INGRID / LIRIS quick look package

An IRAF package is available to help you getting a quick look at your LIRIS data while you are observing at the telescope, but it is not intended to provide fully reduced data ready to be analysed.

For example, when you take an exposure with LIRIS the array is reset and immediately read out. This is called the pre-read image, and is stored in extension 1 of the FITS file (rxxxxxx.fit[1]). After the exposure time has passed the array is read out again. This is called the post-read image and is stored to the second FITS extension (rxxxxxx.fit[2]). The information useful for the observer is the difference between these two images. This package provides a quick way to access this information, and many others as shown below.

Note: If you want to reduce your data later-on with your own tools, you can split the FITS extensions from within IRAF using the imcopy task:
cl> imcopy r223344.fit[1] r223344_1.fits (this is the pre-read)
cl> imcopy r223344.fit[2] r223344_2.fits (this is the post-read)

You can advise the instrument control system to automatically subtract the pre-read for you. It is then no longer available as a separate FITS extension (what task is this?). Better put this somewhere in the general description page!

The INGRID quick look package, its documentation and the information given here were created by for the INGRID imager at WHT. Due to the great similarities of INGRID and LIRIS, you can use it for LIRIS at well.

Concerning data reduction, James Geach has created his own data reduction pipeline (for INGRID) and has kindly made it available to other users. You can go with his package for LIRIS reduction, too. Click here for further info.

1. Using the INGRID / LIRIS quick look package

To use the INGRID quick look package:

• Open an Ximtool (or another display tool) to display the data:
  ximtool& or ds9&
• Open an xgterm, change to the home directory and invoke IRAF:
  cl or ncl
• To launch the package:
  cl> ingrid_ql
  The prompt will change to in>
• To get information about the use of the package:
  in> help usage
• To get a list of the available tasks:
  in> help
• To get more information on using each particular task:
  in> help <task name>

Some of the most frequently used tasks are:

You can also download the INGRID quick look package and install it for your personal use. The information is given here.

2. Useful examples

At the beginning of the night you may want to focus the telescope (see also the LIRIS users guide). For this purpose you should take a series of exposures at different focus settings. Choose a long enough exposure time (8-10 seconds), so that the seeing averages out.

• istarfocus finds the best focus values from the stars visible in an image. For example, if you took a focus series of 9 exposures with variable telescope focus you would then find the best focus with
  in> istarfocus r223344 nim=9

• idispframe displays an exposure you have taken. To display the exposure r223344.fit in frame 1 of your display tool, type the following command. You can then use imexamine to examine this image as usual. All parameters for the display are also available for idispframe.
  in> idispframe r223344 1
  
  To subtract a sky frame from before displaying the image, use the parameters subsky and sky. To subtract the image "sky_h" from "r223344" before displaying, type
  in> idispframe r223344 1 subsky+ sky=sky_h
  
  A quick and dirty way to subtract a sky image is to use an exposure from a different field as the sky image, for example:
  in> idispframe r223344 1 subsky+ sky=r223355
  
  
• idedither is another commonly used task. If you have taken a dither, this tasks combines the images. You can select one of three methods with the match switch to find the offsets for combining the images:
  in> idedither @allfiles allcomb match="wcs"
  
  • match="wcs" This is the most automatic way. The WCS, generated from the telescope pointing information, is used to find the offsets between images.
    
  • match="pick1" This method is half automatic. The first image is displayed and you should pick a star with the 'm' key that will be visible on every following image. The task then calculates where the star should be on any image from the telescope pointing information and uses imcentroid to get the exact offset.
    
  • match="manual" This method is entirely manual. Every image is displayed and you have to pick the same star on every image. The positions of the stars are then refined using imcentroid and used for calculating the image offsets.

  To reassemble a 5 point dither using the "pick1" method and storing the result in a file named "comb", use:
  
  in> idedither r223340,r223341,r223342,r223343,r223344 comb match="pick1"
  
  Of course you can also use lists to specify the input files. The following example would combine all files listed (one file per line) in the file "allfiles" using the telescope pointing information. The output goes into the file named "allcomb":