This photograph of the Gamma Cygni region includes the Crescent Nebula, NGC 6888, in the upper right. It was photographed from a rural California backyard observatory with a friend using his excellent scopes and cameras. I did all processing. I am a novice astro-imager but find the processing of nebulae rewarding, with technical challenges that are quite different from processing nature or portrait photography. The Gamma Cygni region image is a mosaic shot in three sections in order to cover the desired area. The typical three narrowband filters, Ha, SII, and OIII, were used. Mapping those three wavelengths to RGB offers interesting choices. The Hubble team assigns the Hydrogen, Silicon and Oxygen to red, green and blue in a different order than do the Hawaii and Canadian astronomers. Given that there is no “sanctioned” plan, I have chosen to map Ha to red, SII to green and OIII to blue. I sometimes call this my Diehard palette because it produces images with the colors of a good fiery cinematic explosion while still illuminating the dynamics of the cosmic processes at work. For example, oxygen bubbles racing away from the rest of the material show as a cool blue which feels right somehow. Technical Details Each image in the mosaic is a combination of stacks of multiple images shot through each filter. For example, 15 twenty minute images were shot through the Hydrogen alpha filter, aligned, then “stacked” in a process that combines the pixels from all 15 images statistically. This averages out noise while reinforcing true detail. Additionally, 15 “flats” were shot of empty sky and stacked for each filter. This produces a frame that maps the dust motes on the imaging equipment. This is then subtracted from the Ha stack. Additional “dark frames” are shot with the scope covered. These subtract any electronic noise artifacts from all images. The same process is repeated for the SII and OIII filters, each with their own stack of flats and so on. The finished three images are then assigned to the RGB channels in Photoshop or other final processing software. Then the whole thing is repeated for the other two sections of the mosaic. Once the mosaic is assembled, the final processing begins, teasing out detail, reducing noise and polishing the image. I try to do no damage to the pixels, no clipping of values, no cloning or patching. This image involved about 50 hours total for the finishing stage, probably because I am a novice and did a lot of experimenting. The Crescent nebula We chose to also photograph the Crescent nebula with a different telescope at higher magnification to bring out its detail. We used the same process described earlier, but in addition to the three narrowband filters, we shot sequences of red, green, blue and clear for a total of seven filters. I composited that finished image precisely into the Gamma Cygni region mosaic to produce the hybrid wavelength image you see here.
An award-winning artist and animator, Joel Hagen is a founding member of the IAAA. He is a MER collaborator, part of the Opportunity rover science operations team and has produced image products for the Opportunity, Phoenix and Pathfinder missions. A retired computer graphics instructor, Joel works with digital and traditional media as well as 3D printing and print surface treatments. Joel also designs extra-terrestrial life forms and biomes. His work has appeared on PBS, BBC, NHK in Japan, Discovery and National Geographic channels. Go to the artist's portfolio
This artwork is copyright © Joel Hagen. All rights reserved.