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Replacing the Canon 350D’s Internal IR-Block Filter with Clear Glass.

Replacing the Canon 350D’s Internal IR-Block Filter with Clear Glass.

The new breed of digital SLR cameras from Canon are highly regarded for use in the somewhat demanding task of astronomical imaging. While they cannot go as deep as most of the dedicated, CCD-based astronomical imaging systems and are limited to 12-bit signal resolution, they can produce amazing results
Canon DSLR models such as the Digital Rebel XT (a.k.a. the 350D) come from the factory with a pesky internal fixed “low pass” filter that blocks almost all of the astronomically interesting reddish hydrogen alpha (Ha) band, where many nebulae glow brightly. These filters are intended to keep colors true and to block the infrared portion of the spectrum in order to keep the visible portion sharply focused, and in that task perform admirably. However, the response of these filters slopes gradually into the visible band, intruding so much that the filter permits only like 10% of the Ha band to pass through. To solve this problem, the camera can be disassembled to remove the low pass (or IR blocking) filter, permitting the CMOS imaging device to receive and register all available radiation.

But there is downside: once the internal low pass filter has been removed, the camera is useless for normal daytime usage, for two reasons. Firstly, the camera electronics and firmware have been “tuned” to expect the response curve of the factory filter and without it, colors are not recorded properly. Secondly, the lack of the glass material (that the filter is constructed from) in the light path means the distance between lens and imaging device is now incorrect (due to the index of refraction of light through air is different than through glass), and the auto-focus electronics will not focus properly.

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