The Phoenix Butterfly

I spent last week on an imaging trip near Lassen Peak, in northern California. It's a minor miracle that I got an image of the Butterfly Nebula (IC 1318), given how much forest-fire smoke was in the air. The last several years of Lassen trips have been blessed with clear, blue, gorgeous skies, for the most part. Forest fires are par for the course in the area, however, and it was only a matter of time before the dice came up snake-eyes, smoke-wise. In other words, I was bound to lose a Lassen trip to forest fires, someday. That someday was the August 2012 dark-moon cycle... almost. Despite all the smoke (and clouds), there was enough clear sky to image some of the nebulosity around the star Gamma Cygni. I like to think of this as `a butterfly rising like a phoenix from the ashes of a fire-plagued season'.

IC 1318 d and e and LDN 889, a.k.a. the Butterfly Nebula, imaged from Lassen Peak.
Click on the image for a larger version, or click here for full size.

Only a couple of nights in my week-long trip had worthwhile skies, so I had to abandon my plans to image the Swan nebula (M17) and the Triangulum galaxy (M33), and concentrate on a single object that would be near the zenith for most of the night. An object that appears near the overhead point in the sky (the zenith) is seen through the least possible atmosphere. In this case that meant through the least possible smoke, depending on how the smoke was being blown around by the wind.

During northern-hemisphere summer nights, the region of the zenith is dominated by Cygnus, the Swan. Also known as the `Northern Cross', Cygnus is a grand constellation, one of the few that really looks like its namesake. Right at the heart of the swan is the star Gamma Cygni (a.k.a. Sadr). A good deal of bright emission nebulosity and dark dust can be seen around Gamma Cygni, making it a popular target for imagers. I happened to pick up the September 2012 issue of Sky and Telescope right before my trip, and when I had to pick an imaging target in Cygnus, I thought of the Gamma Cygni area. Sue French and Steve Gottlieb had covered this region in two very nice articles in the September S&T, and Rob Gendler's image, accompanying Steve's article, really got me excited about this area.

According to Steve's article, the `butterfly' is formed by two portions of the IC 1318 emission-nebula complex (IC 1318 d and e), in front of which lies the Lynds dark nebula 889, a mass of dark absorbing dust. The bright emission nebulosity forms the wings of the butterfly, and LDN 889 forms the body, complete with a head that sports two antennae! Like other `emission' nebulae, the bright material glows because of the excitation of the hydrogen atoms of which it's made. IC 1318 is a star-forming region, and ultraviolet light from hot, massive, young stars causes the hydrogen atoms to glow, a little like a fluorescent light tube or a fluorescent mineral. LDN 889 consists of microscope grains of interstellar dust, which absorb the light from the nebula. (The sky over the Lassen Peak region often contained clouds of smoke that dimmed the stars in much the same way.)

The Reading fire, one of the fires that turned the blue sky brown for much of this year's trip.
(Image credit: National Park Service, Lassen Volcanic National Park)

Data Acquisition

On two nights, the sky was acceptably transparent for imaging, and I managed to acquire three hours of data through a clear (`Luminance') filter, in 5-minute subexposures. The last night of the trip yielded a very nice sky, thanks to some fortuitous wind patterns, with the Milky blazing bright and `sugary' overhead. Two of my three hours of data were acquired under that sky.

I would have liked to shoot some color data, but equipment issues put an end to that idea.  Perhaps foolishly, I decided to try and `drive' my mount from my laptop. Maxim DL was able to talk to the mount and order it to slew around the sky, but I kept having a problem with `backwards slews' in the western part of the sky. I'd have shot an additional 3 or 4 hours of data on the final, clear night if I hadn't been trying to debug this problem. Oh well, I'll get it sorted eventually, and at least I got three hours of luminance.

Pixinsight processing:

The data for this image followed my standard Pixinsight processing routine for a luminance-only image:

1. Calibrate subexposures with the BatchPreprocessing script
2. Register and stack the calibrated subexposures
3. Deconvolution to sharpen the bright, high-signal-to-noise-ratio (high SNR) areas
4. Multiscale Median Transform to smooth the dark (low SNR) areas
5. Stretch the brightness values of the pixels with Histogram Transformation and Local Histogram Equalization
6. Shrinking (actually more like dimming) stars with StarMask and Morphological Transformation)
7. Cropping, conversion to standard ICC color profile for web publishing, and saving as JPEG.

