M39
M 39- Click here for full resolution
Messier 39 is an open cluster of young stars in the constellation of Cygnus, sometimes referred to as the Pyramid Cluster. The cluster was discovered by Guillaume Le Gentil in 1749, then Charles Messier added it to his catalogue in 1764. It has a total integrated magnitude of 4.6 and spans an angular diameter of 29 arcminutes. It is centered about 1,010 light-years (311 parsecs) away. Of the 15 brightest components, six form binary star systems. HD 205117 is a probable eclipsing binary system with a period of 113.2 days that varies by 0.051 in visual magnitude. Both members seem to be sub-giants. Within are at least five chemically peculiar stars and ten suspected short-period variable stars.
source: wikipedia
NGC/IC:
Other Names:
Object:
Constellation:
R.A.:
Dec:
Transit date:
Transit Alt:
NGC7092
Pyramid Cluster
Cluster
Cygnus
21h 31m 42s
+48° 25.0’
15 September
79º N
Conditions
M39 is best visible during the summer months from June through September. At the observatory it then reaches altitudes of almost 80°. Images were taken on 7 different nights during September 2024, from the remote observatory at IC Astronomy in Oria, Spain. Sessions were planned around the full moon phase.
Equipment
The default rig at the observatory was used. The core of this rig is 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
Imaging under full moon is always a challenge. My typical approach was to revert to narrow-band targets during this time. But even narrow-band targets may sometimes be so close to the moon that even their results suffer. In a recent Astrobin-discussion someone mentioned imaging star clusters during full moon. And that makes a lot of sense. A star cluster, especially an open cluster, has little detail to it other than the bright stars that make up the cluster. And bright stars are not affected a lot by full moon. The Messier catalogue includes many open clusters that may not all be the most thrilling objects. Imaging them under full moon appears the way to go. So going forward I have a couple of open cluster targets setup in the target manager that can be imaged when no other objects are photographed. Here the first result of that approach, with an LRGB image of M39. Overall just over 10h of data was collected.
Resolution
Focal length
Pixel size
Resolution
Field of View (original)
Image center
9378 × 6276 px (58.9MP)
2585 mm @ f/7.3
3.8 µm
0.30 arcsec/px
47' x 31'
RA: 21h 31m 49.322s
Dec: +48° 28’ 35.53”
When doing the annotation of the image, something strange appeared. M39 as object seemed to have only been half captured! Even though all the bright blue stars that are typically considered to be part of M39 all seemed to be captured. It appears that not all catalogues are in agreement about the exact location of the centre of M39. The Messier and NGC catalogues in Astroplanner give coordinates of RA: 21h 31m 42s and Dec: 48° 25.0’. Wikipedia is very close to that as well. But the Simbad/Aladin database shows coordinates: RA: 21h 31m 33.4s and Dec +48° 14’ 49”. That is significantly more to the south than the centre of the cluster appears like. When the image was plate-solved on Astrobin, at first it showed NGC7092 to cover the cluster perfectly, but once the advanced plate solved had finished it had replaced NGC7092 by Messier 39 and moved it to the south, like the PixInsight annotation. By the way, in the annotation from PixInsight, which uses the ViZier database, M39 does not even include bright stars HD205116 and HD205198, which are definitely part of the cluster, as is demonstrated in a recent scientific paper. So all in all I remain confident that the characteristic nature of M39 has been captured. But there is confusion between data sources about the location of the center of the cluster.
Confusion over the center of the M39 cluster. According to the NGC and Messier databases used by AstroPlanner, the center of M39 is perfectly in the center of the visible cluster. The Simbad on the other hand shows the center of the cluster much more south and in the resulting annotation from PixInsight, bright stars like HD205198 and HD205116 fall oddly enough outside the apparent boundaries of the cluster.
Processing
All images were calibrated using Darks (50), Flats (50) and Flat-Darks (50), registered and integrated using the WeightedBatchPreProcessing (WBPP) script in PixInsight. During integration a total of 31 frames were rejected (weight <10%), probably due to bad imaging conditions (cloudiness). The rejections were more or less equally divided over the four filters.
Red, Green and Blue channels had their gradient removed with GradientCorrection. But after combining the R, G and B channels, the resulting image still had a remaining gradient on it. So an extra run of GraXpert was applied to remove that remaining gradient. Further processing consisted of colour calibration (SPCC), deconvolution (BXT), stretching (GHS), noise reduction (NXT) and a mild smoothening of the image with Convolution.
The luminance was gradient corrected with GradientCorrection, which worked well on this image. Following steps included deconvolution (BXT), stretching (GHS), noise reduction (NXT) and adding a little bit of contrast using CurvesTransformation. Stretching of the luminance was done very careful and not pushed in any way. Too bright stars will easily wash out colours later.
The luminance was then blended into the RGB image using LRGBCombination. As per the plan, the colours came through still pretty intense. Just a little bit of touching up the blue colours was done using CurvesTransformation. No further noise reduction was necessary anymore, as this was done already on the individual RGB and Lum images. Setting background levels to 0.07 to align with other images was the last step in the process.
Processing workflow (click to enlarge)
This image has been published on Astrobin.
