NGC 1491
NGC 1491
Emission Nebula in Perseus

Click here for higher-resolution versions:  40% (1561x1509)  65% (2537x2452) 100% (3903x3772)
Click on image to cycle through the four versions of the image (described below)


NGC 1491 is a bright emission nebula visually located in the constellation Perseus. 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 a highly-energetic young star near the middle of the brightest part of the nebula.

The field is thought to be about 11,000 light years from Earth; at that distance, the brighter part of the nebula would be about 100 light years across. Visually, this field is about the width of a full moon (although very dim by comparison).

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 the reddish version (the first, second and fourth all share the same luminance--detail--layer):

(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) Since Ha is in the heart of the red part of the spectrum, and OIII is on the border of green and blue, people often create an image only using Ha and OIII, by using the Ha the red, and OIII as both blue and green (so the shorthand name is "HOO"). That is what I have done here, except I have over-emphasized the OIII (relative to the Ha), resulting in the ghostly, slightly bluish, center (otherwise it would have been far redder). It's interesting how close this comes to the full "true color with Ha and OIII" version, with considerably less data/imaging time.

(iii) 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 impact.

(iv) 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 green from the Ha emissions turned brownish, and de-emphasized in this rendition because they would be so dominant 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.

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 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 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 true color without the Ha and OIII data is pretty, to me, but more muted than the image with Ha and OII data.

The HOO/bicolor version, with the OIII emphasized, gives a different look, with a delicate beauty..


Technical Information:

Ha:OIII:SII:L:R:G:B: 720:600:630:741:255:180:240 (a total of over 56 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, deconvolution of the luminance data using Russ Croman's BlurXterminator, HDR combine of the luminance data; HDR multiscale transform to reduce the dynamic range; multiscale linear transformation for sharpening, 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 September, October and November of 2022. Image posted December 31, 2022.

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

Seeing: Variable.

CCD Chip temperature: -25C

Copyright 2022 Mark de Regt

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