Visual observations of the comet have been giving me the strong impression that there is a slightly brighter, ring-like structure along the outer edge of the coma. Photographs were showing the same effect, but along with other observers, I was wondering if this was a real feature or perhaps an illusion. I used Photoshop to measure the Luminosity values of one of the images posted at CloudyNights.com on Friday to see if the effect was real. I measured the image along an axis through the center of the bright fan-like structure, as well as another axis perpendicular to this.
The results showed no drop in brightness inside the edge of the coma. Instead, there was a plateau in brightness. My guess is that our eyes and brain see the steep rise in brightness both at the edge of the coma and around the pseudo-nucleus and this plateau comes out looking like a dark region.
Below is a chart showing the values I measured. I haven't received permission from the imager to post his image for comparison. But there are certainly enough images posted to this thread at Cloudy Nights to see that the illusion is rather strong. Tom Polakis made similar measurements via the R G B channels of a photo he shot, and his clear presentation can be seen here. His results got me off my rear end to go ahead and post this without the reference image. As Tom said, "Don't believe everything you see."
Click image for larger version.