Room for Improvement

(Pixinsight geekery ahead...)

Naturally, I would have liked to acquire more data, including color data. Processing-wise, I noticed that some small-scale, `salt-and-pepper-like' noise was introduced somewhere in the processing. This probably happened during the Histogram Transformation or the Local Histogram Equalization, despite my use of a luminance mask. The luminance mask was made in the usual way, by applying an auto-STF to a copy of the image (via HT). I wonder if I should have done a more elaborate intensity transformation when I made the luminance mask, so as to protect the dark areas better, and to get a more effective deconvolution in the bright areas.

After the initial star-shrinking, which worked mostly on the small stars, I tried to build a  new star mask for the larger, more bloated stars, but after a lot of experimentation, I hadn't gotten much of a result. I decided to post the image as-is, but I still dream of dealing with the large stars someday.

Eagle nebula in B&W H-alpha

Ever since this summer's imaging session at Lassen, I've wanted to process the hydrogen-alpha data that I acquired as a black-and-white image. I spent this evening working on the data in Pixinsight, and here's what I've come up with so far:

Acquisition:

I shot these data during two nights, using 15-minute subexposures through an Astrodon 3nm H-alpha filter, for a total exposure time of 5 hours. This was with the 80mm refractor and borrowed QSI camera that I described in a previous post.

I've always enjoyed the look of black-and-white hydrogen-alpha images, and I wanted to try and make one myself. Images like this remind me of the days of heroic long exposures on gas-hypered Technical Pan 2415 film... days that I have to admit I didn't experience first-hand. And, frankly, I'm not too sorry about it, although it would make for some nice bragging rights. Me, I'm grateful for CCD cameras and autoguiders, which make the whole thing a lot more do-able, although it's still a fair amount of work.

The real key to an image like this is the narrowband hydrogen-alpha filter. I'm lucky that my friend from Cilice, who loaned me the camera, had invested in a filter with such a narrow bandpass. Besides bringing out all of the lovely emission nebulosity (which would look deep red in a color image), a filter like this makes the stars look very small! That's a very nice `perk', although it makes focusing and framing the image rather time-consuming. No need to shrink the stars in software when you have such tiny stars to begin with! I can't wait to get an H-alpha filter for my SBIG filter wheel, someday. I think I'll go with 3nm - it's worth the extra effort.

Processing in Pixinsight:

Like most CCD image processing, part of my workflow happened while the image was still linear, and then I took it to the non-linear realm with a histogram stretch, where I did further processing.

I started by using the A Trous Wavelet Transform (ATWT) tool to reduce noise, following the example from Juan Conejero's `tutorial post'. I used considerably less aggressive ATWT noise-reduction settings than Juan's example, though. Then I did some Richardson-Lucy deconvolution, again following a tutorial-like post by Juan. A real key to getting Deconvolution to work is the use of Dynamic PSF to model the telescope's point-spread function.

Once I had reduced noise with ATWT and applied a bit of deconvolution, I stretched the image into the non-linear realm using Histogram Transformation. (I just applied the stretch parameters from an AutoSTF into HistoTrans.) As per usual Pixinsight practice, I used the HDR Multiscale Median Transform (formerly HDRWT) to bring down the brightness in the central part of the nebula. I found that increasing the number of wavelet layers to 8 helped bring out detail nicely, and did the best job of `taming' the brightest areas. I did another moderate histogram stretch to increase contrast, and then applied the Local Histogram Equalization (LHE) tool, with a contrast limit of 2.0 and and Amount of .25.

A last, light little shot of ACDR was the last step. I did this with the built-in lightness mask enabled, so as to apply it only to the darkest areas. These areas had had their noise increased a bit by LHE.

Assessment:

I'm reasonably pleased with how this image turned out. I like the way the deconvolution brought out detail around the `Pillars of Creation' and other dusty structures in the nebula. HDRMMT also helped to bring out a fair amount of detail, and LHE pumped up the contrast between adjacent light and dark areas.

Naturally, I'd love to get additional hours of data, to bring out even more nebulosity at a reasonably high signal-to-noise ratio. Maybe next summer!

Oh, I almost forgot: I flipped the image left-for-right, compared to my previous Eagle nebula image. I hadn't realized that the previous image was oriented incorrectly. I think this one matches the published `Pillars' images better.

