Return to 		advanced index Beginners Intermediate Advanced

Data Reduction

When a picture is taken with a CCD it needs to be improved to make it clearer. This is called data reduction. There are four main ways to do this, called dark frames, flat fielding, calibration using standard stars and dithering.

Dark Frames

CCDs have what is called a dark current. This is caused because the CCD charges up slightly even if no light is falling on it. To get rid of this dark current, astronomers take a picture with the shutter of the detector closed, to see how much each pixel charges up. This is called a dark frame and it can then be taken away from the picture of the object to remove the dark current.

Flat Fields

Each pixel of a CCD is slightly different. This makes it difficult to compare different parts of the picture taken. To find out how much each pixel varies, astronomers take a picture of a blank sky just as the Sun goes down, so the stars can't be seen yet. This is then used to correct the picture of an object. This is called flat-fielding.

Standard Stars

Once the picture has been flat-fielded and had its dark current removed it can then be compared to a standard star to find out how bright it is. A standard star is one that gives out a known amount of light, this means that by comparing the picture of an object to a standard star taken on the same night, the actual brightness of the object can be found.


All CCDs may have some bad pixels, there is also a chance that a cosmic ray might hit the detector. To stop this changing the picture, astronomers use a method called dithering. They take a few pictures of the same object, each one in a slightly different position, so that when they are put together the effect of a bad pixel or a cosmic ray is cancelled out.

Click on the links below to find out more about telescopes

Telescopes Introduction

Types of Telescope


Faulkes Telescope

Return to advanced index Beginners Intermediate Advanced

Authors: Carolyn Brinkworth and Claire Thomas

Last updated: July 2001