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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:

How to display images in IRAF using the display task.
How to display subsections of images. Observers viewing WF/PC-1 and WFPC2 data may wish to remove cosmic rays before displaying their data (see page 26-20). The FOC photon-counting hardware does not detect cosmic rays at easily as CCDs, the NICMOS pipeline automatically removes cosmic rays from MULTIACCUM observations, and the STIS pipeline automatically removes cosmic rays from CR-SPLIT association products.

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:

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 &

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.

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:

Use the SAOimage command buttons that control zooming, panning, etc.
Reset the display task parameters. Once an image appears in your SAOimage window, you can use the SAOimage commands displayed near the top of the image window to manipulate or print your image. The SAOimage Users Guide describes these commands, although most are fairly intuitive. Just click on the buttons to scale, pan, or print the image, or to perform other commonly-used functions. On-line help is also available at the system level: type man saoimage in Unix or help saoimage in VMS.
The example in Figure 3.2 shows how you should display an image for a first look. By default, display automatically scales the image intensity using a sampling of pixels throughout the image. During your first look, you may want to experiment with the scaling using the zscale, zrange, z1 and z2 parameters. The zscale parameter toggles the autoscaling. Setting zscale- and zrange+ tells the task to use minimum and maximum values from the image as the minimum and maximum intensity values. To customize your minimum and maximum intensity display values, set zscale-, zrange-, z1 to the minimum value and z2 to the maximum value that you want displayed. For example:

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.

stevens@stsci.edu