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;