Wednesday, June 27, 2012

Globular Star Cluster M3

Harbinger of Summer - that's how I always think of the globular star cluster M3.

A little less than a hundred years ago, Harlow Shapley measured the distances to the globular clusters, and realized they form a spherical halo around a point that lies in the direction of the constellation Sagittarius. That was the beginning of the realization that our solar system is not at the center of the Milky Way galaxy. Globular clusters like M3 are classic summer objects; I've lost count of the number of times I've passed the short summer nights looking at them, through any number of different telescopes. Constellations like Sagittarius itself are rich hunting grounds for `globs' large and small, bright and dim. A trip to the southern hemisphere has, as one of its many treats, views of the huge, blazing Omega Centauri and 47 Tucanae globulars. Simply put, globular clusters are classic summer `eye candy'. Here's an image of M3 that I shot during the  June 2012 dark-moon cycle:

Globular star cluster M3
8-1/3 hours total exposure time
Evenly split between unbinned R, G, and B, shot in 4-minute subexposures.
Click the image for larger version, or click here for full size.

M3 will always have a special place in my astro-heart, since it was the first object I ever saw through a large amateur telescope. It occurred just over 10 years ago, in April of 2002. I went to one of my first Bay Area observing events, at a local hilltop site. I had my little 5" Meade ETX-125, and I was ready and excited to see some deep-sky objects! To my amazement, Bruce Jensen set up an 18" Starmaster dobsonian next to me. I'd never even looked at a telescope that big, at such close range, let alone looked through one. Bruce showed me M3, which was still rising in the east, and I was blown away. There was no going back - aperture fever took hold of me for good! (I'm lucky enough to be able to enjoy my own views through an 18" scope these days, something for which I'm very grateful, even if I'm mostly using my imaging rig these days.)

M3 is one of the farther-west of the bright globulars, so we see it in the (northern-hemisphere) spring, before the other globs are well-placed for viewing in the summer sky. I'll always associate M3 with April, May, and June, when we're enjoying the galaxies of Coma Berenices and Virgo, taking peeks at globs like M3 and M5, and dreaming of the summer Milky Way...

Acquisition and Processing

I shot the data for this image on three nights during the June 2012 dark-moon period, from the same site where Bruce Jensen showed me M3 through his Starmaster all those years ago. I decided to shoot unbinned R, G, and B images, to try and maximize the resolution of the image, and to avoid having to match the histogram of a luminance image to that of an RGB image. In the end, over the three sessions, I got about 36 four-minute subexposures through each filter. As with my other recent images, I used my Orion ED80 f/7.5 refractor on a Losmandy G-11 mount, with a short-tube 80 refractor and StarShoot camera for autoguiding. My trusty SBIG ST-8300 monochrome CCD camera gathered the photons, with a chip cooled to -15C.

Pixinsight processing followed my usual workflow, with deconvolution (i.e. sharpening) of the innermost core of the cluster, as well as smoothing of the background, done while the image was still linear. A wee touch of HDR Multiscale Transform helped to `un-blow-out' the cluster's core. I pumped up the color saturation in the brightest part of the cluster, so as to bring out the differences between the blue and orange stars.

Pixinsight geekery: The main thing I learned while processing this image was the usefulness of the `Amount' slider in Multiscale Medium Transform's noise-reduction routine. As with many of PI's tools, MMT is powerful yet somewhat hard to understand. I don't really know how to set the parameters for its noise-reduction routines, and I've always wanted to be able to increase the amount of noise reduction ever-so-slowly. Well, I should have guessed that the `Amount' sliders in the noise-reduction settings for each wavelet layer will do exactly that. I guessed at some Threshold values, starting with 4 for the first (1-pixel-scale) layer and decreasing roughly by half as I went from layer to layer. Then, having set those Threshold values, I set all of the Amount sliders to 0.1, and ran MMT. There was just the tiniest little bit of noise reduction in the background sky. (I used a luminance mask to protect the globular's stars.) By moving up the Amount sliders
one little increment at a time, I could get what I wanted: A nice, moderate amount of noise reduction.

Room for Improvement

I could have set the black point a little lower, to suppress the remaining background noise a little better. I could also have tried to dim/shrink the bright, burned-out-looking foreground stars. They're a little distracting. But, since the deep-sky object in question is a star cluster, I couldn't find a good way to make a star mask that didn't include stars from the cluster. So, I just left the stars alone and decided to post what I had. I think the thing I like the best about this image is the halo of very faint stars that makes up the outermost part of the cluster. I doubt I can see those visually, even through a large telescope. That's one of the joys of imaging, going deeper than the eye can see!

