Sharpless 86
NGC 6820
NGC 6823
Emission Nebula, Open Star Cluster, and Reflection Nebula in Vulpecula

Click here for higher-resolution versions:  40% (1630x1626)  65% (2649x2643) 100% (4076x4066)
Click on image to cycle through the four versions of the image (described below)


Sh2-86 is a dim emission nebula visually located in the summer constellation Vulpecula. It is predominantly red in the true-color version, because (i) ionized hydrogen emits in the red part of the spectrum; (ii) the vast majority of the light matter in the universe is hydrogen, and (iii) the hydrogen in this image is being ionized (stripped of its electron) by the highly-energetic young stars being formed in this hydrogen cloud. The field also includes NGC 6823, the open star cluster in the center of the image, and NGC 6820, a nebula of uncertain description (it variously is described as essentially another name for Sh2-86, or "a small reflection nebula near the open cluster NGC 6823"; the man who is credited with discovering it described it as "faint, small, round, brighter in the center"). The only reflection nebula I see in the entire field is a tiny thing to the left of the top center of the image, so that probably is NGC 6820 (apparently also cataloged as GN 19.40.3), but (if so) I'm surprised that such a tiny thing would be included in the New General Catalog.

The field is thought to be about 6000 light years from Earth; at that distance, the brighter part of the nebula would be about 50 light years across. Visually, it is about the width of a full moon (although very dim).

I have presented this object in four different formats; I like each one in its own way. This is the order in which they appear as you cycle through (by repeatedly clicking on the photo, waiting for each to download), starting with the reddish version:

(i) A true-color version (the top photo in the stack), with the color created by imaging through red, green and blue filters (with a significant amount of Ha and OIII data blended into various channels, in varying percentages; Ha emissions are in the red spectrum, and OIII emissions are blue-green, so I have blended Ha into the luminance layer and the red channel, and OIII into the green and blue channels);

(ii) 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 emissions as blue (with the Ha emissions de-emphasized in this rendition because they would be so dominant otherwise--but I still keep a lot of green in the nebula). I like the clear blue when a nebula has significant oxygen emissions. This form of combining results in magenta-colored stars, which I have significantly desaturated.;

(iii) This started out as a "Hubble palette" image, but, for purely esthetic reasons, people often transform the green to tan;

(iv) A pure Ha version (grayscale, showing only light in the very narrow Ha band); this is fun for me to gaze at in full resolution, to see all the detail.

These are the most frequent ways images of emission nebulae are likely to be presented, so I thought it would be fun to include all of them, to be able to compare and contrast the different presentations. The "true color" version is, to me, the most beautiful of the lot, so I put it at the top; the narrow-band data adds contrast, depth and detail to the colors and the image, and makes the stars smaller, which is pleasing to me, while maintaining the "true color" nature of the data. The Hubble palette version is interesting, showing the dominance of the Ha emissions, while also showing that there are significant OIII emissions (the blue) and even some SII emissions (the yellow/orange pieces), and it's nice to include the (relatively sparse) sulfur emissions that I spent a bunch of hours gathering.

The grayscale Ha image is, to me, striking in its smoothness and contrast (and the gray, of course, is the color you see when you look at this through a telescope). And the traditional LRGB, without using any narrow band data, also is for comparison purposes, to show the benefits of including narrow-band data in a true-color image (compare to the top image, which has more detail, deeper colors and smaller stars).

This pillars below and to the right of center remind me of the famous Hubble "Pillars of Creation," which I imaged here.


Technical Information:

Ha:OIII:SII:L:R:G:B: 660:630:600:720:180:180:240 (a total of over 53 hours of light-frame exposure time); luminance, red and green exposures were all 15-minute exposures (plus I took a series of 5-minute luminance images); blue all 20-minute exposures; Ha, SII and OIII were all 30-minute exposures. The luminance layer is a blend of the 5-minutes images taken through the luminance filter, the 15-minute images taken through the luminance filter, and the images taken through the Ha filter. The red channel is a blend of the red-filtered data and the Ha-filtered data. The green channel is a blend of the green-filtered data and the OIII-filtered data. The blue channel is a blend of the blue-filtered data and the OIII-filtered data.

Equipment: RC Optical Systems 14.5 inch Ritchey-Chrétien 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, and noise reduction) done in Pixinsight; some cleanup finish work was done in Photoshop CC.

Location: Data acquired remotely from Sierra Remote Observatories, Auberry, California, USA.

Date: Images taken on many nights in June and July of 2022. Image posted August 21, 2022.

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

Seeing: Generally good, with individual calibrated luminance frames varying from 1.9 to 2.1 arcsecond FWHM.

CCD Chip temperature: -25C

Copyright 2022 Mark de Regt

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