M71
M71 - Click here for full resolution
Messier 71 is a globular cluster located in the constellation Sagitta, approximately 13,000 light-years away from Earth. It was first cataloged by Jean-Philippe de Cheseaux in the mid-18th century and later included in Charles Messier's catalog in 1780. M71 has characteristics that initially led astronomers to classify it as a dense open cluster. However, subsequent studies revealed its true nature as a loosely concentrated globular cluster. It has a relatively small size compared to other globular clusters, spanning about 27 light-years in diameter, and contains stars that are roughly 9–10 billion years old. The cluster is rich in variable stars, including RR Lyrae variables, which helped confirm its status as a globular cluster. M71 is also notable for its relatively high metallicity (the presence of elements heavier than hydrogen and helium), indicating that it formed from material enriched by earlier generations of stars. It has an apparent magnitude of 6.1.
source: ChatGPT
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NGC6838
n.a.
Globular Cluster
Sagitta
19h 53m 46s
+18° 46.6′
13 kly
6.1
22 August
71º S
Conditions
M71 is best visible in summer and early Autumn. Maximum altitude reached is 71° in mid August. Weather conditions were not great, resulting in many sessions with relatively exposures. M71 was photographed over 14 nights during October and November 2024 from the remote observatory at IC Astronomy in Oria, Spain.
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” Luminance, Red, Green and Blue unmounted, Moravian filterwheel L, 7-position
Unguided
Compulab Tensor I-22, Windows 11, Dragonfly, Pegasus Ultimate Powerbox v2
Voyager Advanced, Viking, Mountwizzard4, Astroplanner, PixInsight 1.9.2
Imaging
M71 is a typical broadband object, that is not very critical to conditions around moon interference. Several hours of exposures were collected during moon phase of >80%. Luminance images were shot using 3 min exposures, RGB images at 5 min exposure. The total exposure was 14.2h.
Resolution (original)
Focal length
Pixel size
Resolution
Field of View (original)
Rotation
Image center
5600 × 4200 px (23.5 MP)
2585 mm @ f/7.3
3.8 µm
0.30 arcsec/px
28' x 21'
-0.19°
RA: 19h 53m 47.106s
Dec: +18° 46’ 37.63”
Processing
All images were calibrated using Darks (50), Flats (50) and Flat-Darks (50), registered and integrated using the WeightedBatchPreProcessing (WBPP) in PixInsight. All further processing was done in PixInsight, including the use of scripts and tools developed by RC-Astro, SetiAstro, GraXpert, and others. For a step-by-step description of the processing techniques applied, see process flow below.
After completion of WBPP, an anomaly showed in the Green channel. In parts of the image, the stars had a very blocky appearance. It looked like something had gone wrong during the integration process with inappropriate pixel-rejection. I typically use Winsorized Sigma Clipping with pretty strict criteria. So I repeated the process with looser criteria. This did help with the issue, but did not completely eliminate it. An integration using Generalised Extreme Student Deviate (ESD) pixel rejection at default settings worked without any problem. I’m not sure what caused this problem. Perhaps there was an issue with the weighting of the images, or something had happened during collection of (some of) the Green data. Weather conditions were not always great, so perhaps too many borderline Green frames were included. Whatever the cause, it looked like ESD gave a more robust result than Winsorized Sigma Clipping. So going forward I will keep with that.
The original Green integrated image (left) showed some strange artefacts. It looked like a problem with the pixel rejection during integration. Applying the same Winsorized Sigma Clipping with looser criteria (middle) did help, but could not eliminate the problem. Integration using ESD as rejection algorithm using default parameters (right) gave no artefacts at all. And I do have the impression that the overall image is even a bit sharper, but that is hard to quantify.
This was the second time to use MultiscaleGradientCorrection (MGC). In line with earlier experience, the tool seemed to be working straightforward and effectively. The only change this time was the use of a gradient scale of 2048 instead of the default 1024. This seems to result in a more gradual and more appropriate gradient profile.
Processing workflow (click to enlarge)
This image has been published on Astrobin