Wednesday, June 13, 2012

A shout-out to the film folks

Film! I have a soft spot in my heart for film astrophotography, even though I use a CCD camera. Last night, while surfing the web, I checked to see if Jim Cormier, a modern-day film astrophotographer, had posted any new film images. He has, and they're really cool! I want to post some links to his images, so that more people will get a chance to see them.

I've done some film astrophotography - more on this in a bit - but I'm not one of the old-school `film guys' from back in the day. For well over a century, emulsion-based photography was photography, before sophisticated electronic sensors were developed. The art and science of emulsion-based astrophotography produced some beautiful results, through the heroic efforts of many, many research astronomers and amateur enthusiasts. These results depended on things like long single exposures, manual guiding, cold cameras, gas hypersensitization, and the envelope-pushing techniques that David Malin developed at the Anglo-Australian Observatory. Other than a few star-trail images, and a couple of short guided images of Halley's Comet in 1986, I didn't shoot film back in the day. (I was just a kid/teenager at the time, too.) But plenty of people did, and they left a rich, heroic legacy of astro-imaging on emulsion.

The advent of CCDs meant the `death of film', for the most part, since CCDs are so much more sensitive, and have a (generally linear) response to light that makes them more useful for measuring the brightnesses of things. The recent demise of Kodak is perhaps the best-publicized event in the long twilight of emulsion. However, not all amateur imagers have given up on film! There are a few folks out there who really enjoy shooting film, and enjoy the results they get. Naturally, there's some involvement with the digital realm, since we see their images on the web, after all. But at heart, their `sensors' are emulsion-coated materials, and I just think that's cool. They love film, and I admire them for it. I think that the world of film and processing will always have a special place in my heart, probably because I enjoyed darkroom work when I was a high-school student. I worked in the yearbook darkroom, and I set up a small B&W darkroom in my folks' house during high school. (I even developed a roll or two of slide film during graduate school, which was a hoot.)

If there's a `hero of film' in 2012, it's probably Jim Cormier from Maine. He mostly shoots wide-field images, and largely on Ektachrome 200, which seems to have been the `color astro film of choice' during the latter years of film's heyday. At present, his images can be found in several places on the web. Here are some recommended links:

For an image with a great `wow' factor, check out his latest 4-panel E200 Milky Way panorama.

Jim's Blogspot site also shows his images, and he's got a nice post about `My Most Productive Dark-Run Ever'. I love it! (Also note the `hand-corrected guiding'... John Henry, indeed!)

He has a photostream on Flickr, which is worth exploring. Another highlight from his Flickr stream is his 2011 B&W project to shoot parts of the Milky Way, a la Edward Barnard's atlas. Very cool.

While you're at it, you might enjoy Christopher Barry's Kickstarter proposal, to shoot wide-field film images this summer. It looks like he made his funding goal! I eagerly await his results.

I can't quite describe why I get such a kick out of the work of these `film guys', but I just do. I'm really glad that they're sharing their work.

While I'm on the topic of film, I suppose I ought to post a film image of my own. There's a bit more backstory to this film enthusiasm of mine, as it turns out. I could probably write a long series of blog posts about this, but here's a short version: In the late summer and fall of 2011, I did a film-imaging project. I was finishing my MSc in astronomy, and my final project involved a comparison of film-based and CCD-based imaging techniques. The film side of the story got pretty epic, but keep things short, here's an image of M31 that I shot on Ektachrome 200, using a Nikon FM camera body attached to my ED80 refractor. This is about 150 minutes of total exposure time (I forget the lengths of the subexposures), stacked and processed in Pixinsight:

M31, captured on Ektachrome 200 from a Bay Area hilltop site.
Click on the image for a larger version, or click here for full size.

You've probably noticed the curious flares coming off of the brighter stars. Those are actually due to the film scanner I used. (I've examined the slides under a microscope, and the flares aren't present in the slides.) One of these days I'd like to re-scan my slides and see if I can get a better result. Another issue that came up: The red LEDs from my light meter caused the slides to be badly light-struck. Next time I try shooting with my FM, I'm going to take out the light-meter batteries. Pixinsight's Dynamic Background Extraction routine was able to clean up most of this red mess, but it would have been nice if I hadn't had to deal with it.

