M75
NGC 6864
37’ x 25’ | 0.3”/px | 7500 × 5000 px | full resolution
Sagittarius
RA 20h 06m 05s Dec -21° 55’ 14” | 180°
Messier 75, also known as NGC 6864, is a globular cluster of stars in the southern constellation Sagittarius. It was discovered by Pierre Méchain in 1780 and included in Charles Messier's catalog of comet-like objects that same year. M75 is about 67,500 light years away from Earth and is 14,700 light years away from, and on the opposite side of, the Galactic Center. Its apparent size on the sky translates to a true radius of 67 light years. M75 is classified as class I, meaning it is one of the more densely concentrated globular clusters known. The absolute magnitude of M75 is about −8.5, equating to 180,000 times more luminous than the Sun (L☉). The cluster appears not to have undergone core collapse yet. There are 38 RR Lyrae variable stars and the cluster appears to be Oosterhoff-intermediate in terms of metallicity. 62 Candidate blue stragglers have been identified in the cluster field, with 60% being in the core region. Messier 75 is part of the Gaia Sausage, the hypothesised remains of a dwarf galaxy that merged with the Milky Way. It is a halo object with an orbital period of 0.4 billion years to travel around the galaxy on a very pronounced ellipse, specifically eccentricity of 0.87. The apocenter (maximal distance from Earth) is about 57,000 ly (17,500 pc).
source: Wikipedia
Data Acquisition
Data was collected over 9 nights in July and August 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 approximately 12 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.
My experience with globular clusters is that sharpening in BlurXTerminator can best be set to “0” or a very low value, as artefacts can show up very quickly in the fabric behind the stars towards the nucleus of the cluster. That leaves some extra room for sharpening later on in the process. In this case a very mild sharpening was applied using LocalHistogramEqualization towards the end of the processing, which worked actually quite nice.
The rest of the processing workflow followed regular procedures.
Processing workflow (click to enlarge)
This image has been published on Astrobin.
