Dark Skies

At the encouragement of my wife, I am resuming astrophotography. Big Bertha gathers so much light that even without a clock drive on it, it should be possible to get decent pictures of the Moon and perhaps of Jupiter and Saturn with it. With 800 speed film, 1/1000th of a second is a long enough exposure for a quarter Moon--and there's not much motion in 1/1000th of a second. (There's a very nice calculator for this here.)

Film? Why so primitive? Unfortunately, consumer digital cameras really aren't well suited to this, because the lens can't be removed to do prime focus or eyepiece projection astrophotography. In prime focus photography, you use the telescope objective as the lens, so Big Bertha is properly understood as a 2000mm f/4.5 telephoto lens. In eyepiece projection, you put one of the telescope eyepieces between the objective and the camera, substantially increasing the focal length and f-ratio. For example, with an 18mm eyepiece in Big Bertha, it becomes a 11,227mm lens at f/29.5.

It is possible to use a consumer digital camera in what is called "afocal" mode, where you focus the camera on the image in the eyepiece, but I haven't found that to work too well.

There are considerably higher end digital cameras with removable lenses, the digital equivalent to a 35mm SLR, but these were priced in the $800 to $1000 range until recently. (I just checked, and I was startled to see the Pentax body only version now offered for $342, after rebates.) Certainly when the old house in Boise sells, this is on my list of things to get!

Why Pentax? I have a number of Pentax lens from my ME Super that will fit, and I have an adapter for the telescope that is also made for the Pentax lens mount.

Anyway, away from astrophotography, and to dark skies. I was outside last night do star trails, where you aim the camera at the North Star, and set various exposure times: 5 minutes, 10, 20, and 40. As the stars rotate around the North Star, you get photographs like this.

My wife shared my amazement at how dark the sky was at our house. The Moon had not risen yet, and the Milky Way stretched across the entire sky, from north to south. Even the skyglow of Boise wasn't enough to drown it out. I couldn't find the constellation Hercules at first, because it was overwhelmed by all the other stars.

There's a big development going in about ten miles from us, just across the county line, called Avimor. I guess that I better write them a letter, asking them to think about preserving the dark sky when they are picking out light fixtures.

UPDATE: I located the Governmental Affairs guy at Avimor. Yes, preserving dark skies is on their list of objectives, and they will be picking exterior lighting with this in mind. While he has not yet joined the Boise Astronomical Society, he has attended at least one of our events, with his telescope. It's nice to have allies in important places!