A Numbers Game

 (Aren't most things?) Anyway, in the previous images of NGC6823, I was wondering just how dim the nebula is. Of course, this would have to be referenced to my location, since local light pollution would affect how dim the nebula appears. Another way of thinking about this is, light pollution makes the black portions of the sky lighter, thus decreasing the contrast between it and the nebula. Lack of contrast would have the effect of making the nebula look dim. (Find a polar bear in a snow storm type of thing.) Since this is a LRGB composite, and the nebula is red, I decided to use the red channel for the determination. So, what did I get? I looked for the darkest portion of the image and found that it had a mean reading of 599 ADU. (ADU means analog-to-digital unit. In my case, since I have a 16 bit camera, 1 ADU represents 1 of 65536 levels of brightness. 0 or zero equals black, 65535 equals white.) The mean reading in the brightest part of the nebula is 749. That's pretty close. In other words, the nebula is only 150 ADU brighter than the blackest part of the sky (in this image). That's not very bright. It turns out that the nebula is only 0.23% brighter than the darkest part of the sky.