From the Pros: Multi-wavelength Eagle

When I say `the pros', what I'm really talking about are research astronomers. These are the folks who do fundamental research in astronomy, such as the people who make observations with the Herschel infrared space telescope and the XMM-Newton X-ray space telescope. Most of these big-buck research projects are pretty good about remembering the public outreach part of their mission. Here's an example: Today's NASA Image of the Day is a view of the Eagle nebula, captured in two very different wavelengths - infrared and x-ray:

(Image credit: ESA/Herschel/PACS/SPIRE/Hill, Motte, HOBYS Key Programme Consortium)

Having recently spent so much time on processing the Eagle nebula, it's fun to see it in wavelengths that I can't capture with my CCD camera from the Earth's surface.

Naturally, I can't resist including the most famous `pro' shot of a part of the Eagle nebula:

(Image credit: NASA / STScI / Hubble Heritage Team)

I'll bet you've seen this image before. It was acquired by the Hubble Space Telescope in the 1990s, and has been very widely reproduced and distributed. It's probably one of the most famous `Hubble shots' of all time, if not the most famous. (I'll take a moment here to plug a friend's business, where you can buy prints of the Pillars.)

If you look closely in the Herschel / XMM-Newton image, and even in my Eagle image, you can make out these pillars. In fact, they're even visible to the eye, if you use a reasonably large telescope, and you're observing from a very dark site under good conditions. When I've taken my 18" (45cm) scope to observing sites in the northern California mountains in the summer, I've sometimes been able to make out the two largest pillars from the image above. It's tough, but with some practice, an OIII filter, and careful examination of a printed image (using very dim red light, so as not to spoil one's dark-adaptation), they're just visible. It's fun to be able to see something so well known with your own eyes! It's fun to be able to capture it with one's own telescope, too.

UPDATED a couple of hours later...

The Herschel and XMM-Newton missions are run by the European Space Agency, and they've got a nice webpage about these multi-wavelength observations of the Eagle nebula. It includes a video showing the various images and how they correspond to each other. Here's a summary image, which places the various Eagle images next to each other:

(Image Credit: European Space Agency, European Southern Observatory, NASA)

One of my favorite of the `Pillars' images is the near-infrared image; it's the center image in the right-hand column of the mosaic above. It was acquired using one of the giant 8-meter telescopes of the Very Large Telescope observatory at Paranal, Chile. I love the purple color palette of this image:

(Image Credit: VLT/ISAAC/McCaughrean & Andersen/AIP/ESO)

The team that made this image used an infrared camera/spectrograph called ISAAC to collect image data in three wavelength bands, all of which are in the infrared. This means that the wavelength of the `light' in each band is longer than that of visible light - it's beyond our eyes' ability to see. The dust that makes up the Pillars is mostly opaque at visible wavelengths, but infrared `light' can make it through a greater thickness of dust than visible light can. As a result, they can see deeper into the Pillars, or entirely through them in the case of the left-hand pillar. This allows for a clearer view of young stars that are forming out of these clouds of gas and dust.

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!