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
Inspect your output and iterate as needed
Look for under-rejection of cosmic rays, or over-rejection of real features
(e.g. stellar cores) by blink-comparing the output drizzled image and weight map
(drz_sci.fits and drz_weight.fits). You may need to adjust your rejection
thresholds (driz_cr_snr), or grow the rejections and/or reject CTE tails
(driz_cr_grow and driz_cr_ctegrow).
Setting clean=no leaves the many intermediate files in your working directory,
which can be helpful for diagnostic purposes. However, the output drizzled
image and weight map are a good initial diagnostics, so keep clean=yes
until you feel you need to analyze each step more closely.
Send any questions or concerns to the STScI Help Desk (help@stsci.edu).
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