The Astrodrizzle software (see the Drizzlepac Web page) is used to combine images. The main use of the software is to:
- improve the cosmic ray rejection
- optimize the resolution of the image when sub-pixel dithers were obtained (i.e. data taken at the same time)
- combine data from multiple visits (i.e. data taken at different times).
To load the package (after pyraf has been started), issue the command:
>>> import drizzlepac
The main routines you need are:
- astrodrizzle - the main task which replaces MultiDrizzle
- updatenpol - a task to update the headers with new reference file information (for ACS data only)
- tweakreg - a task to determine offsets between pointings (e.g. between images taken at different times); see the separate help for this task
You run astrodrizzle on the _flt.fits or _flc.fits files.
If you have retrieved the data after July 16, 2012, these are the only files you need (with the possible exception of the MultiDrizzle Parameter Table, if you want to duplicate the pipeline results). However, if you have old data on disk, you will also need certain reference files, which can be obtained from the appropriate Instrument web page; see the Drizzlepac Handbook for details.
To run astrodrizzle, issue the command:
>>>epar astrodrizzlewhich will bring up the parameter file for the task.
You should set UPDATEWCS=NO, especially if you have used tweakreg to align images; see the Drizzlepac handbook for details on when to set this parameter to YES. The remaining items can be modified if necessary (see the Drizzlepac handbook for details). Note that you can set the parameters to duplicate the pipeline results by click on the Update from MDRIZTAB button, If you do this, you should change BUILD=NO so you will also get an effective exposure time map as a separate file (otherwise this map, as well as other files, will be separate extentions of a single file).
The remaining parameters can be changed to optimize process for your particular dataset; see the Drizzlepac handbook for suggestions on optimizing the parameters for your particular case
If you want to save the parameters you have set, go to File/Save As, which will create a configuration file. If you want to restore a previously save configuration, go to Open to read in the configuration file.
Once the parameters are all set, click on Execute to run the package.
The data we are drizzling was obtained with the ACS/WFC, so we needed to update the headers to include the new Astrodrizzle keywords before we can run astrodrizzle. With the appropriate reference files in place:
which will bring up the parameter file for the task.
In this case, we can leave everything with its default value and execute.
We can see the new keywords in the headers, as well as the new extensions in the images, so we know the data has been updated. You can now rerun astrodrizzle.
The results of drizzling can be clearly seen.