M13 (Hercules Globular Cluster)

M13 (also known as the Hercules Globular Cluster) is the brightest and largest globular cluster visible in the northern hemisphere. It orbits the Milky Way at a distance of 22,000 to 25,000 light-years. At this distance, the apparent diameter of 20′ corresponds to a true diameter of about 140 light-years.

Image data

FOV:	1.11° × 0.83°
Position (J2000):	RA: 16h41m42s; DEC: 36°28′
Date:	2024-2025
Location:	Pulsnitz, Germany
Instrument:	400mm Newton at f=1520mm
Camera Sensor:	Sony IMX455
Orientation:	North is up (exactly in the image center)
Scale:	1 arcsec/pixel (at full resolution)
Total exposure times:	Near infrared (SDSS I'): 3.3 h Red (SDSS R' + 400-650 nm band-pass): 3.3 h Blue (SDSS B'): 2.1 h

Image processing

All image processing steps are deterministic and none of the algorithms use machine learning (often referred to as “AI”), which tends to generate plausible looking fake details. The software used can be downloaded here.

The image processing steps were:

1. Bias correction, dark current subtraction, flatfield correction, noise estimation
2. Alignment and brightness calibration using stars from reference image
3. Stacking with outlier rejection, background estimation and optimal weighting based on noise estimation
4. Denoising and deconvolution
5. Dynamic range compression using non-linear high-pass filter
6. Color composition and tonal curve correction