73947RE: Nikon D610 First Light
- Feb 10, 2014
Default images (e.g. JPEG) from terrestrial cameras (DSLR, P&S, cell phone, etc.) set color and contrast tones, hues, etc. according to complicated algorithms that vary with user settings (exp time, ISO, white balance and so on). It is no surprise that the camera creates different colorations and contrasts when using different settings.
Basically none of the in-camera image processing is optimal or even appropriate for astro-imaging. There is one dynamic in particular that completely invalidates in-camera (or other terrestrial processing such as Photoshop): the pernicious effects of sky background. As far as the camera is concerned, you are taking a picture of "the sky" (not a nebula or galaxy). If the sky is dark and the object is very bright (e.g. M42) then this may be a somewhat minor issue, esp if the exps are short and/or you do not try to push the image to reveal dim features. But for long exps and/or dim objects the sky becomes the dominate feature and the camera optimizes the sky to the detriment of the target object.
Sky color cannot be corrected via terrestrial processing (in camera or in Photoshop) because the sky does not truly affect color ratios but rather it is an imposition of skewed color upon the object's image. This is not unlike a double exposure: one exp of the object overlaid with another exposure of featureless sky illumination/color. Terrestrial processing struggles to "balance" the image via ratios, which results in uncorrectable color errors. But in reality, astro sky color effects must be subtracted from the image, which is a function not available in terrestrial processing (esp in-camera, though it can be partially emulated via Photoshop). Furthermore, vignette and other flat-field defects can result in terrible color errors such as gradients and blotches and these things cannot be calibrated in-camera and are very difficult to deal with in Photoshop.
The best results are produced by calibrating and processing the RAW data with astro-software. As long as you restrict astro-imaging to very bright objects (preferable in a very dark sky) you can be lazy and let the camera do its thing. But for serious astro-imaging: learn basic astro-imaging dynamics and use appropriate software. There is a range of astro software for DSLR and some of it is free, which might be a good place to start. (It is probably too advanced and intimidating to start with, but I also mention CCDStack, a "serious" astro imaging package that supports DSLR though it is squarely intended for real astro-cameras).
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