EOS 20Da
Canon released the EOS 20Da on 14 February 2005 in Japan. It is a variant of the 20D designed for astrophotography. On 1 June 2005 Canon announced that the 20Da would be made available worldwide.
The main differences between the 20Da and 20D are the replacement of the "hot mirror" infra-red (IR) filter which covers the CMOS sensor on the 20D and live view mode.
In conventional photography, the IR filter is used to make the spectral response of the CMOS sensor more like that of the human eye. In this way, the pictures gathered by the sensor more closely resemble the world as we see it. In astro-photography, many objects of interest emit strongly in the red H-α line, which is heavily attenuated by the IR filter on the 20D. The IR filter of the 20Da passes 2.5 times more light at around this 656 nm wavelength as the filter of the 20D, allowing more fine detail to be revealed in long exposures of emission nebulae. As a side effect, the color balance is slightly altered in normal daylight photography.
It is extremely difficult to focus an extremely dim image with a reflex viewfinder; locking the mirror up and opening the shutter (live view) was introduced in the 20Da to address this issue. When used the camera provides a live image of the center of the field of view to allow focusing on a bright star. Live view for this application requires a lens which supports fully mechanical focus in manual mode, and is not suitable for normal daytime use.
Reviews report that image noise in the 20Da is lower than in the 20D, and equivalent to that of the 5D MkII.
The EOS 20Da was discontinued when the 30D was introduced in 2006. Canon produced no similar model for astrophotography for years, resulting in many astrophotographers making similar aftermarket modifications to later Canon models.
In April 2012, Canon announced the EOS 60Da which is the successor to the EOS 20Da.
Read more about this topic: Canon EOS 20D