Astronomical CCDs

Last month in this space we discussed webcams, this month we look at astronomical CCDs. The original application for CCD circuitry was supposed to be as a replacement for hard disk drives. The CCD (charge-coupled device) stored information as packets of electrons. During the 70’s and 80’s a lot of work was done to get CCDs to replace vidicon tubes by making them photosensitive. By 1974, Fairchild had a 100x100 imager. In 1976, the first article appeared that reported astronomical results from the use of a CCD – the first features seen on Uranus.

Uses

The astronomical CCD can do things that are virtually impossible for cheaper imagers such as webcams.

Deep Space Astrophotography – Long Exposures

An astronomical class CCD camera can take long individual exposures often lasting for several minutes or longer. What makes these long exposures possible is the ability to handle the inherent noise in the camera’s electrical system. Much of the noise is thermal and it is reduced when the camera is thermoelectrically cooled. It is not unusual for an astronomical camera to be cooled 20° C compared to the ambient temperature. Higher end cameras can use water to further reduce temperatures.

Long exposures allow astronomers to do more than just reduce noise and improve the quality of the image. They also make it possible to only use a portion of the spectrum and still have enough sensitivity to record an image. One example of this is called narrow-band imaging popularized by Rob Gendler and others.

Photometry

The low noise capability of CCDs makes it possible to measure the amount of light from deep sky objects. Photometry can be used to measure the magnitude differences in variable stars and even detect extrasolar planets. The method for doing this is usually to get two or more stars in a single frame where one star is the one being measured and the others can be considered constant. The difference between a constant star and the variable star can be plotted.

Self-tracking

Some astronomical CCDs have the ability to track an object while taking an image. One method for doing this is to have a small part of the CCD detector dedicated for autoguiding.

Other CCD advantages

The astronomical CCDs have other advantages over their cheaper brothers:

Most astronomical CCDs are monochrome which gives them greater resolution and flexibility.

Although filters can be used on webcams they tend to be manually operated while astronomical CCD filter wheels can be automated.

Companies like SBIG sell adaptive optics for their cameras which improves the accuracy of auto guiding and reduces the effect of atmospheric aberration.

Astronomical CCDs are built for outdoor use on cold nights. Webcams are built for an office or den.

A spectrograph can be attached to an astronomical CCD. Spectral shifts have been used to detect most of exoplanets discovered to date.

The amateur using these kinds of cameras are approaching the techniques used by professionals although the custom made professional CCDs cost hundreds of thousands of dlollars.