Messier 13 (LRGB)

It’s been almost a month since we had a night that promised to be clear all night through. The last week or so we’ve had weather that taunted us with clear days only to cloud up at sunset. Finally, we got a clear night but of course it came on a Monday making me a bit sleep deprived today. The forecast was actually calling for today to also be clear but that was a bait and switch. By the afternoon, the forecast had turned to clouds and haze for the night. Oh well, I probably need a good night’s sleep tonight anyway.

Messier 13 is a globular cluster. It’s also the first deep sky object I saw in a telescope way back in the mid-90’s. To my eye, star clusters of any type are about the only thing that look better visually through a telescope than they generally do in images. An image can’t capture the dynamic range that the eye can see and this glittering jewel of pinpoint stars in the eyepiece of a telescope can easily turn into mushy blobs in an image.

While it’s somewhat true that if you have seen one globular you have seen them all, this is one of the best ones in the northern hemisphere. It’s also a sign that summer is on its way and while that makes observing nights shorter it also brings warmer temperatures. But, even with all globular’s looking a lot alike there are differences in density and compactness. Each one is subtly different.

A globular cluster is defined as a spherical collection of stars that are bound together by gravity. They hang out in the halos of galaxies and are among the oldest stars in the galaxy. So far as I know their origins are not well understood. In effect globulars are the closest thing that we can see that aren’t in the spiral arms or main body of our galaxy.

M13 is estimated to be more than 11 billion years old and is just over 22 thousand light years away. It’s about 145 light years in diameter and contains several hundred thousand stars. These stars are much closer together than the stars in our galaxy. Imagine what the night sky of a planet orbiting one of those stars must be like!

This was a fairly short integration of approximately 2.5 hours. Since the stars are fairly high signal in comparison to things like nebulae or more distant galaxies there wasn’t much need to capture several hours of sub exposures. In my case I ended up with 176 luminance, 29 red, 26 green and 27 blue. The luminance exposures were 20 seconds each and the RBG exposures were 60 seconds.

Processing was fairly straightforward. The individual LRBG channels were integrated using local normalization and flat darks were used instead of bias frames.

For the luminance:

  • Mure Denoise
  • Dynamic Crop
  • DBE
  • Histogram Transformation

For each R, B and G master:

  • Mure Denoise

For the RBG:

  • Channel Combination to put R, B and G together
  • Dynamic Crop
  • DBE
  • Photometric Color Calibration
  • Multiscale Linear Transform for noise reduction on luminance and chrominance
  • Histogram Transformation

For the LRBG result:

  • LRBG Combination of L to RBG
  • HDR Multiscale Transform to bring out some detail in the core
  • Local Histogram Equilization
  • Multiscale Linear Transform for mild sharpening
  • Curves Transformation for color saturation
  • Curves Transformation for mild contrast boost

Here’s the link to the image on astrobin:

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