Making Colour Images with IRIS - Macquarie Universityphysics.mq.edu.au/astronomy/space2grow/modules/colour/ColourIm… · Introduction – Colour and Astronomical Imaging The electromagnetic

ASISTM Faulkes Telescope - Deep Space in the Classroom http://www.astronomy.mq.edu.au/deepspace/

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Making Colour Images with IRIS

Contents Introduction – Colour and Astronomical Imaging ...................................... 1 Downloads ..................................................................................................... 2 Part 1: Making a Colour Image ..................................................................... 3 Part 2: Display Options ................................................................................. 6 More Colour Projects .................................................................................... 8

Introduction – Colour and Astronomical Imaging The electromagnetic spectrum covers light of all wavelengths from radio to gamma rays (see Figure 1). All these forms of radiation travel at the speed of light. Visible light, i.e. that which is detected by the human eye, occupies just one small window in this spectrum. This window, also known as the optical spectrum, covers wavelengths between 4000 and 7000 Å. Different wavelengths of light appear to us as different colours. White light contains all the colours of the rainbow and may be broken up into different colours by a prism. Light at 4000 Å looks blue and light at 7000 Å looks red.

Figure 1: The electromagnetic spectrum.

In astronomical imaging, it is useful to observe the colour of celestial objects because colour tells us about the temperature and ages of stars and the gases contained within nebulae or gas clouds. Most astronomical imaging is done with CCD detectors, much like the ones used by digital cameras. CCDs are typically sensitive to all wavelengths from approximately 3800 to 7500 Å. However, astronomers don’t just take a colour photo. The detectors used are not designed to work that way. Instead, filters that transmit selected wavelengths or passbands are used to obtain light of specified colours. The detector records the intensity of light it receives, but not the colour. Image files are carefully labelled to note the filter that was used for each observation. Red, green and blue (RGB) filters cover the optical spectrum (see Figure 2). These colours are the primary colours of light and recombining light of just these three colours in equal amounts produces white light. Unequal combinations can produce the complete range of optical colours.


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Figure 2: Passbands of blue, green and red filters.

The Faulkes Telescopes offer a range of different filters (see http://faulkes-telescope.com/education/planning/filters). Note that in astronomy, V (for “visible”) is used to refer to the G passband. The R, V and B filters match the spectral range of the human eye, so recombining separate images taken through each of these three filters reproduces a natural-looking colour image. The intensity of the light seen in an image is a function of the intensity of the light emitted by the object, the exposure time (objects look brighter with longer exposures) and the transmission allowed by the filter.

Downloads From here you can download everything you need to get you started with the Colour Images project. 1. Download and install IRIS This software will allow you to view FITS files from the Faulkes Telescope and combine them to make a colour image. Follow the instructions for Configuring IRIS to set your working path to C:\data. 2. Download the image files Save these files to your hard drive in the directory C:\DATA ngc2681-b.fit, ngc2681-r.fit, and ngc2681-v.fit are three images of the galaxy NGC 2681 obtained with the Faulkes Telescope North. The images were made through B, R and V filters, respectively. For use with IRIS, the filenames must have the extension .fit (not .fits) and all letters should be in lower case. If you download other FITS files or obtain your own data to use with this project, you may need to rename the files accordingly. Differences in the appearance of NGC 2681 on these three images are clearly seen in Figure 3. Part of this is due to the different intensities of the filter transmission, but you may also note differences in the shape of the galaxy’s features. This is because different filters transmit only specific wavelength ranges and block other wavelengths so that, for example, redder parts of the galaxy show up strongly though the red filter but are hardly transmitted at all through the blue filter.

Figure 3: Images of NGC 2681 taken through the B (left), V (middle) are R (right) filters.


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Part 1: Making a Colour Image 1. Open the three images together in IRIS Open IRIS. Select View > (L)RGB. This will open a window that looks like Figure 4. You will need to type the filenames of the three files into the boxes labelled Red, Green and Blue, omitting the .fit extension. Make sure you have the correct colour files in the correct boxes. Initially set the Step to 1. Then click OK.

Figure 4: Loading the three image files into IRIS.

The display will look very dark, with just a few bright spots. Click Auto on the Threshold dialog box and your display should looks like Figure 5.

Figure 5: The three colour images are loaded. The Threshold is set to Auto.

At any time you can zoom the image in or out using the zoom buttons: The left one zooms in, the middle one zooms out and the right one restores the image to its original size. Notice that the three colour images don’t line up exactly (see Figure 6). The brighter stars show up as three different coloured dots, with some overlap. The images are not perfectly aligned because there were small shifts in the pointing of the telescope between the three exposures. Next, you will line up the three images properly. Also note the tiny white spots in


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the image. If you examine these carefully in IRIS, you will see that they are little squares. These are not stars, but defective pixels on the CCD detector.

Figure 6: Zoom in to see a star in detail. The three coloured images overlap but are not properly aligned.

2. Align the red and green images Select View > (L)RGB to re-open the dialog box shown in Figure 4. You will use the up, down, left and right arrows to move the image that is checked in the lower right panel (in this example, the Red one). The step size (measured in pixels) may be altered to move the image by larger or smaller amounts. You may wish to change this to a smaller number, e.g. 0.2, as your images get more closely aligned. Unfortunately, when you click an arrow the image un-zooms. However, you can keep zooming in to check. Figure 7 shows what a star should look like after the red image is lined up with the green one. Notice also that the bad pixels now show up as separate red and cyan dots.

