SAO Guest Contribution

Colour imaging with a CCD

Prof. Michael Bessell is the Associate Director, Research School of Astronomy & Astrophysics, Mt. Stromlo and Siding Springs Observatory, Canberra Australia.  An internationally known astrophysicist who has a particular interest in producing superb wide-field astronomical images for educational & public use, Prof. Bessell has kindly given us permission to reproduce a number of his images on our course CD and website.  In this contribution, intended particularly for amateur astronomers in the course who are interested in CCD imaging, he explains how he goes about taking these images. Those of you do not fit that category will still enjoy seeing his image set displayed all together.

To see the images referred to in the text below, open this link in a new window so that you can refer to the images while reading the text. 
(On a PC, use the right mouse button on the link to achieve this.)

Unlike a video camera or a digital camera which can produces a colour image in a single exposure, the images that I produce are made from superimposing "black and white" images taken through different colour filters. The CCD that most of my images were taken with was a scientific grade SITe CCD (Charge Coupled Device) cooled to 160K and read out slowly.

Individual "black and white" images were taken through B (blue), V (green) and R (red) coloured glass filters or interference filters such as Halpha (wavelength 656.3 nm width 5nm) and forbidden OIII (wavelength  500.7nm width 2.4nm). The different images were made coincident using GEOMAP and GEOTRAN in IRAF (an astronomical data reduction package produced by the National Optical Astronomy Observatories organisation in the USA). Other image processing packages are available.

Usually several images were taken through each filter and the telescope moved slightly between exposures. In this way both the cosmic rays and the bad columns on the CCD could be removed when the images were superimposed and median filtered. The black and white 16 bit fits images were then converted into 8 bit Photoshop pict files using a "freeware" Mac program F2p (FITS to Pict) written and made available by Ralph Sutherland. There are other packages such as xv that can read fits images, although not as well.

The colour images were made with Adobe Photoshop and most images of gas clouds were produced by allocating red to the H alpha (656 nm) image, green to the [OIII] (501 nm) image and blue to the B (435nm) image. Others used the V (green) glass in place of the OIII image.

Three different cameras and scales are represented in the set of images.

The 40 inch telescope at f/8 cassegrain focus provides a pixel scale of 0.6 arcsec and the field is about 20 arcmin square.

Some images were taken with a Nikkor 400mm f/4.5 lens providing 12 arcsec pixels and a field of 6.8 degrees square. MSSSO (Mike Bessell) and the University of Sydney (Anne Green) and their associates are conducting a survey of the Galactic plane with this setup. See http://msowww.anu.edu.au/~buxton/halpha.html for some details.

Finally, the very wide-field images were taken with a 80 mm Hasselblad lens that produces about 1 arcmin pixels and a field of about 32 degrees square.

In all cases the filters were placed between the lens and the CCD requiring refocussing for each filter.

By using the interference filters to isolate the emission lines from ionized gas it is possible to enhance the brightness of the gas clouds relative to the stars by a factor of about 20. The relative brightness of Halpha and O III measures the temperature of the gas and also indicates in some cases where the gas has been ionized by shocks such as supernovae explosions.

The images of the supernova remnants (Crab and Vela) show many green or yellow filaments where Halpha is absent or weak. These indicate the mostly shocked areas, that is, places where the ionization occurred through collisions of gas clouds and not only from UV radiation from hot stars or hot gas.

On the other hand, the yellow nebulae seen around the hottest stars results from the extremely energetic ultraviolet light from these stars (the trapezium in Orion, the eta Carina nebula and the Rosette nebula and the 30 Doradus nebula)

To see an example of how the combination of different filters emphasizes different aspects look at the three images of 30 Doradus. The white image is made up from wide band BV and R images; the red image from Halpha, V and B images; the yellow image from Halpha, OIII and B. The stars are emphasized in the white image and progressively more of the gas in the other images. They are all beautiful and yet they all capture a different part of the physics of star forming regions.

Most of the galaxy images and the wide field images of the Milky Way and the two Magellanic Clouds together are made from B, V and H alpha. In this way all the HII regions show up easily as beautiful red regions, like red beads on strings, in the distant galaxies. This highlights the fact that star forming generally occurs in the spiral arms of gas rich galaxies.
 

Mike Bessell