For those of you scratching your head and going "What the heck is that?" I'll tell you. It's an Astronomik CLS clip filter from Germany. It's an ingenious little device. Because of the way Canon APS-C cameras are made (those that have a "cropped" imaging sensor which is about 3/4 the size of a 35mm film frame) there is a small open space in between the mirror/shutter assembly and the rear of the attached camera lens. The German engineers at Astronomik realized they could design a filter that could "clip in" to this unused space for astronomy purposes (sorry Nikon users--your camera design doesn't allow for internal clip filters).
The CLS filter I got is a light pollution filter. Because there are so many street lights in my neighborhood, not to mention the sky glow from New Braunfels, San Antonio and Austin, astrophotos of more than a brief exposure end up having an ugly, brown fog to them. Yuck. But the CLS filter is coated in such a way that the sodium and mercury vapor light produced by most street lamps and city lights are almost completely blocked. This means longer exposures are possible without the ill effects of light pollution!
My first attempt at astrophotography using the CLS clip filter and my Canon 7D is above, the Orion constellation. I set the camera up on a tripod and shot 20 10-second exposures at 800 and 1600 ISO (I couldn't go for longer exposures because I hadn't set my telescope up, and the camera had no way to track the movement of the start across the sky). I stacked these individual images using Deep Sky Stacker, a nifty freeware program. The result isn't anything near award-winning, but it does show all the relevant stars without the washed-out effect of light pollution. And the Orion nebula is easily visible. I think I'm going to have a lot of fun with this in the future.
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