Sorry for digging up this old thread - but I have an update.
I did some tests with my refractor and the Bahtinov mask with a CCD camera.
It shows clearly the blue filter has a different focus point to the R and G filters - which makes sense for a refractor. In addition - with the blue filter in place, you can see curvature in the diffraction spike, which indicates that the different wavelengths of blue light have different focus points and perfect focus of blue light will never be possible.
By the same token, the Bahtinov mask gives a visual clue to the the width of the pass band width for all filters, include narrowband.
LINK
It would be great to repeat this test with a few achromats and top end APOs as well.
Overall a very useful tool!
James