M12
NGC 6218
46’ x 30’ | 0.3”/px | 9345 × 6157 px
Ophiuchus
RA 16h 47m Dec -01° 56’ | 0°
Messier 12, also known as NGC 6218, is a globular cluster situated in the constellation Ophiuchus. Discovered by the French astronomer Charles Messier on May 30, 1764, it is one of the many globular clusters that orbit the core of the Milky Way galaxy. Located approximately 15,700 light-years away from Earth, M12 spans about 75 light-years in diameter. This cluster is composed of hundreds of thousands of stars, which are densely packed, especially in its core. The stars in M12 are generally older and exhibit lower metallicity compared to those found in the galactic disk, indicating that they formed early in the history of the universe. With an estimated age of around 12 billion years, M12 is among the older objects in our galaxy, providing valuable insights into the early universe and the processes involved in the formation of stars and galaxies.
source: Mistral
Data Acquisition
Data was collected over 7 nights during full moon periods in April and May 2025, using a 14” reflector telescope with full-frame camera at the remote observatory in Spain. Data was gathered using standrad LRGB filters. A total of about 10 hours of data was combined to create the final image.
Location Remote hosting facility IC Astronomy in Oria, Spain (37°N 2°W)
| Sessions | Moon% | Moon° | Hum% | SQM | T°C | Frames | Exposure |
|---|---|---|---|---|---|---|---|
| 20250408 | 88 | 97 | 55 | 18.0 | 10 | 46 | 2h 18m |
| 20250509 | 94 | 56 | 85 | 19.2 | 10 | 9 | 0h 27m |
| 20250510 | 98 | 46 | 70 | 19.1 | 9 | 4 | 0h 12m |
| 20250511 | 100 | 37 | 55 | 19.0 | 10 | 37 | 1h 51m |
| 20250512 | 100 | 30 | 55 | 18.8 | 10 | 50 | 2h 30m |
| 20250514 | 94 | 26 | 80 | 19.5 | 10 | 34 | 1h 42m |
| 20250515 | 89 | 32 | 70 | 20.0 | 10 | 22 | 1h 06m |
| Total | 202 | 10h 06m |
| Frames | Bin | Gain | Exp.(s) | Frames | Exposure |
|---|---|---|---|---|---|
| Red | 1 | 0 | 180 | 36 | 1h 48m |
| Blue | 1 | 0 | 180 | 29 | 1h 27m |
| Green | 1 | 0 | 180 | 31 | 1h 33m |
| Lum | 1 | 0 | 180 | 106 | 5h 18m |
| Total | 202 | 10h 06m |
Equipment
Telescope
Mount
Camera
Filters
Guiding
Accessoires
Software
Planewave CDK14 (2563mm @ f/7.2), Optec Gemini Rotating focuser
10Micron GM2000HPS, custom pier
Moravian C3-61000 Pro (full frame), cooled to -10 ºC
Chroma 2” LRGB unmounted, Moravian filterwheel L, 7-position
Unguided
Compulab Tensor I-22, Dragonfly, Pegasus Ultimate Powerbox v2
Voyager Advanced, Viking, Mountwizzard4, Astroplanner, PixInsight 1.9.3
Processing
All processing was done in Pixsinsight unless stated otherwise. Default features were enhanced using scripts and tools from RC-Astro, SetiAstro, GraXpert, CosmicPhotons and others. Images were calibrated using 50 Darks, 50 Flats, and 50 Flat-Darks, registered and integrated using WeightedBatchPreProcessing (WBPP). The processing workflow diagram below outlines the steps taken to create the final image.
Especially the luminance channel showed many ‘dust-mote left-overs’ even after flat correction. I see that more often especially with images taken at or near full moon conditions. It is as if various dust particles reflect the the bright and angled reflections of the moonlight a bit different than the even light of a flat panel. In the past this was often corrected using the clonestamp tool, but recently I was introduced to the technique of Synthetic Flats.
Synthetic Flats
The first step is to make a clone of the image that needs correcting. Then all signal needs to be extracted from that image, so that the (imperfect) background is left. There are different ways to do that, and almost always require several steps. For this image a generic method was applied that should work in many cases. The first step is to remove all the stars, using StarXTerminator. No need to save the stars, it is the background we’re interested in. SXT will only extract true stars, and often leaves small structures in the image. These can be removed using the tool MultiscaleMedianTransform. As a starting point, choose 6 layers, and disable all layers, except the Residual Layer. Apply the tool and most defined structures will be gone now. What is left is nebulosity, or in this case background glow from the cluster. Here it was not too much and it was all located in the center of the image. The CloneStamp tool was sufficient to eliminate that background glow from the image. The final output from this process is called a Synthetic Flat and is essentially just the imperfect background of the main image.
The final step is now to apply a flat-field correction to the main image. This can be done using PixelMath. The equation used for that is:
$T * mean(SynFlat)/SynFlatThe technique is not always successful. Sometimes it can be difficult to eliminate signal from the background. Sometimes even the synthetic flat does not correct fully for irregularities in the background. But for this image it worked very well and created a super smooth background. See the GIF image below showing the image before and after flat field correction using a synthetic flat.
Creating a clean background that can act as a Synthetic Flat, often several steps are involved. In this case the first step was the removal of stars using SXT (left image). The remaining non-star signal was removed using MultiscaleMedianTransform (center image). As a final step Clonestamp was used to remove more nebulous signal (right image).
Applying a synthetic flat correction to the original Luminance image greatly improved the quality of the image.
When stretching a stars-only object, such as a cluster, I always find it a bit challenging to maintain good star colour. Of course you can run a standard stretch and then crank up the saturation, but that often gives somewhat unnatural results. To maintain colours in the stars I often use processes such as ArcSinhStretch, or GHS in colour-mode. In the new PixelMathGUI script, Mike Cranfield has added a scriplet called SmartStretch. That allows an automatic stretch targeting a certain background value, but it also includes a saturation protection slider. Putting that at 1 for the RGB image maintains the colours in a natural way, a bit like GHS works in colour mode. So by running SmartStretch with the same settings on both Lum and RGB, while setting the colour protection in the latter to 1, provides a very easy way of stretching, also for stars only images like these. Like other automatic stretching methods, both Lum and RGB come out with similar brightness values, so can be easily combined later.
The rest of the processing followed a standard processing workflow.
Processing workflow (click to enlarge)
This image has been published on Astrobin.
