M51 (NGC 5194 and NGC 5195, Whirlpool Galaxy) in H-alpha and continuum light

M51 (also known as Whirlpool Galaxy) are two interacting galaxies in constellation Canes Venatici. The larger one (M51A or NGC 5194) is a grand-design spiral galaxy. Its smaller companion (M51B or NGC 5195) is a dwarf galaxy that is highly distorted from the interaction with M51A.

M51 (NGC 5194 and NGC 5195, Whirlpool Galaxy) in H-alpha and continuum light
Click on the image to toggle H-alpha off (requires JavaScript, H-alpha: on)
In both images NIR (near infrared) is mapped to orange and blue is mapped to bluish cyan, but the first image additionally contains H-alpha light in the red channel. (The only difference is that in the H-alpha variant the NIR data in red channel was replaced by data taken with the H-alpha filter which contains about the same continuum information as the NIR data.)

By toggling between the images the correlation between HII regions and the bluish regions (consisting in young stars) can be visualized.

It seems that M51A contains many young (bluish stars) while M51B and its tidal streams mainly consists in older (orange) stars. Furthermore it can be seen that the dust band of the upper arm of M51A obfuscates M51B, i.e. M51B lies behind M51A.

Image data

FOV: 0.31° × 0.36°
Date: 2020-2021
Location: Pulsnitz, Germany
Instrument: 400mm Newton at f=1520mm
Camera Sensor: Panasonic MN34230
Orientation: North is up (approximately)
Scale: 0.8 arcsec/pixel (at full resolution)
Total exposure times:
H-alpha (3nm): 20.0 h
NIR: 3.7 h
Blue: 3.9 h

Image processing

All image processing steps are deterministic, i.e. there was no manual retouching or any other kind of non-reproducible adjustment. The software which was used can be downloaded here.

Image processing steps where:

  1. Bias correction, photon counting
  2. Dark current subtraction, flatfield correction, noise estimation
  3. Alignment and brightness calibration using stars from reference image
  4. Stacking with masking unlikely values and background correction
  5. Denoising and deconvolution
  6. RGB-composition
  7. Tonal curve correction

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