From Photographs to Spectra

Astronomical images taken using the Anglo-Australian Telescope were all originally captured using a photographic plate method. An emulsion was spread onto rigid glass plates, made from billions of tiny silver salt crystals. Plates could only be covered in one layer of emulsion and so could only produce black and white images. Silver salts are extraordinarily sensitive to light, so that they form an image when light hits them. This image is the negative, which needs to be re-photographed to form the actual black and white photography.                                                            Plate photograph of a galaxy. Image: Amanda Bauer

       A black and white plate photograph of a galaxy. Image: Amanda Bauer

In 1977 David Malin pioneered a colour astro-photography method from scratch using the AAT and UKST telescopes. Malin used three black and white images taken in different filters to capture the red, green and blue light. Careful calibration was required so that the colours appearing in the photographs matched true colours. Each colour plate required up to 90 minutes of exposure time to photograph the faint objects in the night sky. To do this, Malin would sit inside the prime focus of the Anglo-Australian Telescope and manually affix and remove the plates after an appropriate exposure time.

Image of a reflection nebula in M20, taken by David Malin

    David Malin in the prime focus of the AAT. Image: AAO








Left: An example of David Malin's colour astro-photography. Right: David Malin sitting the prime focus of the AAT. Image: AAO.

Since then, the focus of the AAO shifted from photography to spectroscopy, as it was realised that light spectroscopy from astronomical objects could deliver a rich well of information to astronomers. One of the early successful spectrographs was the 2dF-spectrograph, which began regular observations in 1997. Custom-made metal plates (field plates) with holes cut out of it were used to capture light from desired astronomical objects (the holes would line up with galaxies or other objects when pointed at the right place in the night sky).  On the back of the plate, optic fibres plugged into the holes and transported the light to a spectrograph, like the AAOmega.  Now days the optic fibres are positioned robotically, using instruments like 2dF and StarBugs.