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FUNDAMENTALS OF ASTRONOMICAL IMAGING 1 LABORATORY 1: DIGITAL IMAGING WITH DS9 MATERIALS: • Network-ready PC running Windows, equipped with web browser. These are available in the student laboratories on the third floor in the Chester F. Carlson Center for Imaging Science (Building 76) and in Room 76-2205 (enter through 76-2125). If the small laboratories are not in use, you may sign them out at room 76-3211. • DS9 software (available from the website http://hea-www.harvard.edu/RD/ds9) • Images to view and process. Many are available from the Space Telescope Science Institute “Digitized Sky Survey”, which is part of the MAST project (Multimission Archive at Space Telescope), which accesses several astronomical data archives, with the primary focus on scientifically related data sets in the optical, ultraviolet, and near-infrared parts of the spectrum. MAST provides search tools and retrieval support for the following missions at the website: http://archive.stsci.edu/dss/dss_form.html INTRODUCTION: This first lab is intended to familiarize you with the image archive and imaging software. The full laboratory is expected to run for two weeks. DS9 is the latest iteration of the astronomical data visualization program “SAOImage”, which is published by the Smithsonian Astrophysical Observatory in Cambridge MA. It accepts images in the “standard” FITS image format (Flexible Image Transport System), which was developed in the late 1970s as a standard format for interchanging astronomical image data. The format includes a header that contains the relevant “metadata” pertaining to each image, such as the object, telescope used, resolution, plate scale, etc. The format is maintained by a working group of the International Astronomical Union (IAU).http://www.inasan.rssi.ru/iau/iau5/wgfits.html DS9 is available for free for several operating systems: Sun Solaris, Sun Solaris64, Linux, LinuxPPC, SGI, DEC Alpha, Windows 98/ME/NT/2000/XP, and MacOSX (darwin). All versions and platforms support a consistent set of GUI and functional capabilities. You may download DS9 and run it on your own computer. PROCEDURE: Part I: Select a cluster and download the image data: 1. Make a small list of names of star clusters from the on-line catalog of Messier objects: http://www.seds.org/messier. (click on “look at star clusters” which is on the left below “Messier Object of the Week” and “Look at Nebulae”). There are two types of clusters listed: open and globular clusters. We are interested in the open clusters, which typically are “closer” to Earth. The stars in an open cluster are gravitationally linked and “move” as a group, but the angular separations between individual stars are still rather large. Globular clusters are older, farther away, and crammed tightly together, so it will be difficult find individual resolved stars. a. This website is maintained by the Students for the Exploration and Development of Space, which began at the University of Arizona; you might also want to surf around their website (http://www.seds.org), which FUNDAMENTALS OF ASTRONOMICAL IMAGING 2 has lots of useful information. Their website is hosted by the Lunar and Planetary Laboratory (LPL), whose site is http://www.lpl.arizona.edu/. 2. Go to the Space Telescope Science Institute’s Digitized Sky Survey (DSS) website, http://archive.stsci.edu/dss/dss_form.html, and fill in the form to obtain the images of a star cluster from your list. The images were created by digitizing photographic plates on a scanner. To fill in the form and download images: 1. Enter name of the cluster, e.g., “Pleiades” or “M45”, in the “object name” box, and click on “get coordinates”. The Pleiades, also known as the “Seven Sisters” is probably the best known open cluster. It is visible in the winter as the shoulder of Taurus (the bull), rising in the east on December evenings. Most people can see six of the seven brightest stars, but some people can see as many as 13. As an aside, the Japanese name for the Pleiades is “Subaru” – compare the hood ornament of a Subaru to the pattern of the Pleiades. 2. Choose a sky survey to search, and download first blue (λ = 400 nm) and then red (λ = 600 nm) images of the star cluster. (You may need to repeat these steps until you find a cluster for which both blue and red images are available.) 3. Choose the “display” option to preview the available images for the cluster. 4. Once you have decided on a cluster to analyze, specify that you want the images in FITS format (the default), and that you want to “download” (rather than “display”) the images. 