WFPC2 drizzling
See the WFPC2 drizzling overview and general tips, to find a cookbook and/or sample dataset that best represents your dataset, with regards to target placement and pointing strategy.
Cookbook for simple 2-3 point dithers with the target placed on the PC chip (only)
NOTE: You may use the c0m/c1m files you now receive from the archive 'as is', there is no need to convert them to GEIS format using STRFITS as mentioned below. If you have WFITS (c0f/c1f) data, then please follow the instructions for converting your data to GEIS (c0h/c1h) format. These instructions are intended for datasets where a simple 2 or 3 point dither line pattern was used.
De-archive the science and data quality FITS files to your working directory. Make a uref directory and download the distortion reference files (IDCTAB and OFFTAB in your image headers) into it, and define your uref directory (set uref). Convert the files to GEIS format (with strfits), and make a list of input images (list_c0h):
> set uref = "/data/mymachine/uref/" > strfits *c0f.fits "" "" > strfits *c1f.fits "" "" > ls u*c0h > list_c0h (no blank lines!)
If your data are not all from the same visit, or otherwise did not use the same guide stars, you need to refine the image registration.
MultiDrizzle parameters
The following are suggested parameters (with some rationale in the comments) for applying the geometric distortion correction, combining, and cleaning your dataset. Some of the default parameters are not listed here.> unlearn multidrizzle # reset all parameters to default values first multidrizzle.input = '@list_c0h' # input image list multidrizzle.output = 'ngc999_f555w' # target_filter filenaming convention multidrizzle.group = '1' # process PC chip only multidrizzle.build = no # single-extension FITS output multidrizzle.shiftfile = 'shifts.txt' # apply measured delta-shifts, if relevant multidrizzle.static = yes multidrizzle.skysub = no multidrizzle.driz_separate = yes multidrizzle.driz_sep_outnx = 800 multidrizzle.driz_sep_outny = 800 multidrizzle.driz_sep_kernel = 'turbo' multidrizzle.driz_sep_scale = 0.0455 # PC detector pixel scale multidrizzle.driz_sep_pixfrac = 1.0 multidrizzle.driz_sep_rot = INDEF # don't rotate north up for CR-detection multidrizzle.driz_sep_fillval = -9.9 # arbitrary low value; easy to exclude multidrizzle.driz_sep_bits = 0 # exclude all flagged pixels multidrizzle.median = yes multidrizzle.combine_type = 'median' multidrizzle.combine_lthresh = '-8.8' # exclude fill values from median multidrizzle.blot = yes multidrizzle.driz_cr = yes multidrizzle.driz_cr_snr = '4.0 3.5' multidrizzle.driz_combine = yes multidrizzle.final_outnx = 800 multidrizzle.final_outny = 800 multidrizzle.final_kernel = 'square' multidrizzle.final_scale = 0.0455 multidrizzle.final_pixfrac = 1.0 multidrizzle.final_rot = 0.0 # rotate north up multidrizzle.final_fillval = 0.0 multidrizzle.final_bits = 0