M3 Globular Cluster

Click image for full size version

March 1, 2017; AstronomyNow! Magazine, April 2020

Whenever I can’t find something else conveniently placed for imaging in the Spring sky, I can usually find a bright globular cluster or two. M3 (Messier 3) is one of the brighter globular clusters and is not far from M13 in the sky.

Unlike some other objects in the Messier catalogue, this object was actually discovered by Messier, in 1764. It is one of the largest globulars, containing about half a million stars, and it lies about 34,000 light years away and measures about 180 light years across. Are you feeling old today? Consider that the estimated age of this cluster is more than 8 billion years! Then go celebrate your relative youth, no matter how old you are! If you have binoculars, you can see this object easily in a moderately dark sky as a hazy patch. Larger and larger scopes reveal more detail towards the core, and fainter stars around the outside.

If you look closely, you’ll find many galaxies in this field. I’ve posted an annotated image that will help you find some of them. Interestingly, not all of the galaxies are included in the major catalogues.

A previous version of this image, from May 2012, was Astronomy Magazine Online Picture of the Day February 7, 2015, but I think this 2017 update shows much more detail.

Tekkies:
SBIG STL-11000M camera, Baader LRGB filters, 10″ f/6.8 ASA astrograph, MI-250 mount. Guided with STL-11000’s external guider and a 500mm f.l. Lumicon guide scope. Acquistion, guiding and calibrationwith Maxim-DL. Registration, integration and all processing in PixInsight. Shot from my SkyShed in Guelph, Ontario. No moon, above average transparency and seeing.

12x5mL, 4x10mL and 10x1mL (total of 110mL), and 6x5m for each of R,G and B (total of 90m RGB), for a total of 200m LRGB.

RGB
Creation and cleanup: R, G and B masters were cropped and processed separately with DBE. R, G and B were combined to make an RGB image which was processed with ColorCalibration.

Linear Noise Reduction: MultiscaleLinearTransform was used to reduce noise in the background areas. A Linear mask and the following Layer settings for threshold and strength were used: Layer 1: 3.0, 0.75 Layer 2: 2.5, 0.62 Layer 3: 2.0, 0.5 Layer 4: 0.5, 0.2, Layer 5: 0.5, 0.1.

Stretching: HistogramTransformation was applied to make a pleasing yet bright image. During stretching, it was noted that the colours were not properly balanced for the background, so this was adjusted manually using sliders for the individual colour channels, as well as the RGB/K slider. Colour as adjusted using PixelMath to blend the a* and b* channels of the image with the corresponding channels of the 2014 version.

Synthetic Luminance:

Creation of HDR Luminance: Three master L files of different duration subs were created and processed separately with DBE. HDRCombine was used to combine them.

Creation and cleanup of SynthL: The cleaned up HDR-L, R, G and B masters were combined using the ImageIntegration tool (average, additive with scaling, noise evaluation, iterative K-sigma / biweight midvariance, no pixel rejection).

Stretching: HistogramTransformation was applied to make a pleasing yet bright image.

Nonlinear Noise Reduction: TGVDenoise was applied with a mask to protect stars. The image was re-stretched to reset the black point.

Dynamic Range Compression: HDRMultiscaleTransform was applied to a copy of the image using Median mode and a scale of 7, with a mask used to limit the application just to the core of the globular. The resulting image was blended with the original in a ratio of 3:7 using PixelMath with the same mask.

Combining SynthL with RGB:
The luminance channel of the RGB image was applied with LRGBCombine to make a SynthLRGB image. Colour noise reduction was used at default settings.

Final Processing of SynthLRGB:
Background and star brightness, contrast and saturation were adjusted in several iterations using Curves with masks as required.

Image scale is about 1.1 arc sec per pixel for this telescope/camera combination.