![]() SAO ds9 dispalying the SDSS g-band image of the Sombrero Galaxy, zoomed to fit and displayed with ZScale.įirst we’re just going to experiment with the basics of using ds9. coordinates as most astronomy applications do: ds9, SAOImage, SExtractor. These are simple tasks that astronomers use constantly to visually inspect images.Īs a sample image, we’ll use the Sloan Digital Sky Survey‘s (SDSS) g-band image of the Sombrero galaxy ( M104), which you can download here. A class for storing image data along with the pixel scale or WCS information. SAOIMAGE DS9 MAC OS X DOWNLOADĭownload this FITS image and open it via the File > Open menu item in ds9. You can go to File > Display FITS Header to learn everything you could ever want to know about how this SDSS image was made. When you first open the image, you’ll be greeted by an almost totally black screen. This is because ds9 will by default show you the full range of the image – from the brightest to the dimmest point – all on a simple linear scale. Since the sky is mostly dark, almost every pixel will essentially be black. You’ll probably want to change the “lookup table” to something which will better show you the dynamic range of the image. You can do this using the Scale > ZScale menu option. You can go to Scale > Scale Parameters… to see a histogram that will show you the exact brightness profile of the image. This histogram also allows you to constrict the display range so you can, for example, see the bright center of the Sombrero galaxy. Additionally, you can play with the colorbar by holding down the right mouse button and dragging the cursor. Using ds9's scale parameters to constrain the display range of the image to highlight the bright center of the Sombrero galaxy. You can tell that ds9 is only showing you a small portion of the image by the rectangle drawn in the finder window in the upper right corner of the screen. By default, it displays the image on a 1:1 pixel scale. If you want to see the whole image, go to Zoom > Zoom to Fit Frame. By the way, you can access all of the most common menu options like ZScale and Zoom to Fit from the “buttonbar,” which is the double row of buttons right above where the image is displayed. To practice moving around an image, zoom back in by selecting Zoom > Zoom 1. You can recenter on any pixel in the image by clicking on it with the middle mouse button (either in the lower display pane or the small finder window above). Notice that when you mouseover any region in the image, a zoomed-in preview will be displayed in the upper-right corner of the screen. When you move your cursor around, also notice that the current RA/DEC position of your cursor is displayed in the text boxes in the upper-left corner. Note that ds9 can only calculate these coordinates if your FITS image has a World Coordinate System (WCS) specified, as all the SDSS images do.
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