Imaging with Cooled CCD Cameras

Abstract

The latest DSLR cameras can do a very good job of astro-imaging and can, of course, be used for general photography as well, so why go to the expense of buying a cooled CCD camera? The main reason lies in the word ‘cooled’. All imaging chips produce dark current noise which increases with exposure time and is also highly dependent on its temperature, that of a typical chip dropping by half for each drop of 6 degrees Celsius in temperature. So, if the chip is cooled by 30 degrees below ambient temperature, the dark current noise will have dropped by about 5 times, so allowing longer exposures to be taken before dark current noise becomes a problem. Given dark skies that do not suffer from light pollution, this can allow images to reveal faint nebulosity that would otherwise be lost in the noise. When significant light pollution is present, the exposure times, and hence the dark current contribution, have to be less, before the skylight becomes obtrusive, and so cooling does not confer as great an advantage. The latest chips have very low dark currents, and it is rarely worth cooling them down below about –20 C. This temperature can normally be reached with the single-stage Peltier cooling employed in CCD cameras aimed at the amateur market. Chip Size The dimensions of the CCD chip allied to the scope’s focal length dictate the field of view of the resulting images. The chip dimensions in millimetres divided by the focal length in millimetres give the field of view in radians. Multiplying this by 57.3 gives the field of view in degrees. So, obviously, a bigger chip will provide a bigger field of view. For example, my 80-mm refractor has a focal length of 550 mm, and this is used with a CCD camera whose sensor has dimensions of 18 × 13 mm. This gives a field of view of 1.9 × 1.4 degrees.