M17 | Sh2-45

MessierSharplessNebula

31 May

Written By AstroWorldCreations

NGC 6618, Swan Nebula, Omega Nebula
47’ x 31’ | 0.3”/px | 9506 × 6318 px

Sagittarius
RA 18h 20m Dec -16° 07’ | 180°

Annotated image

Nearby objects within a 25° Field of View

Messier 17, commonly referred to as the Omega Nebula or Swan Nebula, is a captivating emission nebula situated in the constellation Sagittarius. The nebula was discovered by Jean-Philippe Loys de Chéseaux in 1745 and later indexed by Charles Messier. Spanning approximately 30 light-years in the longest direction, Messier 17 is located roughly 5,000 to 6,000 light-years from Earth. It is one of the most massive molecular clouds in our Galaxy, and part of a more extensive molecular cloud complex, a very dynamic site of ongoing star formation. At the heart of M17 is the cluster NGC 6618. Originally considered a small cluster, recent observations by the Chandra X-Ray Observatory indicates that the cluster includes som 8,000-10,000 stars, with very active star formation going on at a rate at least 25% than before. It is this star cluster that lights up the surrounding gas clouds so much and makes M17 the spectacular nebula as we know it.
source: Deep Sky Companions - The Messier Objects

Data Acquisition

Data was collection was started in summer 2024, but the opportunity was lost to complete a decent amount of frames. In May 2025 data acquisition was continued. In total data was collected over 17 nights, using a 14” reflector telescope with full-frame camera at the remote observatory in Spain. Data was gathered using 3nm narrowband filters (Ha, OIII and SII), complemented with standard RGB filters for proper star colours. A total of approximately 28 hours of data was 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” Ha, OIII, SII (3nm) and R, G, B 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.

For deconvolution, BXT was used with non-stellar sharpening set to 0.65. Using higher values definitely increased sharpness. But there is so much going in this nebula, that higher settings created structures that looked more like artefacts than real structure. So I decided to use BXT in a somewhat moderate fashion.

Hubble Palette in three simple steps

There are many different ways to create a false colour image using the Hubble palette. And not every dataset responds equally well to each of the different methods. Balancing the relative signal contributions, finding a pleasing colour palette without overdoing it, and maintaining enough detail in the process can all be a bit tricky from time to time, and result in unpredictable outcomes. This time a simple three step process was applied that seemed to work well for this dataset and has merit to become the method of choice. It is a combination of a standardized stretching process followed by steps that define colour and increase detail. The steps were as follows:

Stretch each channel using SmartStretch (scriptlet in PMGUI) to a target background level of 0.125 for each of the channels.
Combine the three channels as SHO and finetune colours using NarrowbandNormalisation (see image for details. Note that in this case the OIII signal was exceptionally strong, so OIII was dialed back a bit where more often OIII is given a bit of a boost. Also because of the high brightness of the blue from the OIII, some of the washed out aspects were corrected using CurvesTransformation, to the image where the non-blue hues were selectively masked out. The amount of SCNR applied is often a matter of taste. For this image, I found the green distracting and took it out by dialling SCNR to 1. For other images, no or lower SCNR might be the better choice.
Brighten and bring out contrast in specific parts of the image using a combination of small tweaks using GHS and HT. LHE was not applied as a tool to bring local contrast, because the effects would be just a bit too unnatural.