, 1ms exposure. It also has a distinct blue/purple tint to it. I wonder what I think about that.

Sun using film filter

The second image is a representative image taken using the glass filter. Again you can easily see the sunspots, but in this case the color is a yellowish orange. Shorter exposures made it look a little red, and longer exposures looked yellower.

Sun using glass filter

From the bunches of images, these two were typical from each filter. Both images use the same exposure settings, so the differences are entirely the differences between the filters. I'm looking for brightness and sharpness. Brightness might be an issue if I find I'm forced to take a long exposure, but both are bright enough (the images above were taken using 1ms exposure time) and neither filters seem to distort the image any, so those are not criteria for choosing one image over another.

Ultimately, I think this is going to be a matter of taste, and not science. Which looks better? To my eye, the glass filter looks better because the color looks more realistic. Technically, the blueish white image from the film filter may be more correct, in that the spectrum getting through is more uniform, but this is not going to be a scientific endeavor and we humans think of the sun (incorrectly!) as reddish yellow. So for the purposes of making pretty pictures that give the right impression, I think I'm going to go with the glass filter.

Of course, the moon (or Venus on the day of the transit) will be black so that's not an issue for either filter. I guess I'm ready to go. Now the planets need to do their part. And by the way, I'll be setting all this equipment up on the observatory deck at the Chabot Space and Science Center on the appointed days. Join us!