SH2-155 (Cave Nebula)

SH2-155, also known as the Cave Nebula, is a mixed ionization and reflection nebula in the constellation Cepheus containing many star formation regions. The ionization sources are young stars of the Cep OB3 association, which are located in a distance of about 2700 light-years.

Click on the images for a full-scale/full-view version.

SH2-155 (Cave Nebula) in H-alpha and continuum
This image is a false color composite where H-alpha (including red continuum) is mapped to red, blue is mapped to green and near infrared is mapped to blue. The latter two channels may contain emission lines. Stars are partially subtracted to improve the visibility of the nebulae.
SH2-155 (Cave Nebula) in continuum light
This false color composite of continuum channels is presented in natural order: near infrared is mapped to red, red is mapped to green, and blue is mapped to blue. All three color channels do or may contain emission lines, meaning the various colors in this image are caused by emission lines, star colors and Rayleigh scattering (which depends on wavelength). In particular, a small fraction of Hα was (re-)added to the red channel (mapped to green), which was taken using a SDSS R' filter in combination with a 400 nm to 650 nm band-pass filter (in order to block Hα). However, without Hα-blocking, this emission line would still be about five times as intense as in the image above.

Image data

FOV (full view in the JavaScript viewer): 1.35° × 0.89°
Position (J2000): RA: 22h56m; DEC: 62°25′
Date: 2022
Location: Pulsnitz, Germany
Instrument: 400mm Newton at f=1520mm
Camera Sensor:4 Sony IMX455
Orientation: North is up (exactly in the image center)
Scale: 1 arcsec/pixel (at full resolution)
Total exposure times:
H-alpha (3.5nm): 14.7 h
Near infrared (SDSS I'): 7.4 h
Red (SDSS R' + 400-650 nm band-pass): 6.1 h
Blue (SDSS B'): 6.4 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. Star subtraction where star positions and intensities are extracted from continuum images
  5. Denoising and deconvolution of both components (stars and residual)
  6. Dynamic range compression using non-linear high-pass filter
  7. Color composition and tonal curve correction

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