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Introduction and Background - Images and Color
The colors we see are the result of how the human eye and brain perceive different wavelengths of light in the visible part of the electromagnetic spectrum - roughly radiation in the range of 380 nm to 740 nm. The ability of the human eye to distinguish colors is based on the varying sensitivity of different cells in the retina to light of different wavelengths.
The retina contains three types of color receptor cells, or cones. Light, no matter how complex its composition of wavelengths, is reduced to three color components by the eye. For each location in the visual field, the three types of cones yield three signals based on the extent to which each is stimulated: red, blue and green. True-color images of a subject are images that appear to the human eye exactly like the original subject would: a blue sky is blue, a red apple is red, and green grass is green.
A false-color image is an image that depicts a subject in colors that differ from those a faithful full-color photograph would show. The term false-color is typically used to describe images whose colors represent measured intensities outside the visible portion of the electromagnetic spectrum. Astronomical images are false-colored images. A false-color image is not incorrect - it is an arbitrary selection of colors chosen to represent some characteristic in an image, such as intensity, energy or chemical composition. The colors selected are representative of the physical processes underlying the objects in the images, and display in a single image as much information as possible that's available from the data. The data are transported into image analysis software where adjustments are made to emphasize the individual features or processes that scientists are interested in - or enhanced to make the images more interesting to the public. See X-ray 101 for more information on how Chandra "sees" X-ray light.
Flexible Image Transport System (FITS) Image File Format
FITS is a digital file format used mainly by astronomers to store, transmit, and manipulate scientific data and images. The FITS format was designed specifically with scientific data in mind and includes a rich, human readable, ASCII metadata header capable of providing very detailed information about the contents of a particular file. These details include very specific information regarding the nature and source of the data, the time when the data were created, as well as photometric and spatial calibration information. In the openFITS project, you will use raw or minimally processed FITS data from the Chandra X-ray Observatory to create your own color images! The FITS Index will be a repository for all of the FITs files used in the project.
A Word on Software and Open Source
The industry standard software for processing & imaging work of this nature is Adobe Photoshop. Photoshop comes in a variety of "flavors" priced according to the needs of the user and is available for both Windows and Mac (unfortunately, not Linux). While we would like to maintain an open source workflow in the openFITS project, there are certain advantages to using Photoshop and the FITSLiberator plugin that will make this difficult. For the first tutorial, and anywhere else that is applicable, the Gnu Image Manipulation Program (GIMP) will be used as this is completely open source, available for all platforms, and will be suitable for the more straightforward images. Although GIMP is perfectly capable of reading FITS files, it is extremely limited in its control over image scaling. As the difficulty level of the images increases, this limitation will force the use of Photoshop with the FITSLiberator plugin developed in conjunction with ESA/ESO and NASA. This plugin gives the user complete control over the appearance of data before it is projected to the screen and most of the scaling information is lost.
Image Scaling: A Quick Lesson In Bit Depth
Any image that you view on your computer screen has a bit depth associated with it. Bit depth refers to the number of colors that can be displayed in any one pixel in your image. The higher the bit depth, the more colors used in the image, and consequently, the larger the file size. In an 8-bit gray-scale image, for example, each pixel within the image can appear as one of 256 different shades of gray. The number 256 comes from the fact that bit depth increases as powers of 2, and 28 = 256. A 16-bit image, therefore contains 216 or 65,536 potential color values for each pixel. Most astronomical images come in the form of 16-bit images, and with over 65,000 color values per pixel, there is more information, or dynamic range in an image than can be displayed on the screen, or viewed by human eyes. Image scaling is used to project the dynamic range of an image into a range that is suitable for display on a monitor. A scale function manipulates the pixel values before projecting them to the screen. The example on the left shows a linear scaling of the supernova remnant E0102 on top of the same image scaled logarithmically.The ability to properly scale your data before working with it is crucial to creating dramatic and interesting images. This is why we will move towards using Adobe Photoshop with the FITSLiberator plugin for the more challenging images.
Installing GIMP and G'MIC
Point your browser to http://www.gimp.org/downloads and download the latest version of GIMP. If you're looking for the simplest installation of GIMP on a Mac using a binary, then use: http://gimp.lisanet.de/Website/Download.html. Note that these pages are for Mac and Linux versions of GIMP. If you need to download GIMP for Windows, follow the instructions on http://www.gimp.org/windows.
G'MIC is a plugin for GIMP that defines a set of various filters as well as image denoising and enhancement algorithms. It includes a very nice GUI implementation of the GREYCstoration contour preserving smoothing algorithm that is particularly well suited to smoothing X-ray data. To install the G'MIC plugin for GIMP, point your browser to http://registry.gimp.org/node/13469 and download the appropriate installation file for your OS. The downloaded file is the GIMP plugin executable and it must be moved to the standard GIMP plugin directory to be seen by GIMP. On a Mac, and with a standard GIMP setup, this directory typically resides in /Users/[user]/Library/Application Support/Gimp/plug-ins. Simply copy the file to this location and the next time GIMP is started up, the GREYCstoration smoothing plugin will show up in Filters->Enhance->GREYCstoration.
Disclaimer: The Chandra X-ray Center and the Harvard-Smithsonian Center for Astrophysics do not endorse any particular vendor's products. Please read all license agreements before downloading any software.
Revised: September 13, 2012