NGC7000 (North America Nebula)
Canon EOS 5D MkII ISO6400 65x30s 254mm Newtonian f/4.8 (f=1200mm)
HEQ5 mount, driven but not guided 2011 Sep 29 23:23:21-00:01:54 UT
From Rookhope 54.8
With an unmodified DSLR this nebula is not as red as most photos show it. That is because most published photos were taken with CCD cameras. They are monochrome devices so multi-channel images are created by combining exposures made through different coloured filters. For photographing nebulae the filters used tend to cover wavelength bands around particular spectral emission lines that emanate from such nebulae. Examples are the Hydrogen Alpha line at 656nm (red) or Oxygen III (fully ionised oxygen) at 501nm (green). This results in false colour photos, unrelated to human vision.
Digital SLR cameras have a glass plate in front of the (usually CMOS) detector that cuts out infrared wavelengths that would otherwise be likely to dominate photos. In Canon DSLRs, like mine, the cut-off comes just past the Hydrogen Alpha wavelength but already the sensitivity is falling at 656nm. So such cameras make emission nebulae look less red.
Our eyes cannot distinguish colour at very low light levels but if they could we would perceive the colours of emission nebulae as similar to the DSLR images rather than the CCD ones.
CCD cameras have one major advantage over DSLR: every pixel counts (well, except for defective ones). In a DSLR there is a pattern of coloured filters over the pixels, as I described here, so software has to interpolate missing data for alternate pixels. CCD cameras used to have other advantages but DSLR sensitivity has overtaken them unless you want to spend a lot of money for doing very serious research.