INGRID quick look package

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INGRID quick look package

An IRAF package is available to help you take a quick look at your INGRID data while you are observing at the telescope. Note that it is not intended to provide fully reduced data ready for analysis.

When you take an exposure with INGRID the array is reset and immediately read out. This is called the pre-read image, and is stored in extention 1 of the 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 extention 2 of file (rxxxxxx.fit[2]). It is the difference of these two images that is of interest to the observer. The ingrid_ql package provides a quick way to access this information.

Using the INGRID quick look package

To use the INGRID quick look package from the WHT control room:

Log onto the WHT data reduction machine as whtguest (with the password provided by your support astronomer).

Open a display tool, such as ds9, to display the data (eg. type ds9&).

Start an IRAF session by opening an xgterm, changing to the home directory and typing cl (or ncl). The prompt will change to cl>.
Type: cl> ingrid_ql. The prompt will change to in>.
Type: in>help usage, to get information about the use of the package.
Type: in>help to get a list of the available tasks.
Type: in>help <task_name> to get more information on using each particular task.

Some of the most frequently used tasks are:

idedither	combine dithered images
idispframe	display frame (post-pre)
istarfocus	find focus from focusrun images
icombine	combine images
ifindron	find readout noise from two dark frames
iframediff	calculate and store post-pre
ilistdark	list dark exposure times needed
istatistics	calculate image statistics
isubtract	subtract two images

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

Useful examples

While observing, the most useful tasks will probably be (i) to display a single image and (ii) to quickly re-construct an image from a number of images taken in a dither pattern.

idispframe
Once you have taken an exposure you will probably want to view it. This can be done with the idispframe task. For example:

in>idisp r123456 1

will display exposure number 123456 in frame 1 of your display tool. You can then use imexamine to analyse this image. All parameters for display are also available for idispframe. Additionally if you want to subtract a sky frame from the image, before it is displayed, you can use the parameters subsky and sky. Eg.

in>idisp r123456 1 subsky+ sky=sky_h

would subtract the image "sky_h" from "r123456" and display the resulting image. 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>idisp r123456 1 subsky+ sky=r123455
idedither
This task can be used to combine a set of dithered images. Changing the match parameter will select one of three methods for computing the offsets between the images:
- match="wcs" This is the most automatic way. The wcs coordinates, generated from the telescope pointing information, is used to calculate the offsets between images.
- match="pick1" This method is semi-automatic. The first image is displayed and you have to pick a star with the 'm' key that will be visible on every following image. The task calculates where the star should be on each image (from the telescope pointing information) and then uses imcentroid to get the exact offset.
- match="manual" This method is completely 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.
Eg. To reassemble a 5 point dither using the "pick1" method, and store the result in a file named "comb", use:

in>idedither r123456,r123457,r123458,r123459,r123460 comb match="pick1"
As usual, 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, and put the output into a file named allcomb:

in>idedither @allfiles allcomb match="wcs"

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Last Updated: 30 January 2008
Ian Skillen, wji@ing.iac.e