About observational astronomy

Astronomers can gain an understanding of objects in the universe by studying them with a telescope. Observational astronomy is the process of collecting and analyzing data to learn about these objects. In this learning project a computer program will be used to view and study images taken with a telescope to learn basic concepts in astronomy.

Introduction

The activities in this learning project are based on the idea of learning by doing. You will learn about astronomy by going through the same process that an astronomer does to analyze data. As you follow the instructions in the activity you should also follow the links to related reading materials.

A learning project is a collection of pages devoted to the Learning of a specific topic or family of topics; and, simultaneously, a group of people that use said pages to collaborate on a learning goal.

This page is the start of a series of related activities that will be created here during the first half of 2007. Visit the planning page to see some ideas on how this project might develop, and check back often to see the progress. There aew also new projects to observe supernova and study extrasolar planets which have just started.

Activities

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. 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.

Browsing the sky

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. Where did this image come from? Aladin allows you to retrieve images from a number of different sources. This particular 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?

Creating color images

• Open another window in your web browser by clicking on this link to the Aladin Sky Atlas home page.

There are three different versions of the Aladin program. The version used in the first activity is the Aladin Previewer. The Previewer is a convenient tool for browsing images but it does not allow for editing the images. For this activity we will need to use the full version of Aladin.

• To get started quickly just click on the Start Aladin applet link to run Aladin in a browser window. (You can also Download Aladin on your machine and run the program from your desktop.)

• Start Aladin and click on the Load... button. In the Server selector window click on SkyView. In the Target box enter M101 and then choose POSS2 Red for the Survey. Click Submit.

The image of M101 will now download and open in the Aladin multiview window. Notice that the image is displayed as a negative, ie. the bright stars appear black and the dark sky appears white. This is an image taken through a red filter. Next we will download a second image taken through a blue filter.

• Go back to the Server selector window and choose POSS2 Blue for the Survey. Click Submit. You can now Close the Server selector window.

We will now combine these two images to create a color composite.

• In the Aladin multiview window click on the icon labeled rgb. In the RGB image generator window use the pulldown menu to select the images for each color. For Red choose the image name that begins Skw DSS2 Red. For Green leave it set to none. For Blue choose the image name that begins Skw DSS2 Blue. Click on the Create button.

A third image will now appear in the Aladin multiview window. This type of image is called False-color because the colors do not exactly match what the human eye would see when looking through a telescope.

Readings

Background information:

• w:Observational astronomy
• w:Telescope

References

Additional helpful readings include:

• Image processing introduction
• Color in Hubble images
• Techniques of Observational Astronomy