Messier 83  -  Southern Pinwheel

Image Acquisition by Christoph Kaltseis / Image Processing by Bernhard Hubl

Full resolution (2035x2035 px  590kB)

Crop in 150% resolution (170kB)

Object data of M83

Object type: galaxy (SBc)
Size: 13.1' x 12.2'
Magnitude: 8.0 mag
Constellation: Hya
Distance: 15 million Ly

Exposure data

Date: 2008-06-02
  Location: La Palma
Telescope: 10" ASA (f=965mm)
Camera: ST4000XCM (one-shot color)
  Binning: Bayer matrix
  Exposure time: 12x8m
Exposure time total: 1h 36m

Image Acquisition by Christoph Kaltseis

Image processing by Bernhard Hubl (Color weights: B-V method)


Image Processing


This is my first processed image of a one-shot color camera. I had to solve the following two problems to prepare the image data for my standard workflow of monochrome CCD cameras:

1. Calibration of the raw data and converting images from the Bayer matrix format to separated Red, Green, Blue and Luminance images.

2. Calculation of the color weights without usage of the standard G2 star method.

The first problem was solved with CCDSTACK. The calibration of ST4000XCM files is possible and the Bayer matrix can be converted in Red, Green, Blue and Luminance. The conversion into the color channels worked very well, but the luminance channel showed elongated stars. So, I used only the color channels. The images of each color channel were registered using the interpolation method Bicubic B-spline, then normalized, sigma rejected and combined using Mean. At this point, I had three images (combined Red, Green and Blue). There was a slight shift between these color images, which was eliminated by another registration. Afterwards the luminance was created using a simple mean of Red, Green and Blue.

The second problem was solved with the B-V method for color weights: A G2 star shows a value of 0.65 for B-V (difference of brightness in mag between blue and green filters of the Johnson system). So, one have to find unsaturated stars in the image which have a B-V value around 0.65. This can be done using the NOMAD1 catalogue in the Vizier database. I selected five stars with a B-V value between 0.62 and 0.68, and a V-R value between 0.3 and 0.7 (V-R=0.52 for a G2 star). The next step was to measure the brightness of the 5 stars in Red, Green and Blue, which was done with the software AstroArt. Then, I could calculate the color ratio for each of the 5 stars. This method seems to give quite reproducible results, because the ratios of the 5 stars were very similar.

All other image processing steps are similar to my standard processing workflow.