NGC 7635
NGC 7635
Bubble Nebula
Emission Nebula in Cassiopeia

Click here for higher-resolution versions:  40% (1612x1616)  65% (2620x2626) 100% (4031x4041)
Click on image to cycle through the four versions of the image (described below)


NGC 7635 is a large emission nebula, visually located in the constellation Cassiopeia. Its defining feature is the large bubble at the center of this image, which is a Wolf-Rayet bubble. The nebula is predominantly red in the true-color version, 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 Wolf-Rayet star inside the nebula. The bubble in the center of the image is created by the high-velocity stellar winds from the huge, hot central star pushing the gas away from the star.

The field is thought to be about 7,100 light years from Earth; at that distance, the bubble would be about 7 light years across. Visually, this field is about the width of a full moon (although very dim by comparison).

As befits such an iconic object, I had imaged this region 22 years earlier, in the very early days of my imaging work. I was (and still am) quite pleased with the old image, which really was the first image I took as I became more comfortable with the entire process. To see what a bit of acquired skill, much better location and much better equipment/software can do, compare this current image to my original one here.

Also befitting such an iconic object, this has been imaged by Hubble (in the Hubble palette, like the third image in my stack above); to see what that marvelous instrument can do with it, click here.

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 (the top photo in the stack), 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 it's also interesting to note the more vibrant star colors that often appear in the RGB version.

(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. With that, the OIII emissions show as pure blue (would be a blue-green otherwise), and the SII emissions show as a ruddy brown/yellow (would really show up much otherwise). I like the clear blue when a nebula has significant oxygen emissions, as is the case here. 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: 900:720:660:540:180:180:240 (a total of 57 hours of light-frame exposure time); luminance was a blend of 15-minute individual exposures and 3-minute individual exposures; red and green exposures were all 15-minute exposures; blue all 20-minute exposures; Ha, SII and OIII were all 30-minute exposures. The luminance layer is a blend of the 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, 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 August and September of 2023. Image posted December 15, 2023.

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

Seeing: Variable, but generally good

CCD Chip temperature: -25C

Copyright 2023 Mark de Regt

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