A Recipe for Cooking Up Astronomical Images
Jayanne English

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Here we give a brief overview of the four visualization stages in order to have a general idea of the process before getting into the technical details. The next section, Technical Instructions for Each Visualization Stage, will describe the process in detail.

Visualization Stages

Visualization manipulation occurs after the raw data have been processed to remove electronic noise. (The processed data are usually in FITS format, which allows one to make measurements from the data.)

Initially work on a low resolution image (i.e. reduce the dimensions by combining pixels together or reducing the number of pixels). Artists call this a thumbnail sketch. Then you can quickly produce a number of strategies (ways of combining colour and other compositional elements) and determine the optimal one. Then you can re-work your strategy on the full resolution dataset.

It should take 1-2 weeks to make a satisfying image.

This example uses HST data in the visual range of the spectrum. (For more about this interacting group of galaxies see Hubble Watches Galaxies Engage in Dance of Destruction.)

  1. Stage 1: Display the intensities in these Black and White data using a mathematical "stretch" which allows the viewer to see very faint and very bright regions simultaneously.
    Original Image Stretched Image
    To larger version To larger version

  2. Stage 2: Assign colour to the black and white images. To make a colour image you need preferably 3 or more sets of data (A dataset is initially monochromatic and in Stage 1 you produce a black and white image for each dataset.)
    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

  3. Stage 3: Combine the colour images:
    Each colour image is placed in a "layer" in an image editting package. Setting the display mode in a particular way allows each image to appear like a transparency (or positive slide) projected onto the other images. The layers are adjusted to reduce noise and to enhance the light/dark contrast and colours. Once satisfied, this multilayered image is copied to a new single layer file, producing the first draft of your colour image.

    To larger version

  4. Stage 4: Final Adjustments. The draft of the colour image is adjusted for contrast, cosmetic defects are removed and an orientation is selected.
    To larger version

The next section will describe each of these stages in more detail. Click here to go to the Technical Instructions for Each Visualization Stage section.


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