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Section Contents


Learning to Use the ds9 Imaging System

Overview & Purpose

In this section you will learn to manipulate images with the ds9 imaging system. Before you go on to the more sophisticated analysis contained in the Activities and Images sections, you should become relatively comfortable and familiar with the commands and functions of ds9 so that you are thinking about the science rather than about the software. The sections below use a Chandra data set from an observation of the supernova remnant Cassiopeia A (aka Cas-A) as the basis for the tutorial. Follow them through until you are comfortable with the controls and options in "ds9". You will be doing the same kind of image manipulation and preliminary "qualitative" analysis that a scientist would do when she or he first gets a data set from an observation.

A Short Summary of the Chandra Mission

When we think of an astronomer doing research, we often picture someone peering through a telescope in a domed observatory building. X-ray astronomers work differently. Their telescopes are not located in a building or even anywhere on earth. Rather, X-ray telescopes orbit the earth in highly sophisticated spacecraft that are carefully controlled by astronomers on the ground. After a star or galaxy or other object is observed by an X-ray telescope, the information gathered about that object is relayed back to earth, where astronomers use computers to do their analysis. Some astronomers prefer to refer to themselves by the title of "astrophysicist". This is because the power of telescopes like Chandra allows scientists to test and explore fundamental physics problems using the objects in the universe as cosmic physics laboratories.

X-ray data is rich and complex, containing quite different information than the image or picture that we can "see" when looking at objects in visible light through a telescope on earth. An x-ray telescope collects x-ray emission (called "x-ray photons") from a star, galaxy, or other source. X-ray photons are much more energetic than optical photons. They have temperatures of millions of degrees Celsius. They allow astronomers to study very energetic objects and events, such as collisions of galaxies, and matter swirling into black holes. Chandra uses x-ray detectors to record different kinds of information (position, energy, time of arrival) about each individual x-ray photon. No single x-ray detector can accurately record all three types of information, so Chandra has two different x-ray detectors, the High Resolution Camera (HRC) and the Advanced CCD Imaging Spectrometer (ACIS), that specialize in recording one or another type of data.

The HRC can record very accurate positions as well as the photon arrival time (i.e., the time that the x-ray photon hit the detector). The ACIS records accurate position and very good energy information (i.e., the energy of each photon is determined within a narrow range). It can provide less precise timing information as well, usually pinpointing a photon's arrival time to a precision of about 3 seconds.

Section Contents

For more information about topics visited in the introduction, visit these quick links to the Chandra Public Education site:
When proposing an observation, astronomers specify one x-ray detector or the other based on what sort of data analysis they think is appropriate for their object. When Chandra carries out the observation, the proper detector is placed at the focal point of the mirrors to record the incoming x-ray photons. For example, timing information is most important for studying properties that appear to vary, like the X-ray emission from some pulsars in supernova remnants (SNR). Position is important if we want to make an accurate image of an object to study special features. Energy is important for making spectra to study, for example, the composition of the elements in the expanding outer shell of an SNR. Thus, to get the most complete x-ray information about the SNR Cas-A, an astronomer would want to see data from observations using both Chandra detectors, the HRC and the ACIS.

Next: Part 1: First Look Qualitative Analysis using ds9

Last updated: 11/14/02


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