M73
NGC 6994
46’ x 30’ | 0.3”/px | 9232 × 6092 px | full resolution
Aquarius
RA 20h 58m Dec -12° 38’ | 0°
Messier 73 (also known as NGC 6994) is an asterism of four stars in the constellation Aquarius which was long thought to be a small open cluster. It lies several arc-minutes east of globular cluster M72. M73 was discovered by Charles Messier in 1780 who originally described the object as a cluster of four stars with some nebulosity. Much later observations by John Herschel could not find any nebulosity. M73 was once treated as a potential sparsely populated open cluster, which consists of stars that are physically associated in space as well as on the sky. The question of whether the stars were an asterism or an open cluster was a matter of debate in the early 2000s. The controversy was resolved in 2002, when M. Odenkirchen and C. Soubiran published an analysis of the high resolution spectra of the six brightest stars within 6 ′ of the centre point. They demonstrated that the distances from the Earth to the six stars were very different from each other, and the stars were moving in different directions. Therefore, they concluded that the stars were only an asterism.
source: Wikipedia
Data Acquisition
Data was collected over 4 nights in July 2025, using a 14” reflector telescope with full-frame camera at the remote observatory in Spain. Data was gathered using standard RGB filters. A total of 6 hours of data was finally combined to create the final image.
Location Remote hosting facility IC Astronomy in Oria, Spain (37°N 2°W)
Sessions
Frames
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” RGB 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.
In the Green master one of the satellite trails had not been rejected properly and was still somewhat visible. Chances are that after combining and stretching this would not pose a real problem, but rather than wait and see, the green frames were re-integrated with stricter rejection criteria. In the end a Winsorized Sigma clipping with and SD of 1.5 for the high signals was needed to eliminate the satellite trail completely. Subjectively it looked like the star shapes of especially the bright stars came out a bit more symmetrical, so perhaps I will use a more stringent rejection upper limit during the integration for other images as well.
While the object is very small compared to the field of view, I did not crop in the image on purpose, to emphasise the asterism among a regular star field.
The rest of the processing went pretty standard and the full processing workflow can be seen below.
Processing workflow (click to enlarge)
This image has been published on Astrobin.
