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Canon to produce special version of EOS 20D for astrophotography  
Wednesday, February 16, 2005 | by Rob Galbraith

Canonís domestic sales unit in Japan this week announced their intention to market a version of the Canon EOS 20D designed especially for astrophotography.

Called the EOS 20Da, the camera will utilize the same image sensor as the standard 20D, but it will be designed for enhanced transmission of a specific wavelength of red light within the visible spectrum. It will also be able to show a live, through-the-lens view on the rear LCD monitor for critical focusing, which is often a necessity in astrophotography.

The camera will be about 250% more sensitive to light in the 656 nanometer band than the standard 20D, says Chuck Westfall, Canon USA's Director of Technical Marketing. That apparently will make it well-suited to capturing photos of hydrogen-rich reddish diffuse nebulae.

Contrary to some published reports on the camera, however, Westfall indicates - and Canon Japanís low-res sample photographs support this - the 20Da will not be particularly more sensitive to true infrared light: the cameraís optical infrared filtering is still expected to roll off hard above 700 nanometers, which is about where infrared begins in the electromagnetic spectrum.

In other words, the cameraís image sensor will still be perched behind a filter designed to limit the amount of infrared light reaching it, just like the 20D; that filter will also be a low-pass type to minimize colour artifacting and moirť. The main sensor-related difference between the 20D and the 20Da will be sensitivity to the 656 nanometer band of the visible spectrum.

EOS 20Da information on Canon's domestic sales web site in Japan

The 20Da will also display a live, magnified through-the-lens view on the rear LCD monitor. This will only work when the reflex mirror in the camera is locked up, and then only for dim scenes (like distant star clusters). Only the aperture and ISO can be used to control the brightness of the view in this mode, since the 20Daís CMOS sensor isnít capable of mimicking shutter speeds electronically (the shutter itself will control the exposure time in this camera, as it does in the 20D).

Therefore, use of this focus-setting function in bright light wonít be an option. When the live view mode is enabled, 5x and 10x central area magnification will be selectable by the user, to allow for critical focus of distant objects (under a variety of circumstances in astrophotography, simply setting the lens to infinity does not ensure sharp focus). Once focus is achieved, it will be necessary to exit the live view mode to begin shooting pictures.

Firm pricing and availability of the EOS 20Da has not been set, though the camera is expected to come available some time later this year in Japan. Canon USA is still evaluating whether to sell the camera in the U.S. market, says Westfall. Undoubtedly, other Canon regions worlwide will be deciding whether to carry the specialty model as well.

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