Call up the levels dialog from the pull-down menu (Image > Adjustments > Levels).Select the eyedropper tool from the tool palette and change the sample size to 31 by 31 average.Double click on the eyedropper tool and set it to 35,35,35, then click it in a blank area of sky background. Adjusting the black point in Adobe Photoshop. Technically, subtracting this colorcast should be done to linear (unedited) data, though I’ve found that it doesn’t make much difference if it’s done after non-linear adjustments are performed. To correct this foreground sky in our images, we can subtract it in Adobe Photoshop. The images of deep-sky objects must pass through Earth’s atmosphere, and the color of the foreground sky is added to them. Even at the darkest observing sites in the world, the sky can be a reddish-brown or sometimes green color due to airglow, a naturally occurring emission of light from Earth’s atmosphere. This color comes from what I call the “foreground sky.” The foreground sky color is created in Earth’s atmosphere, usually due to light pollution at urban and suburban observing locations. The problem is, these long exposures reveal the true color of the nighttime sky, which is usually an ugly red-brown color. Indeed, if you expose so that the sky is black, or even adjust it to be black later in processing, you’ll lose the faintest details in deep-sky objects that are just above the brightness of the sky background. In these results, the sky will not be black. The sky is actually quite bright at observing sites with even mild light pollution.įor deep-sky astrophotography, we need to expose long enough to get the faintest detail up out of the noise of the camera. Even at the darkest observing site in the world, you can still easily distinguish between the sky and the horizon.

The Color of the Night SkyĬontrary to what you might think, the night sky is not black.

It is compromised by light pollution in the 'foreground sky,' which produces the red-brown color. This shot of M42 was taken from a reasonably dark observing site. Now let’s look at the second step alluded to in the column: subtracting the light pollution from your results to get a natural color image. Last month I discussed white balance settings for astronomical images recorded with DSLR cameras.

Fixing blue colorcast with gimp