NGC1499 - California nebula in Perseusby
PascalComment by Pascal: It is difficult to say if there is a "true" color in any astroimage. A realistic answer could be "yes" and "no" simultaneously.
At the specific image the red color depicts the presence of Hydrogen (dominant element in the universe and responsible for star formation). But how much red or pink or purple the hydrogen areas should be in order for an image to be "true" enough? I can't answer to this, because it's somehow subjective. It depends of the sensitivity (or response) of camera's sensor at various wavelengths, use of narrowband filters (Ha, Hb for hydrogen, OIII for oxygen, SII for sulfur, attached within a filterwheel at dedicated astronomical CCD's) and of the processing steps that finally offer pleasure to our eyes.
The cameras I use for this type of shots are modified (IR/CUT filter replaced) in order to be more sensitive to the red color corresponding to hydrogen.
And yes, this kind of photography is quite hard :)
Explaining quickly the difficulty of the whole thing:
- Must have a very precise tracking mount
- Perfect optics
- Perfect calibration of the equipment, in software and hardware (balancing, polar alignment, auto-guiding, etc)
- Good weather (transparency), low temperatures, low relative humidity, low wind, etc
- Low light pollution (and moonless nights..)
- Careful preparation and planning of the targets to be photographed
- Succesfull processing of all the frames (lights, darks, bias, flats)
Message edited by author 2009-12-07 03:45:18. 
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