5. Click “retrieve images” and specify the file name and directory where you want to put the image. Repeat for the second image of the cluster. To process the images: 1. Start up DS9 and read in one of the images you just downloaded. 2. Experiment with manipulating the images using DS9. For example: a. Move the cursor across the image, and note how you can measure the locations and intensities of individual pixels using the real-time display at the top of DS9. b. Change the image display intensity and contrast, by holding down the right mouse button while moving the cursor on the image. c. Change various attributes of the image display (zoom, scale, color) using the buttons at the top of the image. d. Load two images at once: The “frame” option on the top row allows you to read in a second image and display it next to the first. This is a handy way to compare red and blue images of the same star cluster side-by-side. Load the second image via “Frame→Tile→New”. Click on the empty half of the screen and open the second image. e. Print each image to a laser or inkjet printer (B&W is fine). FUNDAMENTALS OF ASTRONOMICAL IMAGING 3 Part II: Image Analysis 1. Determine the pixel scale and field of view of each image by measuring the vertical size of the image in pixels and comparing it to the known size of the field available from the download site. 2. Measure the peak pixel intensities (values) of at least 10 stars (preferably 30-50 stars) on each image. Note that the gray values of pixels in the “black sky” are not zero – to get a good measure of the brightness of that star you must subtract the value of the sky near each star’s image. a. Measure the gray value Pp of the maximum pixel of the star b. Measure the average gray value of the surrounding sky pixels Ps. c. The value to be plotted is ∆P = Pp — Ps. 3. Pick the same stars on each image; try your best to choose stars that you think are part of the cluster, yet are not saturated (overexposed) in either image. A saturated image of a star may be distinguished by noting the “gray values” (“brightnesses”) of several pixels in the brightest part of the star. If these all have the same gray value, then you can assume that the image is saturated. A plot illustrating saturation in shown in the Figure. 4. Make a table for each set of measurements (one table for each image), listing the [x,y] pixel values for each star you picked, the peak pixel value, the pixel value of nearby sky, and the difference (star minus sky), and the logarithm (base-10) of these pixel values. It is easiest to make this table in a spreadsheet program, such as Excel. Your laboratory assistant can show you how to do this if you have not done it before. 5. Graph the ratio of the logarithm of the (sky-subtracted) peak amplitude of the ratio of red-to-blue pixel values for each star to the logarithm of the (skysubtracted) peak pixel value in the red image. That is, plot the measured red value on the x-axis, and the ratio of red/blue on the y-axis. That is, plot ⎡ ∆P ⎤ log10 ⎢ red ⎥ vs. log10 [ ∆Pred ] ⎣ ∆Pblue ⎦ FUNDAMENTALS OF ASTRONOMICAL IMAGING 4 LABORATORY REPORT There is no fixed required length to a lab report; I sometimes (and only half-jokingly) say that a lab report “should be as long as necessary, but no longer.” For this lab, the necessary parts in the report are: 1. Objective 2. Observations: a. Name of the cluster you chose, and the reason you chose it, aside from availability of both blue and red images (was it the concentration of stars in the image? Centering of the cluster in the image? Etc.). Include the basic data from the on-line Messier catalog here: Right ascension and declination, distance, apparent magnitude, and approximate angular size. b. The attributes of each image (wavelength, pixel scale, field of view) and how you determined these c. Overall description of the star cluster imaged (does it take up the whole field of the image? Roughly how many stars seem to be part of the cluster? How many of these are very bright [saturated]?) 3. Analysis and Results: a. Criteria for selecting 20-50 cluster candidate stars b. Hardcopy of both images, with selected stars marked and numbered on each c. Data table for each image (table of x,y peak pixel for 10 stars) d. Graph of red/blue image intensity ratio vs. red image value for your (minimum of) 10 stars 4. Conclusions: a. Do you think all of the stars you picked actually belong to the cluster? Why or why not? b. Is the color (red/blue intensity ratio) of a star related to its brightness? c. What errors or problems in measurement may have affected your results?