Ektachrome 200 is basically gone now, but I was able to buy some on EBay, and a fellow astro-imager gave me several rolls. My leftover E200 is in my fridge, and one of these years I ought to shoot it. Some year, I should devote a fall and a winter to shooting the heck out of M31 and M42 on film. If I can find a 16-bit (or deeper) film scanner that doesn't produce those flares, I'd love to create the best `film-captured' M31 and M42 I can, with help from Pixinsight. Send that good `ol E200 out in one last blaze of glory!

Sunday, June 10, 2012

The M87 Chain and the Pixinsight Zone System

One of the greatest euphemisms in the world has to be the phrase `learning experience'. How often do we sugar-coat our mistakes by calling them `learning experiences'? I'm sure I've done it many times. This image provides an example, but in this case there's a bit more to it than that...

A portion of the Virgo galaxy cluster, with the giant elliptical galaxy M87 at top left, and part of `Markarian's Chain' of galaxies at right. Click the image for a larger version, or click here for full size.
Data Acquisition: Making the best of a bad situation

A few weeks ago, I was doing some backyard imaging, and the Virgo galaxy cluster seemed like the logical choice. Having shot a luminance image of the Leo Triplet not long before, I decided to do another one-night stand, with just luminance, but this time I wanted to shoot `Downtown Virgo'. (The origins of that term and its enthusiastic usage seem to go back to Jay Freeman and Jamie Dillon, two highly-accomplished Bay Area visual observers.) Specifically, I wanted to shoot the portion of the Virgo cluster called Markarian's Chain. It's a standard target, since it comprises a pretty, arc-ing chain of galaxies that stretch from M84 and M86 towards M88. Almost everyone works on an image of Markarian's Chain at some point. By planning it out in SkySafari Pro 3 on my iPad, I could see that if I rotated my camera just right, I could frame most of the chain pretty nicely on my ST-8300 sensor, using my ED80 f/7.5 refractor.

One thought nagged at me, though... What about conventions? As in sign conventions and angle conventions? Sky Safari Pro 3 has a really nice slider tool for rotating the position angle of one's field-of-view overlay, relative to the sky. This allowed me to plan my framing really easily. And when I'm imaging, I can download a frame from the camera, and use MaximDL to plate-solve it, which gives me the image's position angle on the sky. This is really handy, but.... what if these two pieces of software use different conventions for specifying the position angle? Hmm. I could wind up with a frame that's rotated 90 degrees from what I expect.

So, it wasn't a great shock when that's exactly what happened. Here's the framing I had planned on my iPad:

Here's how things actually worked out, since the two pieces of software treated the position angle differently:

Hrm. Rargh. What to do? I could have rotated my camera 90 degrees, but that would mean refocusing and probably re-doing the GOTO alignment. Given the couple of hours available for shooting Downtown Virgo before it went behind some trees, I didn't want to do that. So, I panned around in SSP 3 and looked for an alternative framing. Here's what I wound up with:

That seemed like the best compromise, since it caught part of Markarian's Chain, and included the giant elliptical galaxy M87, the real `heart' of the Virgo cluster. I shot a couple of hours of luminance (in 5-minute subexposures), and called it a night.

Processing: Pixinsight meets the Astro Zone System

A few weeks later, I had a little time to sit down with the data, and after using the very handy new preprocessing script in Pixinsight, I saw the following preliminary result (this is a closeup of two of the galaxies in the Chain):

Autostretched image of two galaxies in Markarian's Chain.

It's probably worth explaining what I mean by an `autostretched' image (also sometimes called an AutoSTF'ed image amongst Pixinsight enthusiasts). PI has a tool called `Screen Transfer Function' (STF), which stretches the brightness values of the image's pixels, solely for the purpose of displaying the image on the screen. It doesn't change the original pixel values in the image file, but it basically creates a temporary copy of the image to display on the screen, with the brightnesses changed so as to make the dim parts of the image more visible. The STF tool has a `Auto' button, which creates an image that nicely shows `what you got'. (I used one of these AutoSTF'ed images in my annotated Leo Triplet posting.) Such an image, though, usually doesn't make for a very pretty picture, since it shows just how noisy the dim background areas and dim parts of your target look. That graininess is a combination of instrumental noise and the eponymous photon shot noise (the latter coming from both the target objects and from the sky.)

