Purpose: To produce three-color composite images of supernova remnants from Chandra X-ray Observatory data showing the areas of low, medium and high energies.
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.
Although computers and software are used extensively, scientists and programmers go through painstaking calibration and validation processes to ensure that technically correct images are produced. The colors used for the images are selected to emphasize specific information within the data. The color selections used by the Chandra X-ray images are usually associated with intensity or brightness, or energy. For example, in the yellow and orange Chandra X-ray image of the Cas A supernova remnant to the right, the white and yellow colors represent the areas of highest X-ray intensity, the orange to red areas represent the areas of lower intensity, and the black represents little or no emission.
The X-ray image of the pulsar 3C58 below shows an image constructed by selecting different X-ray energy bands from the data and representing them with different colors - red, green, and blue. The result is a 3-color composite image. The low, medium, and higher X-ray energy bands of the Chandra data are shown as red, green, and blue respectively. In this particular image, red, green and blue represent X-ray energy bands of 0.5 to 1.0 kiloelectron volts, 1.00 to 1.5 kiloelectron volts, and 1.5 to 10 kiloelectron volts, respectively.
In this activity, two image analysis software programs, ds9 and ImageJ, are used to construct a three-color composite image of the Cas A supernova remnant. The red, green, and blue regions in the composite will show the intensities of low, medium, and high-energy X-rays - as in the composite image of 3C58 to the left. The ds9 software utilizes data sets and astrophysical analysis tools from the Chandra X-ray Observatory. The program uses the same process that X-ray astronomers follow in analyzing the data from Chandra observations. The download instructions to install the ds9 toolbox on your desktop are located at http://chandra-ed.harvard.edu/install.html. The introduction at http://chandra-ed.harvard.edu/learning_ds9overview.html describes the overview and purpose of the software and gives a short summary of the Chandra mission. The tutorial for using the ds9 software is located at http://chandra-ed.harvard.edu/learning_ds9.html. ImageJ is a software program developed by the National Institute of Health and is available for download from http://rsbweb.nih.gov/ij/download.html.
NOTE: It is not necessary to read the tutorial before beginning the 3-Color Composite Images - ds9 Activity . Complete instructions to use the image analysis tools in ds9 are given in the following procedure.
1.Download and install ds9 according to the instructions on the Chandra-Ed web site.
2.Open ds9 and maximize the screen. From the pull down menus, choose Analysis>Virtual Observatories and choose Chandra-Ed Archive Server or New Rutgers X-ray Analysis Server from the list that appears.
3.Click Obs ID 114 ACIS OBSERVATION OF CAS A (first 5K seconds only) in the new window that appears.
4.Use Analysis>Chandra Ed Analysis Tools>Energy Cut to show only the x-rays in the soft band (lo = 0.6 KeV, hi =1.65 KeV). Leave "Display all images in tile mode?" and "Save image for further analysis" checked.
5.When the new image comes up in a second frame (the frame should be lined in blue, if not, click this frame), go to File>Save Frame as FITS. Name this file casared.fits. Choose Frame>Delete Frame to leave only your original image.
6.Repeat steps #4-5 for the medium (1.65-2.25 KeV) and the hard (2.25-7.50 KeV) bands, saving them as casagreen.fits and casablue.fits, respectively.
7.Download and install ImageJ and run the program.
8.Go to File>Open three times to load casared.fits, casagreen.fits and casablue.fits.
9.Go to Image>Color>RGB Merge. From the pull-down menus in the new window, choose the appropriate files to go with each color as shown below and click OK.
10.You now have a three-color composite of Cas A. You can use the following tools to enhance the image to emphasize different features.
- Image>Adjust/Brightness/Color Balance
Use the same method to produce 3-color composite images for the following two objects.
Obs ID 7639 - A Deep Chandra Observation of the Tycho Supernova Remnant
Obs ID 115 - ACIS Observation of Tycho and Kepler
If the Obs ID for the SNR you wish to investigate is not in list in the internal browser window, scroll to the bottom of the page and click on Unofficial Chandra Public Archive. Enter your Obs ID and click "Search" - then click on the Title of a returned observation to load it into ds9.