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Since we were up there, and it happened to be a clear night, we ended up sitting in the car waiting for night to fall. The moon was at waxing crescent, so after a few minutes we had total darkness. The first image is a long exposure astrophoto of the trailhead, with I-93 in the background and the trail driveway up in foreground. Instead of my usual leaving-the-shutter-open-for-an-hour, I decided to implement a different technique that's been floating around the Internet mostly for the purpose of noise control; the idea is to take many short (~2 minute) exposures and combine them in photoshop. The technology that makes this possible is an interval timer, which can be obtained from Canon for about $200, the TC-80N3 timer remote. Alternatively one can get a chinese knockoff for less than $15, which is what I did. Thing works perfectly well. The camera was positioned, some trial images taken, and then the camera put in manual mode (ISO 800, f/5, bulb). The bulb setting is critical, because the interval timer controls the shutter. The interval timer was set to take a 2 minute exposure, with a 1 second delay between exposures. I ended up getting about 20 images (40 minutes) before the 40D's battery croaked (it was freshly charged, too -- 16 degree weather really messes with battery life). The images were combined in photoshop by loading the images as layers of a smart object, and setting the mixing mode to "maximum." Suprisingly straightforward.
One huge advantage of doing it this way was that certain images had some defects in them, like a car that drove past us and then stopped with its lights shining right at the lens. In a pure long exposure scheme, this would just ruin the entire exposure. In this method, the worst case is that a single 2 minute segment has to be discarded (which will result in a small gap in the star trail). In fact in this particular stack I was able to rescue that exposure by cutting off the bottom of that image, so that the star exposure was partially rescued but the car didn't blow out the bottom of the frame.
For more typical astrophotos, We went back up the trail to the lower Cascade Brook where it flowed over a large flat granite section. Since it was mostly frozen we could stand in the middle of this brook and look at the stars. The Milky Way is featured in one image, and Jupiter and the Pleiades in the other.
