All-sky camera

Brief description

(supervised by Giles Hammond)

The past ten years has seen a revolution in the type of astronomical CCD cameras which are available to amateur astronomers. The Charge Coupled Devices (CCD’s) found in the RF Concepts 1004X CCTV camera (SONY ICX255AL) are similar to those that have been used in high cost professional cameras from manufacturers such as Starlight Express. However, there are three areas in which these low cost cameras are inferior to their significantly higher cost professional rivals. Firstly, in their off-the-shelf form the cameras are limited to a maximum exposure of 1/5s by an electronic shutter and are therefore of little use for imaging deep sky objects. With minor modifications to the circuit board it is possible to disable this shutter and control the clock pulses which are used to readout the CCD using the parallel port of a PC. The ultimate length of the exposure is then limited to the fact that the cameras are uncooled and thermal noise begins to dominate (hot pixels can become significant and require the use of dark frames). Finally, the cameras are only 8-bit monochrome which results in the digitisation from black-white in 256 levels of greyscale. When imaging deep sky objects, where significant dynamic contrast can be present, this can become somewhat of a limitation. However, stacking multiple exposures can be used to increase the bit depth of the resultant images.

The aim of this project is to use a modified 1004X surveillance camera as an all sky camera by coupling it to a wide-angle 1.6mm focal length fish eye lens. The camera and lens will be located in a weatherproof housing comprising a metal box/acrylic dome and the output can be uploaded onto the observatory website. Although the camera has already been modified some additional circuitry needs to be built in order to capture the integrated video signal into the PC. Once this has been achieved multiple exposures can be taken and stacked together to produce the final image. This may require some basic dark frame subtraction and some flat field correction. Both the operation of the camera during the day and night (by enabling or disabling the shutter) and the method by which exposures will be taken (i.e. number of exposures and dead time between images) need to be considered. The housing needs to be designed and constructed together with a means of holding the camera. Furthermore the associated circuitry and any heater/fan which may be used to reduce the problem of dewing will need to be considered. Finally the camera needs to be tested for continuous operation both during clear nights (with/without moon), cloudy nights and daytime to find the correct combination of exposure length and shutter control.

 

Resources

 

1/5s 2s 5s 10s