Observational astronomy/Image processing
A CCD (Charge-coupled device) 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 than 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 separate 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 separate 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.
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 DSS. Pick a server to retrieve the images from, for instance DSS from STScI. In the Target box enter M101 and then choose POSS1 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 POSS1 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 includes POSS1_Red. For Green leave it set to none. For Blue choose the image name that begins POSS1_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.
- You can use the Zoom button below the thumbnail image to change the scale of the main image. A zoom setting of 1X will give you the largest image that can be displayed, without loss of resolution. Clicking on the thumbnail will center that spot in the main window image.
The Observational astronomy/Supernova page has images of galaxies where a supernova has been observed recently. In this activity we will retrieve images taken before the supernova occurred, and compare them to the new images.
- Start Aladin and click on the Load... button. In the Server selector window click on DSS and select an image server. In the Target box enter the name of the galaxy which is UGC14 and then choose POSS1_Red for the Survey. Click Submit.
- In the Aladin multiview window click on the icon labeled pixel. In the Pixel mapping popup window click on the Reverse button to change the image from a negative to a positive.
- To find out when this image was taken click on the icon labeled prop and look for Date-Obs: which indicates that the image was taken Aug. 11, 1953.
- Compare the image displayed in Aladin to the image shown at right. Click here (or on the image) to see a larger version and to find out when it was taken (the Date is 2007-01-02T23:03Z.) This supernova was first seen on Dec. 12, 2006, so it will not be visible in the earlier image displayed by Aladin. Do you see the supernova in the recent image? If you are unsure, click here to see another version of this image that is marked to show which star the supernova is.
- More images of this supernova can be found in Category:Supernova Images or at the Supernova 2006sr in UGC 14 webpage. Compare images taken on different dates to see if the supernova has changed brightness. The difference will be very subtle in images taken a week apart. It is more obvious after about a month.
Additional helpful readings include: