Working with a bunch of target images

Today, you will

learn how to save yourself a lot of typing
reduce an entire set of images of Lictoria

Simple shell scripts

After a night of work, you may have 10, or 100, or 1000 target images. Each one needs to be cleaned. It would take a long time to do all this manually...

     sub 1107a.001 dark15.fts
     div 1107a.001 flatv.fts flat
     sub 1107a.002 dark15.fts
     div 1107a.002 flatv.fts flat
     sub 1107a.003 dark15.fts
     div 1107a.003 flatv.fts flat
      ...
     sub 1107a.060 dark15.fts
     div 1107a.060 flatv.fts flat

Fortunately, there are ways to perform repeated operations automatically. One can

use shell commands at the command line
create a shell script
create a script in some other language (e.g., Perl)

The shell is the program which interprets the commands you type; it expands wildcard characters, spawns processes to execute the programs you specify, keeps track of jobs running in the background, and so forth. There are several shells for Unix-like systems; the one you are using on iceberg is called the Bourne shell.

Very simple way to repeat commands in the Bourne shell: try the examples!

using for with a list of words

      for i in 1 2 3 
      do
        echo $i
      done

using for with a list of files

      for i in *
      do
        echo "next file is $i"
      done

Script files

You can save a set of shell commands in a file for later use. For example, if you create a file called testme.sh which contains these lines:

     #!/bin/sh
     # execute a simple loop
     for i in 1 2 3 4 5
     do
         echo "current value of i is $i"
     done

     exit 0

you can execute them all simply by typing

     sh testme.sh

Shell scripts can be pretty complicated. Read the tutorials listed above to learn more about them.

Cleaning a set of target images

So, suppose you have a set of images you need to clean. You can use a little shell loop to apply the same commands to each image in the set (but don't do this yet!):

     mn flatv.fts
     for i in 1107a.0??
     do
        sub $i dark15.fts
        div $i flatv.fts flat
     done

I like to add a little note to the FITS header of an image as I process it, so that I know whether the image is still raw, or dark-subtracted, or fully reduced. The XVista command comment allows you to add a comment to the bottom of a FITS header. Try this:

     buffers 1107a.001
     comment 1107a.001 "added a comment" 
     buffers 1107a.001

You can get a little more fancy, too:

     buffers 1107a.001
     comment 1107a.001 "added a comment on `date` " 
     buffers 1107a.001

So, I recommend using a little loop like this:

     mn flatv.fts
     for i in 1107a.0??
     do
        sub $i dark15.fts
        comment $i "subtracted dark frame dark15.fts on `date` "
        div $i flatv.fts flat
        comment $i "divided by flatfield frame flatv.fts on `date` "
     done