Author: Richard Beare, Jessica Barton

Introduction:

This is a straight forward project to image a number of star clusters in color. Star clusters come in two forms: Open and Globular - they are collections of a million or so stars. Globular clusters orbit around our Milky Way Galaxy and along with individual old stars, they form a sort of halo around our galaxy. Open Clusters are found mainly within the spiral arms of galaxies.

Planning

• Book an observing slot, preferably when the Moon is not visible.

• Once you know when you will be observing, you will need to choose a number of clusters to observe that will generally be higher than 30 degrees. To plan your session in advance, you can use sky charts or planetarium software. In a half-hour session you should be able to obtain about 5 images once you’ve taken slewing (moving the telescope) and exposure times into account.

• You can choose to observe open or globular clusters, or some of each.
• Background Information – You can read about star clusters before your session.

Observing

Use the telescope to get color images of each galaxy you chose to observe. Use an exposure time of approximately 30-60 seconds for a globular cluster and 60 seconds for an open cluster. When your observing session is over, you will able to save your images to your computer. They will be zipped and in .fits format so you will need special software to open them.

Analysis

• If you wish, you can print the images out using the best print quality available to you. But do not worry if you cannot print the images out.

• Examine each cluster carefully, looking at the overall shape, the colors of individual stars, and any other features you can see.

• Describe your observations in your own words.

Evaluation

Think about the following questions:

• What factors might have limited the quality of the images you obtained, for example poor atmospheric seeing conditions, bright moonlight, twilight?

• Are there any ways that you would improve on what you have done if you were to do the project again using the same targets? For example, would you use different exposure times or different filters?

• Can you think how you might learn more about the clusters you have imaged by using different filters (broadband and narrow band)?