Eridanus Bubble

Image versions: 600×1005 Hα1200×2010 H&alpha600×1005 HαRGB1200×2010 HαRGB

  The Eridanus Bubble is a 25° area of interlocking arcs of Hα-emitting filaments, driven by stellar winds and supernovae. Its age is estimated as 5-6 million years. It is much fainter than its more prominent eastern neighbor, Barnard's loop.
Click on the map to the left to see its location in the the sky (star map generated with XEphem).

10 narrow-band H&alpha images were combined into a mosaic covering approx. 18°×30°. The faint arcs required strong contrast stretching, which made careful equalization of the sky background and star intensities essential. Since the images were taken from sites with differing night sky qualities, they exhibit strongly varying background levels, as well as sky-glow gradients near the horizon (see this "raw" mosaic).

For each of the 10 Ha frames, several hundred stars (more precisely, their integrated flux and local background levels) were extracted using the SExtractor software, which was also used for removing local gradients. Matching stars in the overlap regions were identified with a Perl script. The appropriate flux scaling factors and image backgrounds were then obtained via a least squares fit.

A 2-frame color mosaic was assembled from images taken with a Canon EOS 350D digital SLR camera. In the color mosaic, the R channel was "enhanced" with the Hα data ("lighten only" mode); in addition, contrast-stretched copies of the Hα image were superimposed on the G and B channels, resulting in a "semi-false-color" HαRGB image.

Camera Ha: Starlight Xpress SXV-H9, Color: Canon EOS 350D
Lens Ha: Minolta 1.4/50 mm lens, stopped down to f/2.4
Color: Canon 1.8/50 mm, stopped down to f/4
Filter Astronomik Ha (13 nm FWHM)
Mount Losmandy G-11 / Vixen Super Polaris DX
Guiding SBIG ST-4 autoguider
Dates 10 Jan., 08 Oct. and 29 Oct. 2005, 24-25 Feb. 2006
Color: 25 Feb. 2006
Exposure Ha: each frame 2x15 min, Color: 6x5 min
Site 2005: Sch÷bendorf, Brandenburg, Germany
2006: Cederberg Observatory, South Africa
Processing Bias, dark frame, flat field correction, registration and digital development: IRAF;
mosaic assembly: SExtractor, Match, custom software and Swarp;
post-processing: Gimp

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