| Here is an urban
image taken from my driveway.
IC1396, also known as the Elephant Trunk Nebula is a reasonably bright
emission nebula in Cepheus. This shot was taken using an Optolong
L-eNhance light pollution filter from my urban light polluted skies. I have been struggling to get images I was happy with while using the filter as you can see from my previous attempt at this image using the same filter last month. The issue was excessive noise that seemed to require total exposures that were approaching the present age of the universe to tame. Years ago I designed and built my own CCD camera, easy to do when your day job gives you ready access to the tools and equipment necessary for the task. After that I purchased several different CCD cameras as the price of these devices became more reasonable, finally settling on a DSLR for my imaging. As a result of my engineering background and experience with cameras and sampling systems, the noise I was seeing in the data did not make sense. Initially, sub-exposures were doubled from five to ten minutes with some minor improvement and the histogram looked reasonable so I was at a bit of a loss to explain why such long total exposures were required. It turned out that at least 15 minute subs are required to keep the image data out of the camera noise and a slightly higher ISO setting, 1600 instead of my usual 800, was required to reduce the camera pattern noise far enough to be swamped by read noise. The fact that the histogram looked normal was a function of switching from BYE to Sequence Generator Pro. SGP displays the histogram of the raw data and Canon DSLR's add a fixed 2048 value to the data as part of the bias. This makes the raw histogram look like it is well separated from the left edge when, in fact, it is actually clipped. Properly subtracting the bias in post processing clearly showed the problem, with heavy clipping of the red channel bringing up the camera noise. Click on the image to see a higher resolution version, then click again on the image to return to this page. |
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