NGC 896

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NGC 896 NGC 896
IC 1795
Fish Head Nebula
West End of the Heart Nebula
Emission Nebula in Cassiopeia

Click here for higher-resolution versions: 40% (1630x1630) 65% (2649x2649) 100% (4076x4076)

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

NGC 896 is the brightest part of IC 1805 (a large emission nebula also known as the Heart Nebula), in which star-forming is taking place. This nebula is often called "the Fish Head Nebula," because of its obvious resemblance to the front part of a fish (think, e.g., a grouper). An emission nebula is predominantly red, 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 near the new stars is being ionized (stripped of its electron) by the highly-energetic young stars being formed in the star cluster.

The dark areas surrounded by reddish nebulosity are "dark nebulae," predominantly consisting of interstellar dust clumped densely enough to obscure light from the other side from reaching us. Some of this are pretty dramatic!

This image is a "close-up" of the western part of the much larger nebula. I imaged the entire (almost) Heart Nebula 16 years earlier, with a smaller telescope (and chip); you can see in that larger field of view why the entire nebula is called the "Heart Nebula." Click here to see the older version.

This Heart Nebula is about 7500 light years from Earth and is roughly 200 light years across; the portion shown in this photo is about 70 light years across. Visually, it is about the same width as a full moon.

I have presented this object in five different formats (each image is labeled in the lower left corner), each a fairly common way to process this kind of data-set; I very much 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):

(i) A true-color version (LRGBNB, 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, without using any narrow band data (LRGB, all color is derived from data collected through red, green and blue filters, and the luminance layer does not contain any Ha component), as a comparison to the result when loaded with NB data. I always like the richer colors (especiall in the stars) of the LRGB version.

(iii) A bi-color version (HOO, almost true-color version; the third photo in the stack), in which "red" is ionized hydrogen emissions (Ha), and the green and blue channels both are doubly-ionized oxygen emissions (OIII), which are blue-green in color. It's a fair substitute for having red, green and blue channels, but, because the green and blue channels are the same, the star colors are a bit wonky. I used the Ha master as the luminance (detail) layer. I processed it to emphasize the presence of OIII emissions (the blue-green" parts).

(iv) A version in the Hubble palette (SHO; 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. The luminance layer is the Ha master. Because the green from the dominant Ha emissions can make this look odd, people often "neutralize" the green, turning it into a tan. I have done that with this image, as well as increasing the contribution of the OIII (blue) and SII (red) data. Consider that everything red in the first two images would be green in this one if I had not largely ignored the Ha data; the yellow/brown parts of this one show sulfur emissions, and the blue show oxygen emissions.

(v) 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 in the structure.

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 which includes narrow-band data is, to me, the most beautiful of the lot (generally speaking, I prefer "true-color" to false color), so I put it at the top; the narrow-band data adds contrast 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 Hubble palette version is also very pretty, 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 the gray, of course, is the color you see when you look at this through a telescope).

Technical Information:

Ha:OIII:SII:L:R:G:B: 660:720:600:765:270:225:180 (a total of 57 hours of light-frame exposure time used in this photo); luminance exposures were a mix of 3-minute and 15-minute exposures; Red were 15-minute exposures; green were all 15-minute exposures; blue all 20-minute exposures; Ha, SII and OIII were all 30-minute exposures. In the LRGBNB version, the Ha data were blended into the luminance layer; the Ha data also were blended into the red channel, while the OIII data were blended into both the green and blue channels.

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), sometimes guided by a semi-functional 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 and noise reduction both done with Russ Croman's marvelous tools) 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 December 2024 and January 2025. Image posted June 27, 2025

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

Seeing: Quite variable

CCD Chip temperature: -25C

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