About observational astronomy

Astronomers can gain an understanding of objects in the universe by studying them with a telescope. The view through the telescope is captured using a camera. A CCD imaging system is a type of digital camera that is very sensitive to the dim light from distant objects. The CCD camera is attached to a telescope and the images are stored on a computer where they can later be analyzed. In this learning activity a computer program will be used to view and study images taken with a telescope to learn basic concepts in astronomy.

Activity

Astronomical images are stored in a computer file format called FITS. These images can be opened and viewed by using computer software that understands this file format. We will start by using a program called the Aladin Sky Atlas to retrieve images taken with a telescope and study them.

• Open another window in your web browser by clicking on this link to the Aladin previewer program.
• In the text box enter the name of the object that we are interested in studying. Type M101 and then click the Plate view button.

This image shows an area of the sky that includes a galaxy. The image was taken as part of the Palomar Sky Survey. The galaxy is the small spiral shape near the upper left. The galaxy is much more distant then the individual bright stars in this view. To get a closer look at the galaxy lets take a look at a more magnified view.

• In the Aladin window click on the back button to return to the search page. Then click on the Default image button to retrieve a closeup image of M101.

A galaxy is composed of stars and the brighter parts of the image indicate places within the galaxy where there is a greater concentration of stars, for instance in the spiral arms. The common name for this object is the Pinwheel Galaxy. Astronomers also refer to it using a designation in an astronomical catalog. The Pinwheel Galaxy is number 101 in a list of objects catalogued by astronomer Charles Messier.

• Now use Aladin to retrieve a color image of M101 by clicking on the RGB image button.

The cameras used by astronomers only capture greyscale images. To produce a color composite image requires taking seperate exposures through color filters - usually red, green and blue. The three exposures are then combined to produce a final color image. In the color image of M101 you can see that the three seperate exposures are not perfectly lined up which is why there is a purple border around the edge.

• Click on the New query button in Aladin. Just below the search text box there is a list of examples. Retrieve images of some of the other galaxies listed such as the Triangulum Pinwheel by clicking on M33.

Take a look at types of galaxies and galaxy classification as you look at the images. Try to classify each galaxy. Is it spiral, elliptical, or another type?

Discussion

After you have taken a look at the images, post questions here to start a discussion...

Readings

• w:Observational astronomy
• w:Telescope

References

Additional helpful readings include:

• Image processing introduction
• Color in Hubble images