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.

More Of The Same

 In this case I'm referring to NGC6823. A cold front has moved through Georgia, leaving somewhat clearer skies. I wanted to use N.I.N.A to try positioning the scope to the same coordinates as the previous image of NGC3823. However, the updated version was, shall we say, uncooperative. I ended up using Stellarium to move the scope to the region, then plate solved the image I took to be sure I was where I thought I was, and it came back as being very close. I decided to go with that and see how close it actually was when I added two nights worth of data together to see if there was a substantial improvement. (I'll let you decide what you think.) The previous blog entry was 7 minutes (one minute at a time) in each of the four filters, L,R,G, and B. Below is that same data, plus an additional 10 minutes in each of the four filters; so 17 minutes in each filter for a total of 68 minutes total exposure. Post processing was different for each image, but I don't think that was a major factor in the outcome.

 

NGC6823, two nights worth of data. MUCH larger nebulosity showing.