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!