A Recipe for Cooking Up Astronomical Images
Jayanne English

Back to Table of Contents

Now that you have an overall idea of what the visualization process involves, you will learn about the technical details of each stage of this process.

Visualization Stages - Technical Notes

Stage 1: The stretch:

I assume here that you have a number of exposures through different filters for the same object. Each of these monochromatic datasets needs to be converted to a black and white image. Combining images increases the RGB value per pix and the higher the RGB value the more white the pixel. So the trick is to ensure that "whites" in an individual image are stretched so that they are more grey. That is, your individual image should be dark. One has more control using astronomical software so most imagemakers start with one of these. I recommend that you use "kvis" from the Karma package.

Software comments
IDL This costs money and you have to do your own coding.
RGBSUN in IRAF Requires trial and error for the thresholds and you can only combine 3 filters.
kvis Free in the Karma suite. http://www.atnf.csiro.au/computing/software/karma/. This let's you select thresholds in real time via histograms. As well as linear and log scaling, does square root which is good for nebulae. It has additional algorithms in its pseudo-colour option (best is greyscale3). Also it exports your scaled image to Portable Pixel Map format which is accepted by all many packages.

In Karma, using kvis, you can reduce an image by loading a fits file with the filter option turned on. Select the number of pixels to "skip" (which is actually "add together"). Adjust your image and then export this to a new ppm image. If you are using another package for stretching the intensities, then a good format to save the file as is tiff format, if it is available.

Original Image Stretched Image
To larger version To larger version


Stage 2: Layering and Assigning Colours:

Next pump your output from the Stage 1 (above) into a manipulation package such as GIMP or PhotoShop. Your work is done in "layers" (rather than in colour channels) -- so open up the Layers Dialog Box in your manipulation package. Create a layer for each Stage 1 black and white dataset image. About "layers" in general:

Using GIMP Inserting images into layers
  1. In Gimp open up an image.
  2. Then open up a new image with the background set to black by selecting the foreground or background as appropriate. Ensure the Layers, Channels, Paths dialog box is open.
  3. On your original image, right clicking to get the menu options, find Edit --> Copy visible. On the new image, Edit --> Paste. This will put the B&W image into a layer.
  4. In the Layers dialog box, click on the words "floating section" and give the layer a name. This will also change layer into something you can edit.
  5. Set the mode algorithm, in the Layers, Channels, Paths dialog box. IMPORTANT: in order to see each of the layers, not just the top one, you need to select a relevant mode. "screen" is a good algorithm for combining images; it is set on each of the layers (or you won't see the one underneath).
  6. Repeat for other images.
  7. Save this layered image as an ".xcf" format file.
Assigning Colours

Then for each layer you assign a colour. This you do using the Levels tool.

  1. Click on the name of a layer in the Layers, Channels, Paths dialog box so that it is blue which means active.
  2. Go get the levels tool. [Image --> Colors --> Levels tool]
  3. Change the top menu in the levels tool from value to a colour and adjust the OUTPUT levels to get your requested colour. (For example, if you want your layer to be green, then change the menu to Red and drag the right slider in the output levels to zero... repeat this for Blue... your layer should now be Green).
  4. Repeat for each layer.
  5. Save this as a .xcf format file.
  6. Adjust values and colours until you are happy with the results. For example, if one filter image is particularly noisy (textured), this can be reduced by applying to that layer a gaussian filter with the width of the noise (e.g. a few pixels).
  7. Save changes as a .xcf format file.

Some advise: Make copies of your B&W layers and work on those so you don't have to insert images again. Sometimes turn off the other layers (by clicking on the eye icon) to check your colours.

This is very iterative. Enjoy!

Other options: Some imagemakers work on one RGB image (i.e. 3 filters) and then layer in other filters at this stage. For an example, at http://heritage.stsci.edu/public/apr1/h301filt.html is an RGB image from 3 filters with other filters layered on top.

Filter Black and White Stretch Image Colour Assigned to Image
Ultraviolet To larger version To larger version
Blue To larger version To larger version
Visual To larger version To larger version
Infrared To larger version To larger version


Stage 3: Combine the Layers

After you are satisfied in general with your colour selection, and have saved it as an .xcf file, then you flatten the image, using the Layers dialog box options, into a single tiff file with a different name in tif format.

Even better, open a new image (with a black background), Edit --> Copy Visible the display of your .xcf file and then Edit --> Paste into the new image. Set mode to screen and flatten the new image and save as a single tiff file. To flatten you use the submenus under Layers. Layers --> Merge Visible and then Layers --> Flatten.

To larger version


Stage 4: Remove the Cosmetic Defects.

Use the image manipulation tool options (like levels) for final colour and contrast adjustments. Use the clone tool to remove chip seams and cosmic rays. Chose your orientation.

Save this file as a tiff (no compression) or, in a pinch, a 100% quality jpeg.

To larger version

Back to Table of Contents