• Macquarie Home »
• Faculty of Science »
• Physics and Astronomy »
• Astronomy

Asteroid Investigation

Making a Colour Image with IRIS

Another activity you might like to undertake is to create a colour image with your data. Not only will you produce a stunning colour image, you can use this exercise to teach students about what happens when different coloured light combines. It can be used (depending on the data) to show that stars have different colours and lead on to a discussion of why this is so. You can use this exercise to show the direction the asteroid is moving, provided you know which colour was observed first.

Note: In order to do this, you must have 3 successive images of the same object in the same field, one taken through the B filter, one through the V (green) filter and one through the R filter.

1. Start IRIS by double-clicking on the program icon.

2. Change the settings.

   Select File » Settings... In particular, you must indicate in the Working Path where your data is located. Your data files must have the extension .fit, .fts, or .pic for IRIS to find them AND the names must only contain lower-case letters. If your data files have capitals in the filename or the extension .fits (this is likely if you have used the Faulkes Telescope), you need to rename them using lower-case letters and replacing the .fits with .fit.

   
   Figure 20: IRIS Settings.

3. Load and superimpose the images taken through the B, V and R filters.

   Select View » (L)RGB... You need to put the filename (drop the .fit extension) in the appropriate box and click OK.

   
   Figure 21: (L)RGB control panel.

   This will load and superimpose the images. The screen will probably appear black initially and in order to see the resultant image more clearly, click Auto in the Threshold window. DO NOT at any point click on any of the four coloured buttons in this Threshold window. Alternatively, you might want to play with the sliding bars in the Threshold window to obtain a clear view of the objects in the image. (Note: the Threshold window sets the brightness at which the pixels in the image are shown to be white — in this case at a brightness of 32767 — and the brightness at which the pixels in the image are shown to be black — in this case a brightness of 0).

   
   Figure 22: Threshold window.

   If you look closely at the stars in this superimposed image, you may be able to distinguish the image of the stars taken through the red filter (red dots), the image of the stars taken through the visual filter (green dots) and the image of the stars taken through the blue filter (blue dots). How much the telescope has moved between each image will determine how closely aligned the stars are at this stage. You may also notice that the image itself has a general hue (i.e. the background is not black - in this case it has a reddish hue). This will depend on how much light was let through each filter (in this case, the light coming through the red filter was stronger than that coming through the green or blue filters) and may not be the case if you have previously selected Modified Equalization (refer to the Colour Images teaching module).

   We are now going to shift each of these individual images (red, green, blue) so that they line up on top of each other. This process is quite subjective but you should be able to obtain a reasonable result.

4. Line up the images.

   Select View » (L)RGB...

   Panel A indicates which image you are manipulating (red, green or blue). Panel B is the controller where you move the image. The Step field in Panel C is where you indicate by how much you want the image to move at once.

   
   Figure 23: Lining up the images.

   Start with the visual (green) image (Green is checked in Panel A) and try to align it with the red image. Do a coarse adjustment to begin with of 2 pixels (input 2.0 into the Step field in Panel C). Now, using the controls in Panel B, move the green image until it lines up better with the red image. What you are trying to do is make the stars round and their centres white (when you combine red, green and blue light — you end up with white light). You will need to adjust the Step to be much smaller as the alignment improves. Repeat this for the Blue image. Do not click OK until after you have completely finished lining up the images.

   Once you are satisfied with your alignment, click OK.

   
   Figure 24: Aligned!

   You will notice in this example that the stars have a reddish ring around the outside. The red images are slightly larger than the others. This may be due to observing conditions, different exposure times for the different coloured images or variations in the transmission of light through the different filters. Unfortunately, there is not a lot we can do about this.

5. Adjust the White Balance.

   In an effort to eliminate the reddish cast the image has, select View » White Balance Adjustment... Here again it's a matter of playing with the controls until you achieve the look you want. You may also want to play with the Gamma Adjustment, Contrast adjustment and Saturation adjustment under this View menu. There is no rule about how to make the image look its best, it really is a matter of you tweaking the controls. It's best to be able to see your asteroid as you are making the adjustments.

   
   Figure 25: Adjusting the white balance.

6. Save your colour image.

   Once you are satisfied with your image and you are ready to save it, select File » Save... This gives you multiple options for your save format. If you want to use the coloured image in IRIS (e.g. to make a mosaic out of several images) then you need to select .pic format. Otherwise, you must save it in a format compatible with your other software (e.g. tif, jpg). DO NOT save it as a FITS file or your work will be lost.

   
   Figure 26: Saving your image.

Here is the final image showing the asteroid (one blue image, one green image and one red image) moving against the backdrop of stars.

Figure 27: The final image.

Return to the main page for this module