M94 in H-alpha and continuum light

Messier 94 (NGC 4736) is a spiral galaxy in constellation Canes Venatici.

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M94 in H-alpha and continuum light
(requires JavaScript, H-alpha: on)
In both images NIR (near infrared) is mapped to red, yellow is mapped to green and blue is mapped to blue. In the version with H-alpha that emission line is shown in reddish orange.

By toggling between the images (click on the button) it can be seen that the HII regions (reddish) correlate with bluish regions. That's because the HII gas clouds typically contain many young (blue) stars which are also responsible for the ionization. Furthermore the HII regions also emit [OIII] and H-beta light which is collected by the blue filter.

The Galaxy has two ring structures. The inner ring with an apparent outer diameter of about 2' (arcminutes) is an active star formation region and contains a lot of young blue stars and HII regions. The broader outer ring has a diameter of about 16' and is very faint.

Image data

FOV: 0.47° × 0.47°
Date: 2020-2022
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): 18.3 h
NIR: 3.2 h
Yellow (540nm to 650nm): 6.4 h
Blue: 3.2 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, dark current subtraction, flatfield correction
  2. Alignment and brightness calibration using stars from reference image
  3. Stacking with masking unlikely values and background correction
  4. Denoising and deconvolution
  5. Color composition
  6. Dynamic range compression using non-linear high-pass filter
  7. Tonal curve correction

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