File -> Open menu and
select an image to
display. You should acquaint yourself now with the general operation of ds9
by trying buttons and exploring the menus.
Notice that as you move the mouse around the image there will be a magnified piece of the image in the upper right hand corner of the window. The coordinates in pixels will appear on the upper left, and the value in the pixel under the cursor will be displayed.
A left click produces a circle that marks a ``region'' of the image. For
now, this is not useful, but it's likely that you will do this
by accident anyway! To make the selected region mark disappear, use
the Region -> Delete All menu option.
Press the right mouse button and move around the image to change the
the display intensity and contrast. The data in the images are values from
0 to about 32000. These are mapped to a gray scale or to colors on the
screen through a transformation that you have control over. Try the
Scale button and explore the new menu options. The Color
option gives you a selection of false color maps. Usually the gray scale
for images of the type we have here is a better choice.
The Zoom option allows you to zoom in to a region of the image,
or to center on the entire image. A center mouse click on a point in
the image, or on the preview window left of upper right will center
the image at that point. As you scan around the raw images you will notice
that the stars cover a few pixels. There are also ``hot'' pixels that
stand out as single bright spots that are not stars. These are
regions of the CCD that produce slightly higher dark current than
their neighbors. We ``dark subtract'' to remove the effect of this
non-uniform bias.
One of the more useful image display commands is in the Scale menu. Select this
option, and go down to click on Scale Parameters ... This brings up a new window
called ``Pixel Distribution'' that shows the range of signal strengths in the image.
The red bar at the left is the lower limit of the range displayed in the image window,
and the green bar is the upper limit. Sometimes there will be a few pixels that are
negative. This is an artifact caused by pixels that are actually too intense, causing the
program to read a large positive number as negative. Use the mouse, click and hold on the
red bar, and drag it over to the right. If you drop it at the lower edge of the
distribution for most image pixels, the image display will be compressed to the new range.
Notice the prompt response in the image when you do this. Similarly, go over to the
right, click and hold on the green bar, and drag it back to the upper limit of interest in
the pixel distribution.
For the moment, exit from ds9 while you process the images.