photo vs. visual question

How come emission nebula typically look red in photos but grey or a little green or a little blue in the eyepeice? It can't just be a filter on the camera because all the stars in the photo are typical star colors.

In addition to Harold's notes on eye spectrum sensitivity, I believe if you do a short enough exposure the nebula still comes out grey to green. Colors are hard to detect in low light (look around a dark room and see how grey your furniture suddenly looks). Green is just the color that jumps out first, after grey which is no color. To expose the nebula enough to bring out the reds, in effect you overexpose the green areas until they're a bright yellow or white.

What the other guys said are good answers. Another that fits right in with theirs is that the eye has two types of sensors--Rods and Cones.

One of them (and I can never remember which--rods or cones) is good with low light, but only sees in black and white and grays. The other sees in colors, but needs more light to get functioning.

When looking at astronomical objects, there is generally not enough light to get the color sensors going. That is why things come out colorless.

Alex

In addition to Harold's point about the sensitivity of film vs.eyeballs to the spectrum of light:
Astronomical images taken using film or CCD generally require very long exposure times. This allows photons to accumulate on the CCD chip or film emulsion, thereby recording a lot of information that is basically processed once. The end result being the vivid colors we see in the final image.
Conversely, the eyeball [retina]/ brain combination is constantly processing the images we see. In other words shorter exposures are being taken and processed. Which doesn't allow much time for the accumulation of photons, so what we see especially in low light situations lacks color and definition.


Charles Packard

Another point to consider besides the poor red sensitivity of the human eye, is that these days most high quality DSO images are done with dedicated CCD cameras. These cameras are black and white cameras and color is achieved by using a filters and multiple exposures. The color balance as well as the exposure length for each color is the choice of the photographer.

Another issue is that these cameras can go deep into the infrared far past what the eye can see.

jon