Saturday, December 24, 2011

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:


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!

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