B2 Using a webcam or video imager

A webcam is a cheap CCD camera that delivers a continuous stream of video to the computer by USB or FireWire connection. These are commonly used for low-cost videoconferencing. For astronomy, the lens is unscrewed and replaced with an eyepiece tube adapter (Figure B.2). Because the IR-blocking filter is in the lens, which was removed, it is desirable to add a UV/IR-blocking filter at the end of the eyepiece tube.

Webcam adapters are sold by many telescope dealers. You can also buy them directly from independent machinists who make them, such as Pete Albrecht (www.petealbrecht.com).

Alternatively, you can also buy your video camera ready made. The Meade Lunar/Planetary Imager (LPI) and the Celestron NexImage are basically

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Figure B.1. Rotation of Mars in 1 hour. Each image is a stack of frames selected from a 3000-frame video sequence taken with an 8-inch (20-cm) telescope with negative projection giving f/30.

Figure B.2. A webcam modified for astronomy (my Philips ToUCam Pro, vintage 2003). Top: Assembled and ready for use. Bottom: The components: webcam with lens removed, eyepiece tube adapter, and UV/IR blocking filter. Parfocalizing ring around adapter is optional.

Figure B.3. Video astronomy. (a) Single frame of a video recording of Saturn made with an 8-inch (20-cm) telescope at f /20. (b) The best 500 frames (out of 805), aligned and stacked. (c) After wavelet filtering.

webcams. Any astronomical CCD camera, cooled or not, will work if it has a continuous live video output to the computer.

The camera goes in place of the eyepiece of a telescope working at f /10 to f /30; I use an 8-inch (20-cm) Schmidt-Cassegrain with a Barlow lens. For easy focusing, I suggest using the Moon as your first target. To further speed focusing, I've put parfocalizing rings on my webcam and an eyepiece so that they match; I do rough focusing with the eyepiece, center the object in the field, and then switch to the webcam. The field of the webcam is very small, so centering is important.

To record video, you can use the software that came with the camera or any number of astronomical software packages. Create a file in AVI format. The camera controls will be much the same no matter what software you use, since they are provided by the manufacturer's drivers. Turn auto exposure off, set the speed to 15 frames per second, set the exposure to 1/25 second and the gain to medium, use the full resolution of the camera, and see what you get. Adjust

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Figure B.4. Wavelet filtering in RegiStax. Sliders allow selective enhancement of details of different sizes.

exposure and gain as needed. Err on the side of underexposure, since images tend to gain a little brightness during wavelet processing.

I usually record about 1000-3000 frames (1-3 minutes). The rapid rotation of Jupiter will start to blur detail if you record for more than about a minute (this is a very rough guideline); with Mars, you have more time.

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