M9
Messier 9 - Globular Cluster - Click here for full resolution
Messier 9 is a globular cluster in the constellation of Ophiuchus. It is positioned in the southern part of the constellation to the southwest of Eta Ophiuchi, and lies atop a dark cloud of dust designated Barnard 64. The cluster was discovered by French astronomer Charles Messier on June 3, 1764, who described it as a "nebula without stars". In 1783, English astronomer William Herschel was able to use his reflector to resolve individual stars within the cluster. M9 is one of the nearer globular clusters to the center of the galaxy as is around 5,500 light-years from the Galactic Center. Its distance from Earth is 25,800 light-years.
NGC/IC:
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Object:
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Dec:
Transit date:
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NGC6333
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Globular Cluster
Ophiuchus
17h 19m 11.78s
–18° 30′ 58.5″
14 July
34º S
Conditions
With a declination of -18°, Messier 9 is always a bit of a challenging object to image from the Northern Hemisphere. From the observatory in Spain the highest altitude is 34°. The target was imaged across several sessions, where it made use of the last few hours of the night, just before dawn.
Equipment
The default rig at the observatory was used. This is built around a Planewave CDK-14 telescope on a 10Micron GM2000 mount, coupled to a Moravian C3-61000 Pro full-frame camera. The RoboTarget module in Voyager Advanced allowed automatic scheduling to find some time-slots just before dawn on five successive nights.
Telescope
Mount
Camera
Filters
Guiding
Accessoires
Software
Planewave CDK14, Optec Gemini Rotating focuser
10Micron GM2000HPS, custom pier
Moravian C3-61000 Pro, cooled to -10 ºC
Chroma 2” RGB unmounted, Moravian filterwheel “L”, 7-position
Unguided
Compulab Tensor I-22, Windows 11, Dragonfly, Pegasus Ultimate Powerbox v2
Voyager Advanced, Viking, Mountwizzard4, Astroplanner, PixInsight 1.8.9-2
Imaging
The object was always low on the horizon, and the default limit of 30° altitude had to be stretched to 24° to have at least a small imaging window at each session. Still the data was not great, with HFD values often in the 10-12 pixel range (3.5-4 arcsec). This means a bit of detail in the center is lost, but generally a globular cluster is quite forgiving when it comes to bad seeing.
Resolution
Focal length
Pixel size
Resolution
Field of View
Image center
9456 × 6310 px (59.7 MP)
2585 mm @ f/7.2
3.8 µm
0.30 arcsec/px
47' x 31’
RA: 17h 19m 11.881s
Dec: -18° 30’ 46.57”
Processing
All images were calibrated using Darks (50), Flats (25), registered and integrated using the WeightedBatchPreProcessing (WBPP) script in PixInsight. During the process quite a number of frames were rejected, mostly of the blue filter.
Processing followed a pretty standard pattern. Gradients were removed from Individual colour channels before putting them together and calibrating the colour with SPCC. The cluster has a bright center, and an attempt was made to keep as much structure in there as possible. Interestingly BXT did not result in great benefits, and could even create some artefacts if not careful. To keep maximum amount of detail, three different stretching methods were used, standard HistogramTransformation (HT), GHS and Statistical Stretch (SS), a relatively new script from Seth Astro. HT and SS resulted in fairly comparable results, but over-brightened the core a bit, which then had to be reduced again using HDRComposition. This gave some unnatural looking images. Best results were obtained with careful GHS stretching. The highlight protection feature and the colour blend mode were both very valuable for a good result.
Overall the tone of the image was very warm. So much so that it started to look very unnatural, even though SPCC was applied. I’m wondering if this had something to do with the choice of white-balance (average spiral galaxy). Perhaps this should have been something else. The way I solved the issue was to shift the white balance to the blues by bringing down the CIE *b slider in CurvesTransformation. In order not to create deviations in the background, this was done under a star mask which was essentially a luminance extract of the same image. To complete the operation, some selective increase of saturation of the blues was applied. The image looked now a lot more natural, and with some standard contrast increase and noise reduction the final result was almost there.
As with all recent images, the background was set to 0.7 using Background Neutralisation. And finally, the core cluster was brightened up just a tiny bit using CurvesTransformation while a gradient mask from the GAME script was applied. A final boost of overall saturation completed the processing.
Processing workflow (click to enlarge)
This image has been published on Astrobin.