At this point, my big goal was to do some noise reduction, and try to make the noisy, grainy-looking parts of the image look a little better. In this I was aided by Jordi Gallego's new presentation on noise reduction in PI. There's a lot of good information in this document, but I was particularly intrigued Jordi's slides 51 through 53, particularly #53. In this slide, he shows that one can make masks for applying different noise reduction settings to different parts of the image, such as:

  • The dark background sky, which has the lowest signal-to-noise ratio (SNR), and is thus the `grainiest'-looking part of the image.
  • The dim parts of the deep-sky object(s), which have fairly low SNRs, and thus mostly need smoothing and noise reduction.
  • The bright parts of the deep-sky object(s), which have high SNRs, and thus can tolerate some sharpening, such as through deconvolution.

Aha! This is basically the same concept as Ron Wodaski's Astro Zone System. I borrowed a copy of this book from a fellow Bay Area observer a couple of years ago, and found it to be very interesting. Sadly, the book has been out of print for some time, but I was one of the lucky folks at the 2011 Advanced Imaging Conference who managed to get one of the copies Ron gave away. (Thanks, Ron!)

After a little fiddling around, I realized that PI's Range Selection tool works best on images that have already been stretched into a nonlinear state, so I made a copy of the image, applied its AutoSTF settings to Histogram Transformation, and applied that to the copy. I then used Range Selection on this stretched copy.

First, I made a mask that covered up the stars and galaxies, leaving only the dark background sky to work on:

After a little fiddling around, I stumbled on some settings in Multiscale Median Transform that smoothed the background reasonably well:

I was pleased with this result! It's not perfectly smooth, but I'm calling this a win, so far. Then, I made a mask for the `mid-SNR' zone, which included the fainter outer parts of the galaxies:

And then, by pulling back on my MMT noise reduction settings, I was able to smooth those areas somewhat. Next I made a mask to isolate the cores of the galaxies, for sharpening via Deconvolution:

After mid-SNR-range smoothing and high-SNR-range deconvolution, I had this image:

The brightness levels you see here are `Auto-STF' levels, and even with the noise reduction, they're not really good for posting on the web. So, since the image was still at a linear stage (i.e. not really brightness-stretched yet), it was time for a Histogram Transformation, some star shrinking, and a horizontal flip to match the correct appearance of this area on the sky:

Room for Improvement:

I think this was a good proof-of-concept project, for the Range Selection / `Pixinsight Zone System' approach. My masks could use some work, though. When I examine the image closely, I can see that some of the dim parts of the galaxies got left out of the masking process. Also, the various processing steps left an artificial ring around M87. There really are such things as ring galaxies, but M87 isn't one of them. I'm very interested in refining my touch with Range Selection, and to trying out the new Adaptive Stretch tool! A week or so after shooting these data, I managed to shoot Markarian's Chain with proper framing, and so we'll see how things go with this new data set.

Monday, June 4, 2012

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.

Friday, June 1, 2012

The Silver Coin galaxy

Just a quick image posting today... I don't have as much time as I'd like to write about this object.

I was going through some files the other day, and I realized I had this image of NGC 253, the `Silver Coin' galaxy, sitting on my hard drive. Might as well add it to Photon Shot Noise!

At the moment, I don't have a lot of information about the details of how I acquired it and processed it. I definitely used my Orion ED80 refractor, and I was using a camera with the Kodak 8300 monochrome chip, shooting through a Luminance filter. I think the subexposures were 15 minutes long. I was using my Orion Sirius mount (which has since been retired for solar-observing duty at school), and I was pleasantly surprised that my polar alignment and autoguiding were good enough for 15-minute subs. I seem to recall acquiring the data on one or more cold nights at a Bay Area hilltop observing site, sometime in the last year or two.

As with the acquisition, the details of processing are a bit hazy at the moment. I'm fairly sure I processed this in Pixinsight, and I recall being pleased at how much detail I was able to bring out. This is due to the reasonably good SNR, which came from taking lots of relatively long subexposures over one or two nights. I either used Deconvolution for sharpening, or perhaps an ATWT-based sharpening and noise-reduction workflow.

If I recall correctly, one of the nice things about NGC 253 is its relatively sharp `edge'. It doesn't have much of an extended, low-surface-brightness halo around it, at least not in my subexposures. As a result, I wasn't tormented by the desire to bring out lots of surrounding faint stuff. Such faint stuff around a galaxy is often hard to make look decent, since it requires either a mountain of exposure time, or a miraculous touch with the noise-reduction routines. I seem to recall that NGC 253 pretty much ends where you see it ending here, and so I didn't have any significant `halo struggles'.