http://www.canonrumors.com/2012/04/c...rophotography/
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The simple answer is that they boosted the sensitivity of the sensor in the part of the visible spectrum where most astronomical objects emit light. By doing this, as opposed to increasing the sensitivity to the entire visible spectrum, they are able to better control iso noise in the final output. What I don't know is if they did this via hardware or software.
So is it a one hit wonder? Is it ONLY good for astrophotography?
I don't know that.
Edit: So after doing a little reading, I would guess that since they removed the IR filter (that's how they boots the sensitivity to the Hydrogen-α line), that it's functionality as a "regular" camera would be somewhat limited. The previous model (the 20Da) rendered a red tint to "normal" unfiltered exposures. That's logical since the IR filter blocks some red wave lengths in addition to IR. Normally we don't notice since the cameras software compensates for this in the final image output. Color correction could be achieved thru post processing (setting the white balance) or by using an infrared filter over the end of the lens.
http://www.imaging-resource.com/NPIC...ARISON_1_S.JPGhttp://www.imaging-resource.com/NPIC...ARISON_2_S.JPG
The biggest difference is the removal of the IR filter. A lot of nebulae emit data in the Ha wavelength, which are cut out by the IR filter in standard DSLRs. With the IR filter removed, a lot more of the nebulae data can be captured. Today, many astrophotographers modify their DSLRs manually to remove the filter.
More details, including comparison here: http://www.astropix.com/HTML/I_ASTROP/DSLR_HA.HTM
Thank you both for the schooling!