Finding Charts from SAOTNG or DS9 using XPA

An example of a finding chart created with SAOTNG. The image, taken from a small portion of a digitized POSS-II F plate, has been reduced with the SKICAT pipeline to produce a catalog of detected sources. In the example above we have used SAOTNG to overplot a subset of these sources using the isophotal ellipse derived for each source. Additionally, we have color coded these ellipse markers to indicate object class: galaxies are painted in BLUE, stars are painted in RED, and unclassified sources are painted in GREEN. This example was generated with a simple script and input "regions" file (both of created by a small f77 code). The image seen here was produced using the GIF option in the SAOTNG image display tool. In this example, we generated an 1024 square image with 450 ellipse markers in less than 20 seconds. Hence, one can easily generate such a finding chart in the form of an HTML document that can be viewed anywhere!

How can I painlessly make an SAOTNG finding chart?

Before embarking on a time-wasting campaign to write a piece of code that created image-based finding charts with overplotted ellipses, I asked Pat Shopbell (pls@astro.caltech.edu) about SAOTNG, a new upgrade to SAOIMAGE written by Eric Mandell (eric@head-cfa.harvard.edu) at CfA. He recommended that I look into using the XPA tools in SAOTNG to generate such a plot. The XPA mechamism (X Public Access) provides a way for the user to issue commands to any X-based image display tool (like SAOTNG!). It is described well in a brief paper by Mandel and Swick (1994) (see ASP Conference Series Vol. 61, -- ADASS III). Simply put, there are two fundamental tools: xpaset for sending commands to saotng, and xpaget for receiving information from saotng (i.e. getting cursor poistions, marker characteristics, etc...). With only a moderate amount of work I was able to use this method to generate a type of finding chart like that shown above. The great thing about this approach is that after you have drawn your finding chart, you are left with the power of SAOTNG to refine it, i.e. changing the zoom, the image display scale (log, linear, ...), the color table, and much more.

Here is an example demonstrating the use of this powerful method:

Step 1: Generate a script (here called r0031_F0539.scrpt) containing the XPA command lines.

#
echo "file r0031_F0539.fits" | xpaset -W SAOtng
cat r0031_F0539.regns | xpaset SAOtng regions

Step 2: Generate a file containing the marker information for the objects to be marked on your input FITS image.

# filename:    r0031_F0539.fits
# format:       pixels
+ellipse(111.1,230.1,3.3,2.8,160)  # red
+ellipse(111.1,277.1,4.0,3.8,182)  # blue
+ellipse(120.1,413.1,3.7,3.1,62)  # blue
+ellipse(251.1,455.1,4.0,3.4,168)  # red
+ellipse(254.1,296.1,2.5,1.8,193)  # blue

Step 3: Make your script executable and run it!

  % chmod 777 r0031_F0539.scrpt
  % saotng &
  % r0031_F0539.scrpt

How do I paint different ellipse markers?

The regions file shown above is easy to understand. After the first two header lines, each object to be marked is designated on a single line. You may specify different marker types (i.e. circle, square, etc), but the example above I use ellipses. The five arguments specified after the symbol designation (ellipse) are the X, Y center (in pixel units), the major and minor axis length (in pixels), and the position angle of the major axis (in degrees). To demonstrate the position angle specification, I give the example below.

A demonstration of how to draw ellipses with different position angles. The regions file used to generate these markers is shown below.

The regions file for drawing different position angles.

# filename:    r0031_F0539.fits
# format:       pixels
+ellipse(723.1,487.1,70.3,30.8,0)  # red 
+ellipse(723.1,487.1,70.3,30.8,45)  # blue
+ellipse(723.1,487.1,70.3,30.8,90)  # green