The Incredible Shrinking NebulaI love the name “The Incredible Shrinking Nebula”, but really struggled to find the origin and reason behind the name. It is very small, the striking spherical shell is only 30” across Like most named objects, the name is based on the visual appearance, and as far as I can see it originates with Stephen O’Meara in his book “The Secret Deep”. He explains that at low power visually the nebula appears to extend to a greater extent east and west. But as one increases the power, one resolves the stars on the east and west sides, revealing that the nebulosity appears smaller than it seemed, and the more the magnification the smaller the nebulosity appears to be. He also called it “Lipstick Marks on a Mirror”. Okay, I admit, I love this name as well, though in my image the lipstick is way off the lips. Finally, it is also known as the Snowball Nebula.
It is officially NGC 6804. William Herschel discovered it, but he though nebula were unresolved stars, calling it a star cluster. It wasn’t until 1917 that Pease correctly identified it as a planetary nebula. The guesstimate is that it is 4200 l-y away, but PN nebula are notorious for not having good distance indicators. The central star is a 14.5 magnitude O9 star, very difficult visually but easy to capture photographically.
This PN has many shells, perhaps one of the more complex PN. The annular “lips” are the brightest, but also the spherical shell is very well defined and one of the most studied shells, though only about 0.5” in size, similar in apparent size to our best view of Mars! There is a faint halo as well, though strongest in ansae north and south, which brings the total size to 50”, about 1 l-y at the assumed distance. The halo is visible in my image—but even at 28 hrs I could not push it further without introducing way too much noise. Apparently the halo is actually very mottled in appearance even in the deepest images, which may be contributing to the noisy look, see the article by:
Balick, B., Guillermo, G., Frank, A., & Jacoby, G. 1992, ApJ, 392, 582CaptureCapture was with SharpCap. Due to my low noise camera and high light pollution, I only need 1 minute subs to ensure read noise is insignificant. Look up Robin Glover's videos if you want to learn more about optimal sub exposure selection.
This target is not popular, it is very small and not that bright, so there are few images on Astrobin, and none that have tried to go deep that I found. I generally have two to three projects in progress at once, so I need many months to complete each target. In this case I got lucky with many good nights on my targets, which enabled me to go quite deep with this PN. The improved S/N helped as a result, and the relatively short exposures, were both helpful to bring out detail.
ProcessingI calibrated and stacked using DeepSkyStacker with a 2X drizzle, as I find drizzle increases my resolution (and also magnifies any tracking errors, of course), but without any S/N reduction. I processed the image twice in StarTools, once for the nebulosity and once for the stars. In both cases I used background extraction, digital development, HDR, deconvolution, initial color, and an inverse gamma for contrast to the background. In Photoshop, I separated the stars and nebulosity in both images using StarXterminator. I used APF-R (the multi-scale unsharp mask used by NASA) to bring out detail in the nebulosity separation, followed by NoiseXterminator for final noise reduction and giving some detail boost as well. I did final color and curves adjustments to both the star and nebulosity layers, adding the stars back to the nebulosity using a linear dodge. Note parts of the PN were quite strong in Ha, though many images show the PN as mostly teal color. I went with keeping the red, as it seemed more natural and true to the data. Note I have subdued the stars significantly as in a close-up crop stars tend to bloat. The final image is cropped to about 26% of the original image, to best appreciate the detail.
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