NGC 7822

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NGC 7822 NGC 7822
Sh2-171
Emission Nebula in Cepheus

Click here for higher-resolution versions: 40% (1638x1638) 65% (2662x2662) 100% (4096x4096)

Click on image to cycle through the four versions of the image (described below)

NGC 7822 is a large, very dim, emission nebula, visually located in the constellation Cepheus. The nebula, a star-forming region, is predominantly red in the true-color versions, because (i) ionized hydrogen emits in the red part of the spectrum; (ii) the dominant emissions captured in this photo are from hydrogen being ionized (stripped of its electron) by the energetic young stars being formed inside the nebula.

The field is thought to be about 2,900 light years from Earth; at that distance, this field is about 31 light years across. Visually, this field is about the width of a full moon (although very dim by comparison). This nebula is very close, visually in our sky, to click Sh2-170
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; each is labeled in the lower left corner), starting with a reddish version:

(i) A version in the Hubble palette (at the top of the stack, labeled "SHO" in the bottom left); 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. Because Ha emissions so dominate this nebula, I have significantly de-emphasized them in this rendering. With that, the OIII emissions show as pure blue (would be a blue-green otherwise); the SII emissions in this nebula are very much in the same place as the Ha emissions, which explains the yellowish color. Bright stars tend to be magenta in color with this palette; some people substitute "normal" colored stars, but I just leave them that way, since all the colors are false anyway. I like the clear blue when a nebula has significant oxygen emissions, as is the case here (though certainly the oxygen emissions are fairly faint). This form of combining results in magenta-colored stars, which I have significantly desaturated.

(ii) A true-color version (the second photo in the stack, labeled "LRGBNB" in the bottom left), 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).

(iii) A true-color version (the third photo in the stack, labeled "LRGB" in the bottom left), with the color created by imaging through red, green and blue filters only (no Ha or OIII data included). It is interesting to see how much the addition of the Ha and OIII increases the detail, but I like the more-colorful stars in this version.

(iv) A pure Ha version, showing only emissions from the ionized hydrogen atoms in the nebula.

These are four of 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.

Technical Information:

Ha:OIII:SII:L:R:G:B: 630:600:600:660:180:165:300 (a total of over 52 hours of light-frame exposure time); here's a chart showing the various subexposures I used in the image:

Hydrogen Alpha: 21 thirty-minute
Oxygen III: 20 thirty-minute
Sulfur II: 20 thirty-minute
Luminance: 40 fifteen-minute, and 20 three-minute
Red: 12 fifteen-minute
Green: 11 fifteen-minute
Blue: 15 twenty-minute

The luminance layer is a mix of the luminance-filtered images and the Ha-filtered images; the red channel is a mix of the red-filtered images and the Ha-filtered images; the green channel is a mix of the green-filtered images and the OIII-filtered images; the blue channel is a mix of the blue-filtered images and the OIII-filtered images. The SHO version uses the SII 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 STX guider, 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, noise reduction with NoiseXTerminator; deconvolution using BlurXterminator, HDR combine of the luminance data) 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 November and Decsmber of 2025. Image posted March 25, 2026.

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

Seeing: Uniformly horrible, with single calibrated luminance subexposures ranging from 4 to 5.8 arcsecond FWHM; luminance master deconvolved to about 1.2 (using BlurXTerminator)

CCD Chip temperature: -25C

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