NGC 6888

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NGC 6888 NGC 6888
Crescent Nebula
Wolf-Rayet Bubble in Cygnus

Click here for higher-resolution versions: 40% (1368x1462) 65% (2224x2377) 100% (3421x3656)

Click on image to cycle through the six versions of the image (described below and labeled on the lower left corner of each image)

NGC6888: This is a (very dim) nebula formed by the action of the central star (the very bright star at the center of the nebula) expelling its outer gases as it begins the process of dying. At the first stages of its death throes, the star became a red super-giant, and gently puffed out its outer layers, approximately 250,000 years ago. Then it became a type of star called a Wolf-Rayet, a rare, short-lived super-hot star, which started expelling gas at a very high rate (an amount of mass equal to our sun's entire mass each 10,000 years), and at very high speeds, which caused the expelled gas (including the gas expelled earlier, which was hit by the higher speed winds from the Wolf-Rayet phase) to assume something of a shell shape. It is thought that the central star will become a supernova sometime in the next million years.

The nebula is about 4,700 light years from Earth and is roughly 16 by 25 light years in dimension. The dark, squarish spot in the middle of the nebula is real, not an artifact; I am guessing that it is a "Bok Globule" in the nebula.

Just as an observation, I find it interesting how differently the ionized hydrogen (Ha) and the doubly-ionized oxygen (OIII) have been flung out from the star in such different ways.

I have presented this in six (yes, 6!) different formats (each image is labeled in the lower left corner); this is the order in which they appear as you cycle through (by repeatedly clicking on the photo, waiting for each to download):

(i) A version (the top photo in the stack, labeled "LRGBNB" in the lower left corner)), with the color created by imaging through red, green and blue filters (with a significant amount of Ha and OIII data blended in to all channels and the luminance/shape layer); the effect of weaving the Ha and OIII data into the broadband data (L, R, G and B) is to enhance the red emissions from the Ha and blue/green emissions from the OIII;

(ii) A bi-color version (almost true-color version, labeled "HOO"; the second photo in the stack), in which "red" is a mix of predominately ionized hydrogen emissions (Ha), and green and blue are doubly-ionized oxygen emissions (OIII); this works (giving fairly accurate colors) in this nebula because the vast majority of the emissions are in Ha or OIII, and Ha is in the red part of the spectrum, while OIII is blue-green; I used the luminance (shape/detail) layer from the LRGBNB image in this image, also;

(iii) A true-color version (labeled "LRGB"), with the color created by imaging through red, green and blue filters (with no narrow band data included);

(iv) A version in the Hubble palette (a lot of the Hubble photos, including and especially the famous "Pillars of Creation," are made with this set of filters, since it's a useful set for scientists to see what's actually happening), which shows SII emissions as red, Ha emissions as green, and OIII as blue (because Ha is so dominant, the nebula would be much greener were I not to emphasize the OIII; this is why there is relatively little straight green, although you can see a lot of green in the middle parts, with the blue; there is very little SII emission in this nebula); I used the luminance (shape/detail) layer from the LRGBNB image in this image, also;

(v) A pure Ha version (grayscale, showing only light in the very narrow Ha band), and

(vi) A pure OIII version (grayscale, showing only light in the very narrow OIII band).

The top version is, to me, the prettiest. It shows approximately the colors of the object, except that the wispy blue/green stuff has been somewhat enhanced relative to the red; the "almost true-color" (HOO) version is shown because it is a short-cut way to get a pretty picture, without taking so much time gathering data from 7 filters, so is very popular with astrophotographers (especially in light- polluted areas, because the narrow-band filters filter out the effects of most light pollution (and it shows the dominant emissions--Ha as red and OIII as blue-green--quite graphically in one frame, though I'm not a fan of the blue/green stars; the LRGB version has no data taken through a narrow-band filter included, and the difference from the LRGBNB version (the top one in the stack) is remarkable (seeming to mix the red and blue into a purple in a lot of the nebula; the Hubble palette is just because Hubble does it that way, and it's interesting to add the (relatively sparse) sulfur emissions; and the two grayscale images just because they're so pretty (and it is interesting to see how different they are).

This is the second time I have imaged this target; the first time was when I was just starting with this hobby, with fairly elemental equipment (at least by comparison), and from my own yard. If you want to see if there has been any improvement, click here.

Similarly, I gathered this data in 2019, and processed it as well as I could then. But skill improves (perhaps), and tools improve (undoubtedly), so I thought maybe it would be worth re-processing the same data set. I think it was. See for yourself: Click here

Technical Information:

Ha:OIII:SII:L:R:G:B: 540:600:420:885:280:240:415 (a total of over 56 hours of light-frame exposure time); luminance exposures were a mix of 5-minute and 15-minute exposures; Red a mix of 5-minute and 15-minute exposures; green were all 15-minute exposures; blue a mix of 15-minute and 20-minute exposures; Ha, SII and OIII were all 30-minute exposures.

Equipment: RC Optical Systems 14.5 inch Ritchey-Chretien carbon fiber truss telescope, with ion-milled optics and RCOS field flattener, at about f/9, and an SBIG STX-16803 camera with internal filter wheel (SBIG filter set), guided by an SBIG AO-X, all riding on a Bisque Paramount ME German Equatorial Mount.

Image Acquisition/Camera Control: Maxim DL, controlled with ACP Expert/Scheduler, working in concert with TheSky X.

Processing: All images calibrated (darks, bias and sky flats), aligned, combined and cropped in Pixinsight. Color combine in Pixinsight. Some finish work (background neutralization, color calibration, deconvolution, lessening the dynamic range and noise reduction) done in Pixinsight; some cleanup finish work was done in Photoshop CC.

Location: Data acquired remotely from Deep Sky West Remote Observatory, Rowe, New Mexico, USA.

Date: Images taken on many nights in July, August and September 2019. Data reprocessed in November 2024, and image posted November 7, 2024.

Date: Image scale of full-resolution image: 0.56 arcseconds per pixel.

CCD Chip temperature: -25C

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