Figure 7: The red and green images are aligned.

3. Align the blue image with the others Next, repeat this process to align the blue image, by checking Blue in the (L)RGB dialog box and clicking the arrows. When properly aligned, your star images should look something like the one in Figure 8.

Figure 8: The three images are aligned.


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When you are completely happy with your alignment, click OK. You should be able to see the galaxy and the bright stars in the image. Any bad pixels will now look like three separate dots of red, green and blue. Where you see a single pixel of just one colour (there is a green one at lower right of Figure 8), this may be caused by a cosmic ray. Cosmic rays are high-energy particles travelling through space. Sometimes they strike Earth and, sometimes, they’ll land on your CCD detector! The detector “sees” the cosmic ray the same way it “sees” photons of light, by measuring a disturbance that it records as an electrical signal. Hence, cosmic rays frequently show up on CCD images, but they’re a bit of a nuisance for astronomers! 4. Save your combined image 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 should save it in a format compatible with your other software (e.g. .tif or .jpg). Note: If you save the result as a .fit file and try to reopen it later, you will not see the colours. FITS files only record pixel intensity and if you try to save in this format and re-open the file later, you will only see black-and-white. 5. Record your image registration information If you wish to repeat the alignment later so that you can experiment with various display options, you should record the offsets of the images. Re-open the (L)RGB dialog box and record the DX and DY values in a notebook for each of the images you have moved (see Figure 9). When you wish to repeat the alignment: • re-open IRIS; • open the dialog box; • enter the filenames; • enter the offset values for the appropriate image (e.g. for the red image, check the Red

button) into the DX and DY boxes; • click Apply to enter the settings for each image; • click OK to display the aligned colour image.

Figure 9: Entering the DX and DY offsets for the Red image.


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Part 2: Display Options 1. Improving the colour balance from the command window The different levels of transmission through the filters mean that your image background looks reddish-brown rather than black. To change the background to black and, at the same time, adjust the colour balance throughout the image, hold the left mouse button and drag out a small rectangle somewhere on the background far away from the galaxy (see Figure 10).

Figure 10: Selecting a section of the background.

Then open a command window by clicking the icon. Click onto the line after the > symbol, type black and press Enter. If you get an error message at this point, you may need to set your image size to x1 (alongside the zoom buttons described earlier). After this step, your image should appear to have a black background. You can also adjust the colour of the bright objects in the image. Drag another tiny box over the centre of one of the brighter white-looking star images, making sure not to include any background. Then type white on the command line. Your image will now look something like Figure 11. Save this image in a standard image format (such as .jpg or .tif) with a new name.

Figure 11: Left: The combined images of NGC 2681 with background adjusted to black and bright stars adjusted to white. Right: a detail view of the galaxy.


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You will notice there is a big difference between the images in Figures 10 and 11. One problem with colour astronomical images is in knowing what the real or accurate colour of an object is. For more details about this problem, you may wish to look at the following link: http://www.astropix.com/HTML/I_ASTROP/COLOR.HTM 2. Adjusting White Balance using slider controls To perform an operation similar to that in the previous step, select View > White balance adjustment. You can move the sliders on this dialog box to adjust the colours to something pleasing.

Figure 12: The White Balance sliders.

3. Modified Equalization One other quick option you may wish to try is Modified Equalization from the View menu. Note: you CANNOT UNDO this operation to go back to the previous view. This will show up the outer extremities of the galaxy, and you will also see the diffraction spikes from the bright stars. Diffraction spikes are caused by structures inside the telescope (see how the secondary mirror is supported at the top of the telescope in the photo Gallery) and the wave-like behaviour of light when it encounters these obstacles.

Figure 13: The same image displayed with Modified Equalization.


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4. Threshold Options and False Colour On the Threshold dialog box, there are icons that perform quick operations on the display.

The one on the right: inverts the display (i.e. shows the opposite colours or converts from black-on-white to white-on-black, depending on what you have to begin with). This is reversible by clicking the same button again. Note 1: You CANNOT UNDO the other operations performed by these buttons. Starting with an image like Figure 11,

clicking the button will display something like Figure 14 – what is known as a false colour image. Note 2: If you wish to save this image, you’ll need to take a screen capture, as saving from IRIS as a .jpg will produce a black-and-white image. Figure 14: An image in false colour.

5. False Colour and Modified Equalization

Starting with an image like that in Figure 14, selecting View > Modified Equalization will produce an image similar to the one in Figure 15.

Figure 15: A dramatic false-colour image using Modified Equalization.

You may also wish to refer to the IRIS user manual (PDF file) which gives you more details about fine-tuning of colour astronomical images.

More Colour Projects • If you wish to make more colour images with archive data, refer to these teaching

modules: Asteroid Investigation; Interacting Galaxies. • In this exercise, you worked with the primary colours of light – red, blue and green. In

Figure 6 you can see yellow and cyan where the green image overlaps with the red one and the blue one, respectively. How can you make magenta?

• Magenta, yellow and cyan are the secondary colours of light. Where else are these

colours used?

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Making Colour Images with IRIS - Macquarie Universityphysics.mq.edu.au/astronomy/space2grow/modules/colour/ColourIm… · Introduction – Colour and Astronomical Imaging The electromagnetic