What is an image?

The SDSS telescope uses a CCD (charged coupled device) camera to record images of the sky. A CCD is a device that turns light into electronic signals. When light hits a CCD camera, the CCD records a signal; the brighter the light, the stronger the signal.

The SDSS's camera consists of six columns of five CCDs each

The SDSS's CCD camera is one of the most sensitive ever built. The SDSS camera uses 30 CCDs, arranged in six columns, to record images. Each column contains 5 CCDs that record a strip of the sky in five wavelengths of light. Each CCD consists of a 2048 by 2048 grid of pixels, or single points in the image.

The electronic image that the CCD records consists of a grid of these pixels. Each entry in the grid includes the pixel's x-coordinate, its y-coordinate, and the number of photons, or "counts," that hit that pixel during the camera's exposure time. Image processing software reads the data and assigns each pixel a shade of gray (or a color) depending on the number of counts.

Here is an example of how the data might look for part of an image from one of SDSS's CCDs:

An image processing program would assign a color to each value in the grid. For example, the integers 0 and 1 might be black. The integers 2 and 3 might be a very dark gray, up to the integers 9 and 10, which could be white. The program could also display each integer as a different color. Below is one possible way of displaying this image:

You could then look at this color pattern with a computer image processing program.

A real image would have many more pixels and would use substantially more than 10 colors. The way that image processing programs display the image is the same, however.