I have been looking at this tutorial from Astostuff on taking off the hot mirror on a D70, but cannot figure out from the page where the color mosaic filter is located.
Is it possible to remove the color mosaic filter off the sensor? Does converting the camera to IR involve removing the filter? Will I achieve any significant gains by removing the color mosaic filter? Does the color filter not absorb IR photons?
It's possible to remove the CFA (colour filter array) but extremely difficult. There is at least one company that will do it (Maxmax as asalamon74 states). Doing this is entirely unneccesary for IR conversion, in fact one of the things I like about IR photography is playing with the faint colours that result, which requires the CFA.
As to why you'd want to remove it... sensitivity. Each of the colour filters filters out approximately two thirds of incoming light. Removing them would give you a greyscale camera that's 1.6 stops more sensitive, so where you'd have to use ISO1600 normally with this camera you could shoot at ISO500
Also you wouldn't have to demosaic the resultant images so you'd get an increase in sharpness similar to what you get with a Sigma Foveon camera.
If you do a deep IR conversion (880nm or shorter) then you lose colour information anyway so you have nothing to lose by removing the CFA.
If you remove the color filter array then (theoretically) you'll have a B&W DSLR. Removing the filter is quite complicated, there are a few companies offering DSLR B&W conversion like maxmax. Check their webpage they have quite good sample photos.
I recently looked into the response curve for the bayer patterns and I only came across one camera that gave the curve through the NIR region.
Note that this curve is the sensitivity of the particular sensor with bayer attached (q(cmos)*q(bayer)). The NIR region is pretty much the same as the monochrome version of this camera, which means the bayer pattern doesn't do anything in the NIR region. Assuming that the bayer filter material is the same across the line, you don't need to remove it. Just remove the NIR cut filter.
If your NIR light is centered around 850nm, then all the pixels will have equal sensitivity and you can make a nice NIR image by horizontal and vertical binning, without removing the bayer pattern.
At least from anything approaching a practical viewpoint, the color filter is not removable (you can probably get it removed, but removing it yourself is mostly out of the question). It's part of the sensor itself, in front of the sensor wells, but behind the micro-lenses. To remove it, you'd have to remove the microlenses, remove the color filter, and then put new microlenses back on.
While it's fairly easy to fabricate a sensor without a color filter to start with (and that has been done), removing one from an existing sensor would be more difficult -- to the point that it's mostly impractical.
The color filter array is typically right on top of the sensor, under an infrared filter panel. After removing the IR filter, the CFA can be mechanically scraped off of the sensor:
It's not necessary to remove the Bayer filter as part of an IR conversion. In my testing, the Bayer filter appears to be transparent to IR.
Compare the curves of the spectral sensitivity and QE of various wavelengths of light on a CMOS sensor. You can easily tell that the curves match almost perfectly.
Removing the Bayer filter usually removes the micro-lenses layered on the filter as well. So, Bayer removal will tend to reduce low-light performance, and create pixel vignetting, especially with shorter focal length lenses.
This answer should not suggest that Bayer removal is a bad idea for full-spectrum and B&W conversions, however.
