Sharpless 155
Cave Nebula
Emission Nebula in Cepheus

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


Sharpless 155 is a large emission nebula, visually located in the constellation Cepheus. It is also called the "Cave Nebula," for reasons that escape me (yes, I see the "hole," but...). The nebula 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 2,400 light years from Earth; at that distance, this field is about 26 light years across. Visually, this field is about the width of a full moon (although very dim by comparison). Because it's so diffuse, it has a low surface brightness despite nominally being rated at magnitude 8; it was not discovered until 1959!

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 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 true-color version, 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.

(iii) 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. Because Ha emissions so dominate this nebula, I have significantly de-emphasized them in this rendering; and because green is a bit gaudy, I have partially converted the green to a ruddy tan color. 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.

(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:

The "true color" version which includes the Ha and OIII data 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 image, and makes the stars smaller, which is pleasing to me, while maintaining the "true color" nature of the data.

The true color without the Ha and OIII data is pretty, to me, though less dramatic than the image with Ha and OII data. I like the subtle changes of colors as you scan the image.

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.

And the pure-Ha version is always dramatic, to me, showing as it does so much structure.


Technical Information:

Ha:OIII:SII:L:R:G:B: 600:600:660:450:180:180:240 (a total of over 48 hours of light-frame exposure time). The luminance layer is a blend of 30 fifteen-minute images taken through the luminance filter and 20 30-minute images taken through the Ha filter. The red channel is a blend of 12 fifteen-minute images through the red-filter and the Ha-filtered data. The green channel is a blend of 12 fifteen-minute images taken through the green filter data and 20 thirty-minute images taken through the OIII filter. The blue channel is a blend of 12 20-minute images taken through the blue filter 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, 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 October of 2023. Image posted May 7, 2024.

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

Seeing: Variable, but generally good

CCD Chip temperature: -25C

Copyright 2023, 2023 Mark de Regt

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