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3.2 Displaying HST Images

This section will be of interest primarily to observers whose datasets contain two-dimensional images, as it explains:

3.2.1 The display Task

The most general IRAF task for displaying image data is the display task, the best choice for a first look at HST imaging data. To display an image, you need to:

  1. Start an image display server, such as SAOimage, in a separate window from your IRAF session, either from a different xterm window or as a background job before starting IRAF. To start SAOimage, type the following in any xterm or other system window:
          saoimage &

  1. Load the images.tv package from the window where you're running IRAF:
          cl> images
im> tv

Several different display servers, including SAOimage, SAOtng (the next generation of SAOimage), and Ximtool, can be used with IRAF. SAOtng may be retrieved via anonymous FTP from sao-ftp.harvard.edu in the directory ~ftp/pub/rd. Ximtool may be retrieved via anonymous FTP from iraf.noao.edu in the directory ~pub/v2103-beta. Ximtool is particularly handy if you want to blink images.

  1. Display the image with the IRAF display task, using the syntax appropriate for the file format (Chapter 2 explains how to specify GEIS groups and FITS extensions):
          tv> display fname.c0h[2] 1 (GEIS group 2)
tv> display fname.fits[11] 1 (FITS extension 11)
tv> display fname.fits[sci,3] 1 (FITS extension sci,3)
Note that when using display or any other task on GEIS images, you do not need to specify a group; the first group is the default. However, when working with FITS files you must specify an extension, unless the FITS file contains only a single image in the primary data unit and has no extensions. Figure 3.2 shows how to display group two of a WF/PC-1 image .

If you want to display all four chips of a WF/PC-1 or WFPC2 image simultaneously, you can create a mosaic with the STSDAS wmosaic task in the hst_calib.wfpc package. Type help wmosaic for details.

Figure 3.2: Displaying an Image

Modifying the Display

There are two ways to adjust how your image is displayed:

im> disp w0mw0507v.c0h 1 zrange- zscale- z1=2.78  z2=15.27

Notice in Figure 3.2 that when you run display, the task shows you the z1 and z2 values that it calculates. You can use these starting points in estimating reasonable values for the minimum and maximum intensity display parameters.1

If you want to display an image with high dynamic range, you may prefer to use logarithmic scaling. However, the log scaling function in SAOimage divides the selected intensity range into 200 linearly spaced levels before taking the log. The resulting intensity levels are rendered in a linear rather than logarithmic sense. You can often obtain better results if you create a separate logarithmic image to display. One way to create a logarithmic image is with the imcalc task:

im> imcalc x2ce0502t.c1h x2ce0502t.hhh "log10(im1+1.0)"

If the peak pixel in your original image contained 2000 counts, for example, you would then display the logarithmic image with z1=0 and z2=3.3.

3.2.2 Working with Image Sections

Sometimes you may want to display only a portion of an image, using the syntax for specifying image sections discussed in Chapter 2. Your specified pixel range should give the starting point and ending point, with a colon separating the two. List the horizontal (x axis) range first, followed by the vertical (y axis) range. For example, to specify a pixel range from 101 to 200 in the x direction and all pixels in the y direction from group three of a GEIS format image, you would use a command such as:

tv> display image.hhh[3][101:200,*] 1

To specify the same pixel range in the second SCI extension of a NICMOS FITS image, you would use a command such as:

tv> display image.fits[sci,2][101:200,*] 1

If you specify both a group and an image section of a GEIS file, the group number must come first. When displaying sections of STIS and NICMOS FITS images, you must specify the extension, and the extension designation must come first.

Figure 3.3 shows examples of displaying an image and an image section.

Figure 3.3: Displaying Sections and Groups of an Image

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1 Type help display within IRAF to obtain more information about these parameters.

Copyright © 1997, Association of Universities for Research in Astronomy. All rights reserved. Last updated: 11/13/97 16:26:00