Annotation Script - What did I capture in my image?

Here's another version of the Leo Triplet (luminance) image. This one has been overlaid with the results of Andres Pozo's plate-solving and annotation scripts. Thanks to Andres's hard work, I can take my image and `see what I captured':

The Leo Triplet, annotated. Click on the image for a larger version, or click here for full size.

Andres started a thread in the Pixinsight Forum back in March (see the link listed in the previous paragraph), and he's posted a number of updates to his scripts since the thread started. His scripts do two very useful things:

1) One script `plate-solves' the image. This basically means figuring what part of the sky has been captured in the image, and assigning a set of on-sky coordinates to each pixel in the image. (This is nicely described in Chapter 9 of Berry and Burnell.) By attaching metadata to the image (as part of something called the `FITS header'), the plate-solving script allows the annotation script to look at the image, and figure out the exact location (on the sky) of each pixel in the image.

2) The next script looks up objects in a set of online catalogues, and overlays symbols and coordinate lines on the image.

The whole thing is very slick, and after only one false start, I got Andres's scripts to work. The Annotation script overlaid the locations of objects from these three catalogues:

The Messier catalogue: This is a list of nebulous-looking objects in the sky, compiled by the 18th-century comet hunter Charles Messier. It's a list of roughly 100 bright deep-sky objects visible from mid-northern latitudes. The two big, bright galaxies in my image are Messier objects 65 and 66.

The NGC and IC catalogues: These catalogues were first compiled by J.L.E. Dreyer in the 19th century, and they list thousands of objects beyond the Messier catalogue. The great 18th-19th-century astronomer William Herschel found about 2500 of the objects that provided the initial `nucleus' of the NGC. Amazingly, Steve Gottlieb (a Bay Area observer) and others have been double-checking the NGC/IC catalogues visually!

The Principal Galaxy Catalogue: This list of about 70,000 galaxies was published by a group of French astronomers in the 1980s. Many of the faint `field galaxies' that an imager is likely to capture will turn out to have PGC designations.

Looking at my Leo Triplet image, it seems like I got pretty much all of the overlaid PGC galaxies. In other images that I've shot recently, about which more anon, the boundary between `what I got' and `what I couldn't get' occurs in the PGC galaxies. This isn't really surprising, since a large catalogue like the PGC includes objects that span a large range of apparent brightnesses. If I had more time, it would be interesting to compile lists of the PGC galaxies that I did and didn't get, so as to characterize the depth of my image. How deep can a 3-inch f/7.5 refractor with an amateur CCD camera go in a night or two? Andres's script offers a way of estimating this.

I'm pleasantly surprised at how much I enjoy looking at the image with the annotation overlays. They give me a sense of what's in this part of the sky, and somehow they add depth and richness to the image. Naturally, the `pretty picture' version of an image probably shouldn't have annotations like this on it, but it's nice to be able to make an annotated version easily. The two versions complement each other, I think.

The Orion nebula: Reworking some year-old data

If you've not yet had the opportunity to look at M42, the great Orion nebula, through a telescope, you owe it to yourself to try and find an opportunity to do so. Even though today's entry is part of an `imaging blog', M42 is the kind of object that's beautiful any way you look at it. As long as you've got a clear sky, and are (hopefully) away from city lights, you can see this nearby star-forming complex in some way, regardless of the gear you've got.

I just now spent a moony evening reworking some unbinned R, G, and B data that I shot about a year ago. Every winter, it's the same old routine: Try to get some decent data on M42. Something always gets in the way, though. In early 2011, it was bad weather and camera issues... a story for another time. I managed to shoot some unbinned L, R, G, and B data, but not a heck of a lot. To the best of my memory, the data for this image don't amount to much more than several hours total. Since it's now February, and I'm not 100% sure if I'll get in a decent M42 dataset in 2012, I thought I'd fool around with this old stuff from last year. See if I could make something semi-presentable.

After an evening spent in front of the computer, I happened to go outside, and as I was walking back in, I looked up, and there he was: Orion, the hunter. The constellation was just passing across the meridian, with the bright gibbous moon due north of it. Even with lights in my eyes, and under a city sky, I could make out the bright stars that delineate the pattern: Betelgeuse, Rigel, Bellatrix, Saiph, Alnitak, Alnilam, Mintaka. And there was the sword of Orion, with the middle `star' being M42. This object is so bright that it (or at least the stars in and around it) can be seen under almost any sky, it seems.

Unlike most deep-sky objects, M42 is worth looking at with virtually any optical instrument. The belt and sword of Orion are great in binoculars. Small telescopes show the nebulosity. Large telescopes under dark skies provide one of the few `imaging-like' experiences in visual observing. A greenish color can even be seen in the brightest part of the nebula, in a big scope. The details just go on for days and days.

