M4
M4 - Click here for full resolution
Messier 4 (also known as NGC 6121 or the Spider Globular Cluster) is a globular cluster in the constellation of Scorpius. It was discovered by Philippe Loys de Chéseaux in 1745 and catalogued by Charles Messier in 1764. It was the first globular cluster in which individual stars were resolved. M4 is a rather loosely concentrated cluster of class IX and measures 75 light-years across. The structure consists of 11th-magnitude stars and is approximately 2.5' long and was first noted by William Herschel in 1783. At least 43 variable stars have been observed within M4. It is approximately 6,000 light-years away, making it the closest globular cluster to the Solar System. It has an estimated age of 12.2 billion years.
source: wikipedia
NGC/IC:
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NGC6121
Spider Globular Cluster
Cluster
Scorpio
16h 23m 35s
-26° 31.5’
30 Jun
26º S
Conditions
M4 has a low declination of -26°, which makes the conditions to observe from Europe somewhat challenging. The highest altitudes are reached in June-July, but the cluster never reaches altitudes higher than 30° from the remote observatory at IC Astronomy in Oria, Spain. Images were taken on 5 different nights towards the end of June 2024. Dropout rates for images were high due to atmospheric turbulence and subsequent poor image quality.
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 automated the process to find optimal time-slots during astronomical night.
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” LRGB unmounted, Moravian filterwheel L, 7-position
Unguided
Compulab Tensor I-22, Windows 11, Dragonfly, Pegasus Ultimate Powerbox v2
Voyager Advanced, Viking, Mountwizzard4, Astroplanner, PixInsight 1.8.9-3
Imaging
One of the key objectives with star clusters is to resolve as many stars as possible, also in the center. Sometimes shorter exposures are preferred to achieve this goal. For M4, my default luminance exposure of 180s was good enough to resolve all the detail. Also some 180s shots for the red channel were tested, but the default of 300s for broadband colour filters worked well, so the 180s shots were discarded. Overall just over 5h of data was collected, which was more than enough to get proper colour and detail.
Resolution
Focal length
Pixel size
Resolution
Field of View (original)
Image center
8424 × 5514 px (46.4 MP)
2585 mm @ f/7.3
3.8 µm
0.30 arcsec/px
42' x 27'
RA: 16h 23m 34.988s
Dec: -26° 31’ 21.75”
Processing
All images were calibrated using Darks (50), Flats (25) and Flat-Darks (50), registered and integrated using the FastBatchPreProcessing (WBPP) script in PixInsight.
Red, Green and Blue channels had their gradient removed (GraXpert), combined, calibrated (SPCC), and deconvolved (BXT). Stretching was done by two runs of GeneralisedHyperbolicStretch (GHS). The first one was run in Colour mode instead of RGB mode, with an 80% RGBBlend. This preserves the colours a lot better than regular RGB stretching. Stretching of the RGB image was done very mildly, to keep brightness low and colour intensity high. Noise was removed with NXT and the final bit of smoothing of colour was achieved with a mild convolution.
GraXpert was also used to remove a small gradient in the luminance image. BlurXTerminator was used to sharpen the image. It was used with the same settings as the RGB image, to keep star shape/size the same between RGB and Lum. Stretching was done with HistogramTransformation (HT). Also here great care was put into not clipping the centres of stars too much. It resulted in a pretty dark image, where CurvesTransformation was used to boost the fainter stars in the background, to give a bright start image overall.
Luminance was added to the RGB image using LRGBCombination in default settings. Image was cropped a tiny bit to focus on the cluster itself. Background levels were adjusted to 0.07 using BackgroundNeutralization, to align with other images.
Green stars
When the cluster was processed and I looked around the image to look for any interesting details, it appeared that there were a couple of distinctly green stars in the cluster. The general consensus appears to be that green is not a colour that would typically be seen in the night sky, the first assumption was that something had gone wrong. But after various attempts to color calibrate using different methods, examine individual color stacks, closely examining the stretching and blending process, and even comparing absolute brightness levels at star centers, nothing strange had come up. I posted the result in this thread here on Astrobin, and the most likely explanation is that this is not an artefact. Instead, these stars are variable stars.
Two distinctly green stars near the center of the cluster, which turn out to be RR Lyrae variable stars.
The magnitude variation of RR Lyrae, the prototype of RR Lyrae variable stars
One participant in the thread had found that these stars were NGC 6121 SAW V16 and NGC 6121 SAW V9 respectively. These are RR Lyrae variable stars, named after RR Lyrae, the brightest in its class. RR Lyrae variable stars are mostly found in globular clusters, and are used as standard candles to measure exact distances. Their period is in the order of 0.5-2 days. So if you take images on subsequent nights with different filters, each filter might catch the star in a different stage of its period of brightness fluctuation. So a green star means that the star does not actually have a green colour, but that the star was at its brightest when the green data was collected.
An interesting phenomenon that I had not heard of before. One of the participants in the discussion had demonstrated this phenomenon in M3, another globular cluster with lots of RR Lyrae variable stars. He collected just Luminance data in 2h blocks, and then assigned each of these blocks to another colour channel. The RR Lyrae variable stars showed indeed up as individual red, green and blue stars in a field of otherwise white stars.
Colour Balance
As the green stars showed up, I was convinced that something had gone wrong with the colour calibration. So I spent quite a bit of time comparing different methods. And there was one thing that I learned from that, which might be worthwhile sharing here. With Photometric Color Calibration (PCC) and SpectroPhotometric Color Calibration (SPCC) available in PixInsight, I typically gravitate towards SPCC as being the more accurate one. But an argument that can be heard sometimes is that its complexity may not be worth the extra gain in accuracy. It does indeed involve some extra settings, which may come across a bit more complicated. It requires a plate solved image, selection of a representative reference galaxy, choice of sensor and filters, etc. On the other hand, once you’ve put in your typical imaging conditions and saved the process as an icon on the desktop, it is rather simple to apply.
For the current image, I looked at SPCC and compared it to the standard color calibration. Standard color calibration assumes that the whole image is on average white. This may be true for a small galaxy in a field full of random stars. but for a globular cluster this may be a wrong assumption. Clusters are known to contain a lot more old and red stars, so the ‘real’ colour of a cluster should be fairly reddish. Instead, standard color calibration results in a pretty bright cluster with lots of blue-ish stars instead. And when looking around on Astrobin, many clusters are pretty blue-ish. The question though is if this is the right colour.
Three methods of colour calibration compared. On the left the standard ColorCalibration, with the target image itself as the white reference. In the middle SPCC using Average Spiral Galaxy as white reference and on the right SPCC with S0 Galaxy as white reference. The final image was processed using the latter method.
Above is the comparison between standard color calibration (left) and SPCC (right) using Average Spiral Galaxy as the reference. And indeed, the standard color calibration comes out with much cooler colours. The warmer tint of the SPCC calibrated image is likely to be more accurate. One of the choices to make with SPCC is what to use as reference, and changing this option can have a big effect on the white balance. I have looked at a lot of examples, and found that taking ‘S0 galaxy’ as a reference give a slightly cooler coloured image than ‘Average Standard Galaxy’ (right image). As for personal taste, I prefer that look a bit better, so I often choose S0 Galaxy as the reference point. But be your own judge and feel free to experiment with this setting.
Processing workflow (click to enlarge)
This image has been published on Astrobin.
