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!

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