Color and Digital Imaging in Astronomy

 

 

Take a look at the image below.  It’s a photo I took of the three colored magazine boxes.  Can you tell which box reflects red? Green?  Blue?

 

 

It might surprise you to learn that our eyes only detect three colors…red, green, and blue.  It’s our brain that combines them in different amounts to make all the colors of the rainbow—Red, Orange, Yellow, Green, Blue, Indigo, and Violet (ROY G BIV).  If you don’t believe me, take a close look at your television screen at home…you’ll notice only three different colored pixels, R, G, & B!

 

If you take the digital image above and separate the color channels using a computer program you’ll get the three images below:

 

 Red Channel     

 

 Green Channel   

 

 Blue Channel     

 

Take another look at the yellow box.  Notice that it reflect both red and green light—when you eye detects red and green at the same time your brain sees yellow!  Check out the colored light projector in the room…

 

 

 

That’s neat, but what does all this have to do with astronomy?! 

 

A lot!  One more thing…

 

What do astronomers look at with their telescopes?

 

What does a star’s color tell them about that star?  To answer that…check out the following light bulb!

 

 

 

Can you determine which light bulb filament (the glowing thin metal wire inside) is hotter?

 

How do you know which is hotter?  As something gets hotter, what colors does it start glowing?

 

(I’ll tell you a secret…all of you are hot enough to emit a type of light that you can’t see with your eyes…do you know what it is?)

 

Now let’s look at the barely glowing bulb in red light and blue light:

 

 Red Light                     Blue Light

 

The relatively “cool” filament doesn’t emit much blue light, just red. 

 

 

 

Now let’s look at the brightly glowing bulb in red light and blue light:

 

 Red Light                    Blue Light

 

The relatively “hot” filament emits both red and blue light.

 

 

 

Astronomers use a star’s color to help determine how hot the surface of the star is.  In fact they have an entire classification scheme based on color and surface temperature!

 

Now you be an astronomer by looking at the star field below.

 

This color picture was taken from the Hubble Space Telescope and shows a cluster of stars.  Notice the color differences in some stars.

 

 

Use your computer to look at the separate color channels of the picture above.

 

Looking at separate color channels using GIMP at UW:

  1. Right-click your mouse somewhere on the blank part of the screen and select "New Terminal".
  2. Change to the proper directory by typing “cd  /d/papc0#/Astrocamp” where “#” is the number on top of your computer monitor.
  3. Type “gimp” and press enter
  4. Go to “file” and “open” the file named “3ColorStarField.jpg”
  5. Right-click on the image and go to the “Layers” menu
  6. Choose “Layers, Channels & Paths”
  7. Click on the “Channels” tab in the new popup window
  8. Now click on the icon shaped like an eye to turn on and off the red, green, and blue parts of the image.
  9. When you have answered the following questions with a partner, exit the program.

 

Looking at separate color channels using Paint Shop Pro 8:

    1. Start Paint Shop Pro 8.
    2. Go to File pull down menu and Open the correct image—3ColorStarField.jpg
    3. Go to Image pull down menu and then choose Split Channel
    4. Choose Split to RGB and then examine the red, green, and blue color channels
    5. Answer the following questions with a partner.

 

Looking at separate color channels using Photoshop CS:

      1. Start Photoshop CS.
      2. Go to File pull down menu and Open the correct image—3ColorStarField.jpg
      3. Go to the Channels Tab on the Layers Palette (F7 if it is not already on the screen)
      4. Now click on the icon shaped like an eye to turn on and off the red, green, and blue parts of the image.
      5. Answer the following questions with a partner.

 

Answer the following questions with your partner:

 

  1. Which stars in the field are the hottest?

 

  1. What do you notice about the bright star on the far right of the image? 
  2. Is it hot or cool? 
  3. How do you know?

 

  1. If you were an astronomer and only wanted to take photographs of very hot starts, while filtering all the cool stars out of the image, what type of color filter would you use on your telescope?

 

Check out the following website to learn more about astronomers and color filters:

 

http://hubblesite.org/sci.d.tech/behind_the_pictures/meaning_of_color/index.shtml

 

 

 

Now try and make your own RGB color combined image using photographs taken through various filters (using IDL at UW).

 

1.  Right-click your mouse somewhere on the blank part of the screen and select "New Terminal".

2.  If necessary, change to the proper directory by typing “cd  /d/papc0#/Astrocamp” where “#” is the number on top of your computer monitor.

3.  In the window that pops open, type "idl" and hit enter.

4.  Type "tctool" and hit enter.

5.  Click on the button that says "Read Red Image". Scroll down under "files" and select "M13R.fits". Click "OK".

6.  Click on the button that says "Read Green Image" and select "M13V.fits". Click "OK"

7.  Click on the button that says "Read Blue Image" and select "M13B.fits". Click "OK".

8.  Click on the "Make True Color" button, and watch the image appear.

9.  Play with the "Min" and "Max" for each color.  Remember after you change a number you must hit "enter" and select "Make True Color" again to see your change.

10.  Repeat steps 4 - 8 using the other two image sets, "M51" and "M27". For the M27, the image with "HA" is red, "OIII" is green, and "U" is blue.

11.  If you make a picture and want to keep it, let us know! Each person can print one picture to take home.

 

M13 - The Hercules Globular Cluster

M51 – The Whirlpool Galaxy

M27 – The Dumbbell Nebula

 

You can also use these programs to color combine images taken through filters:

 

In Paint Shop Pro 8:

1. Open the three images you would like to color combine.

2.Go to the Image pull down menu and choose Combine Channel

3.  Then choose Combine from RGB…

4.  Select which image belongs to what color channel and then click OK.

5.  You may now save the new image or make further adjustments with the software.

 

In Photoshop CS:

1.  Open the three images you would like to color combine.

2.  Use the Channels Palette (F7 if not already on screen) and select the arrow for the pull down menu

3.  Select the Merge Channels… option

4.  Change the Mode to RGB Color and click OK

5.  Specify which file goes with which color and click OK

6.  You may now save the new image or make further adjustments with the software.

 

 

Extensions:

  • Encourage students to take digital images of objects at home or school such as computer monitors, televisions, incandescent and fluorescent lights and colored objects like posters.
  • Ditto, but take the images through filters
  • Separate or combine images as appropriate