For a generation of astrophotographers, emission nebulae have always been red. At least, that's how they show up on Ektachrome film, which is very sensitive to the wavelength of hydrogen-alpha (656 nm), at which nebulae shine brightly.
But DSLRs see nebulae as blue or pinkish. There are two reasons for this. First, DSLRs include an infrared-blocking filter that cuts sensitivity to hydrogen-alpha. Second, and equally important, DSLRs respond to hydrogen-beta and oxygen-III emissions, both near 500 nm, much better than color film does.4 And some nebulae are actually brighter at these wavelengths than at hydrogen-alpha. So the lack of brilliant coloration doesn't mean that the DSLR can't see nebulae.
4 The Palomar Observatory Sky Survey also didn't respond to hydrogen-beta or oxygen-III; those spectral lines fell in a gap between the "red" and "blue" plates. This fact reinforced everyone's impression that emission nebulae are red.
DSLRs can be modified to make them supersensitive to hydrogen-alpha, like an astronomical CCD, better than any film. The modification consists of replacing the infrared filter with one that transmits longer wavelengths, or even removing it altogether. For more about this, see p. 133. Canon has marketed one such camera, the EOS 20Da.
In the meantime, suffice it to say that unmodified DSLRs record hydrogen nebulae better than many astrophotographers realize. Faint hydrogen nebulae can be and have been photographed with unmodified DSLRs (Figure 2.5).
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