As an imaging project, M42 presents an almost limitless field of challenges and rewards. With modest equipment and short exposures, one can still get something. Advanced imagers have gotten some incredible results.

Processing in Pixinsight:

This image certainly isn't incredible, but I'm glad that I was able to squeeze a bit of detail out of such data as I had. I spent a fair amount of time on this in Pixinsight, and eventually I gave up on trying to combine the luminance data with the color data. Both my RGB image and my Luminance image were the result of high-dynamic range combinations, for which I'd shot long- and short-exposure frames. Matching the histogram from the L image to the histogram from the RGB image seemed to be taking forever, with little end in sight. I bailed and just went for the RGB.

Getting a good color balance was really tricky, and I just couldn't get it quite right. The stars in the linear image had all sorts of blue and cyan issues, and by the time I got them to look semi-normal, the blue color in the nebula was pretty well gone. I could have (and should have) worked that problem harder, but since this was a `let's see what we can get out of this stuff without too much struggle' project, I didn't sweat it that hard.

I did a bit of Richardson-Lucy deconvolution while the image was still linear, but nothing drastic. I would have liked to have gotten a better sharpening result, but I found that I kept getting bright `wormy' artifacts if I wasn't careful. I think that a really good deconvolution would be pretty substantial project, even with the help of Dynamic PSF.

After histogram stretching, I had to spend a fair amount of time finding the right parameters for an application of HDR Multiscale Transform, to knock down the over-brightness of the area around the Trapezium. Once I got that area tamed, it was rather washed out, as usual. Some additional luminance masking and an extra saturation boost in that area helped a bit, although it left some purple haze around the Trapezium stars.

There's plenty of room for improvement in this image, but I'm glad that I can at least post some sort of M42 image. I hardly feel like `an imager' without one. With a little luck, maybe I can finally get a decent set of data later this month and in March. It would be nice to really go deep on this thing, and under good seeing. We'll see how it goes!

As per usual, there's an amazing image of the object from the Hubble Space Telescope.

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.

M33: Two nights at Dino

Here's M33, the Triangulum galaxy:

(There's probably an issue with orientation or `flipping' of the image, but since I've stared at it for so long in this orientation, this is becoming `how it looks to me'.)

This image has me thinking about two `themes':

1) The pleasures of imaging from a nice dark site, like Dinosaur Point.

2) The difficulties of getting good data on M33, the Triangulum Galaxy.

I shot these data on two successive Saturday evenings, October 22 and 29, 2011, from an observing site called Dinosaur Point. It's a boat ramp on the San Luis Reservoir. The reservoir is part of California's enormous system of water projects, which control floods, supply water, and supply electricity. One function of the San Luis reservoir is, essentially, as a giant electrical storage battery. Water gets pumped uphill into the reservoir at night, when electric rates are low, and the water is drained downhill (through generators) during the day.

Dinosaur Point has long been a favorite winter dark-sky site for Bay Area observers. It tends to be too windy during the warm months. But in the late fall and winter, if the `tule fog' from the nearby Central Valley hasn't covered it, Dino can be a very dark site. I really enjoyed setting up there and imaging M33; the sky was nice and dark. One night, in the wee hours of the morning, we even saw the adaptive-optics laser beam from Lick Observatory, shooting towards some object in the south.

It's very important to note, though, that observing access to Dino is subject to some very specific conditions. If you're a Bay Area observer who hasn't been there, make quite sure that you've read and understood the `gatekeeper' access protocol! You can also check the TAC list and the TAC Observing Intents page to see if a gatekeeper is going. Don't just go there without checking all of these details first!

I acquired these data with the same rig as the last couple of shots - my Orion ED80 refractor (80mm f/7.5) with the SBIG ST-8300M CCD camera. I shot unbinned luminance data, and 2x2 binned color data through R, G, and B filters. If I recall correctly, I think I have a couple of hours from each filter. That would make for 8 or so hours of total exposure time, give or take.

I think that M33 has some potential to be a frustrating object for beginning astro-imagers. Typically, I think a lot of us undergo a pattern like this: a) We get a CCD camera during the summer, and by autumn we have a basic understanding of how to use it. b) During the fall, we shoot M31, which is so bright that we can get a decent signal-to-noise ratio over most parts of the galaxy, without too much trouble. c) Next, we say to ourselves `Aha, look what's nearby - M33! There's another big bright galaxy just waiting to be shot!' As it turns out, however, M33 has a lower surface brightness than most of M31, and it's tough to build enough SNR to get a good image. Unless you're using an optical system with a very fast focal ratio, M33 is going to take a long time to build a decent dataset.

