Image Processing - Tricolor Images
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# Making a Tricolor Image

To make a color image of an object, you need to combine three images of the object. One will be assigned to the blue band, one will be assigned to the green band, and one will be assigned to the red band. However, because the SDSS camera is stationary while the sky moves, images through the different filters were taken at slightly different times. So before you make a tricolor image, you will need to align the pictures.

The SDSS uses the green wavelength picture for the blue band, the red wavelength picture for the green band and the infrared wavelength picture for the red band. These assignments were chosen because they are the middle three filters.

 The Green Filter The Red Filter The Infrared Filter Combined to make a tri-color image

Exercise 2. Make a tricolor image of NGC 1087 using infrared, red, and green images.

First, align the three images. Do this by finding the location of a star near NGC 1087, and using it as a landmark to shift the three pictures.

To find your landmark star, first open the green image. Look for a star near NGC 1087 that you can clearly identify in all three images. Use your mouse to draw a small box around the star. Right-click inside the box and select "PSF." Record the x and y coordinate of the star (these are coordinates in the image, not in the sky). Do the same for the red and infrared images.

Next, figure out how to shift the red and infrared images to match the green. Find the difference between the x coordinates in the red and green image (xr - xg). You need to know the direction of the shift, so don't forget the sign! Do the same for the y coordinate (yr - yg), and then for the x and y coordinates of the infrared image.

Re-open the r image. Click on the Geometry menu and select Shift. Enter the difference between the r and g images, but reverse the sign (so you can shift the r image to align with the g image). Save the image under a new file name. Do the same for the infrared image.

Next, click on View and select (L)RGB. Type the name of the infrared file in the red box, the name of the red file in the green box, and the name of the green file in the blue box. Do not include the .fts extension. Click OK. Adjust the max and min brightness values until you see the image clearly.

What do you see?

You can also adjust the color balance. If you think your image looks too blue, you can decrease the intensity of the blue image. If you want more red, you can increase the intensity of the red image.

 Exercise 3. Click on View and then Colors Stretching. Experiment with the alpha and the intensity and see what happens. NGC 1087 is in SkyServer's Famous Places. Adjust the alpha and intensity until your image closely matches the image in Famous Places. Record the values for the colors and max and min brightness you used.