Hmmm... I wonder... since LRGB combinations in Pixinsight are supposed to be assembled from non-linear images, I wonder if I could get the histogram of the RGB image into the right kind of shape to match this one, and use this B&W H-alpha image as the luminance for an LRGB combine?  Hmm... I ought to check that out.

Eagle nebula 2011

One can't very well have an `imaging blog' without eventually posting some images. Here's the first image I think is good enough to be worth putting out there. This is Messier object 16 (`M16'), the Eagle nebula. I shot these data in the summer of 2011, from a trailhead parking lot in Lassen Volcanic National Park, California.

If I feel the image supports it, I like to make three crops:

1440 x 900 (15" MacBook Pro)
1200 x 800 (13" MacBook)
1024 x 768 (iPad and many older monitors)

I'm a Blogger newbie, so I'm still trying to figure out how to provide links to all three sizes, in a way that's convenient for the reader.

Acquiring the data:

I shot this image during the late July / early August dark-moon cycle, from a site at about   8200 feet (2500 meters) above sea-level. (It's silly, but I sometimes like to point out that it's about as high as ESO's Paranal observatory in Chile.) Being able to image from a dark, isolated site at a reasonably high altitude is really helpful for building signal-to-noise ratio in one's image. Under those conditions, there is very little light pollution contributing to the photon shot noise, so most of that noise just comes from scattered starlight in the Earth's atmosphere, the Earth's natural airglow, dust in the plane of the solar system, and the intergalactic background light. (See Brian Skiff's articles on the night sky background, nicely organized and hosted on Jerry Lodriguss's site, for a nice explanation of why the night sky isn't perfectly black.) In addition to the dark sky, the chilly nighttime temperatures make it easier for the camera's Peltier cooler to keep the CCD chip reasonably cold. (I shot these data at -20 degrees C.)

This Lassen session was blessed with excellent weather - I basically had 8 clear nights in a row. This allowed me to capture multiple hours through each color filter. This image was made from unbinned R, G, B, and H-alpha data. I shot Luminance data, but didn't use it in the processing for this version.

For this session, I was using my telescope (an Orion ED80 semi-apochromatic refractor on an Orion `Sirius' mount), while borrowing a QSI 583 CCD camera from my friend at Cilice Astrophotography. Here's the setup I was using at the time:

Processing:

If I have the time and energy to do much with this blog as time goes by, I'll hopefully have a lot more to say about Pixinsight, the software that I used to make the image. I've bee sufficiently impressed with the work that's been done by Pixinsight users - especially the many APODs from RBA - that I decided to try and learn the software. This isn't an easy task, as I've discovered! The story of the Pixinsight learning curve will, hopefully, be a big part of what I'll describe in this blog, if I have the time.

For this image, I did the following things in Pixinsight:

1) Calibration and stacking of the raw CCD images. (See, for example, Vicent Peris's tutorial on master calibration frames, Jordi Gallego's PowerPoint presentation on image integration, and the new Pixinsight reference documentation on image integration.)

2) Dynamic Background Extraction (DBE) for each of the R, G, and B channels, RGB combination, Background Neutralization, and Color Calibration. (See, for example, the new instructional video that walks the viewer through these tasks, as well as Harry Page's excellent set of video tutorials.)

3) While still at the linear stage, I used the A Trous Wavelet Transform tool to reduce background noise, following Juan Conejero's example.

4) Histogram Transformation to stretch the image from the linear domain (and thus largely invisible on-screen) to a brighter, more visible version.

5) Saturation of the red M16 nebulosity.

6) Reduction of green color noise with the Selective Color Noise Reduction tool.

7) Star shrinking (with Morphological Transformation) and star desaturation.

Room for improvement:

The unfortunate thing about being `an imager' is that when I look at my images, I can only see the deficiencies! There's a lot about this image that I'd like to improve, and hopefully my Pixinsight skills will one day improve to the point where I can make those improvements.

The main thing I'd like to change about the image is the `star carpet'. This is a real problem for objects in the plane of the Milky Way. There are so many stars in our galaxy's disk that it's nearly impossible to shoot something like a Messier object in the MW plane without the field being full of stars. As one stretches the nebulosity, the stars get stretched, too, until they become bright enough to dominate the field, rendering the details of the nebulosity less visible.

I'd like to reduce the distracting effect of the `star carpet', and I'm trying to learn some techniques to help me do that. These include RBA's tutorial on star shrinking, and Vicent's recent NGC 7023 tutorial. Hopefully one day I'll be able to post another version of this image with fewer distracting foreground stars!