This dataset really isn't long enough, but I decided to go ahead and try to process it anyway. I probably won't be able to shoot M33 again until summer or fall 2012, so here's what I've got, so far. With a considerable amount of time invested in Pixinsight, I was able to get something semi-presentable.

Processing in Pixinsight:

I started with the usual calibration routine, using light, dark, bias, and flat-field frames, and I extracted the small amount of light-pollution gradient that one gets at Dino. This gave me linear (i.e. unstretched) luminance (L) and color (RGB) images. These images had the usual background-neutralization and color-calibration corrections applied to them. Then it was time to get a little more from the linear images. First, a bit of noise reduction using the Multiscale Median Transform tool. Then I used the new DynamicPSF module to build a model point-spread function for each image, and fed that PSF into a gentle application of regularized Richardson-Lucy deconvolution. This helped to bring out a bit more detail in the central part of the galaxy.

Then it was time to go non-linear with each image. I did this the easy way: For each image, I did an auto-STF (Screen Transfer Function), and applied each of those auto-STFs to instances of the Histogram Transformation tool. This gave me stretched images that had very similar histograms - and that's just what the LRGB combination tool wants.

If I recall correctly, I did a bit of SCNR (Selective Color Noise Reduction) to take out some of the `galaxy green' in the RGB image, before performing the LRGB combination. I increased the saturation a bit when making the LRGB image, and used Pixinsight's magic Chrominance Noise Reduction routine.

With the LRGB image in hand, it was time to perform two parallel lines of attack, which would later be combined:

1) Compress the dynamic range a bit with HDR wavelets, so as to take away some of the `over-bright dominance' (for lack of a better term) of the central part of the galaxy, and then punch up the contrast with Local Histogram Equalization.

2) Try my hand at the mystical `multiscale processing', a la Rogelio. I split a copy of the LRGB image into large-scale and small-scale components, following the general method of Rogelio's and Vicent's multiscale tutorials. I didn't to anything extra to the smallscale image; I just didn't have the mental energy. But I did some Histogram Transformation (and possibly HDRWT, IIRC) to the large-scale image, brightening the midtones and re-setting the black point. Then I combined everything back together with PixelMath:

a) The LRGB image
b) The LRGB image that had been HDRWavelets-ed and LHE-ed
c) The smallscale image
d) 0.25 * the stretched-even-more largescale image.

Following this recombination, I made a Star Mask (with default parameters), and used Morphological Transformation to dim/shrink the small and medium-sized stars. At that point, I said `Stick a fork in this sucker, it's done. Put it on the blog.'

Room for Improvement:

When I look at this image, it seems to me like it's still afflicted with a bit of `galaxy green', but when I applied an additional round of SCNR to it, it didn't seem to change. Some of the stars also wound up looking a bit pink, but at this point, I'm too tired to fight about it.

Next, there are the big, bloaty stars. These are the bane of all my images. My temptation is to blame them on the small aperture of my telescope. An 80mm scope will have a big, fat point-spread function, and if I want tiny stars, I'll need a bigger scope. That's probably true, to some extent, but I'll bet it's not the whole story. I am beginning to suspect that the big, halo-y stars are a consequence of the fairly severe stretching that the image has undergone. M33's dim, and it takes a lot of stretching. This probably brings the outer parts of the PSFs up to an objectionable brightness. With a longer total exposure time, I could probably get the faint parts of M33 to show up without as much stretching. (Of course, this raises the question of whether those outer portions of the PSFs would show up, too... hmm...) I'd love to figure out how to shrink those stars, so that it looks like I used a bigger scope. After a lot of fiddling around with Star Mask and Morphological Transformation, however, I haven't found a way. It remains a dream.

With more integration time, I think I could show more of the faint outer portions of M33. I'd love to get in night after night on this object, and really punch out every part of this galaxy. M33 is full of resolved stars and HII regions like NGC 604. I often think of M31 and M33 as the closest thing we've got the Magellanic Clouds up here in the NoHem, and it would be nice to make the deepest, sharpest images of them that I can.

Naturally, many people have gotten some very nice, very deep images of M33. One of my favorites is this one by Stephane Guisard, because he shot it from the Atacama region of Chile - exactly the `wrong' place to get a good image of M33. Shows you how good places like Paranal are! And of course, there's a nice Hubble image of NGC 604, the most prominent star-forming region in M33. (In my image, the way I've got it oriented, NGC 604 is down and to the right of the galaxy's center, above two prominent, bloated orange field stars.)

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!