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WFPC2 Drizzle Cookbooks

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 sub-sampled data with the target placed on the WF chip/s

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 an optimally sub-sampling dither box 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 any part of your target falls on the WF4 chip, the data were taken after 1 March 2000, and you are working with c0h/c1h files, ensure that the WF4TFILE and WF4TCORR keywords are present and populated. The WF4TFILE keyword will be present (it should not be 'N/A') and WF4TCORR should be 'COMPLETE'. If this is not the case, you need to re-retrieve your data.

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 sample 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
multidrizzle.clean = yes
multidrizzle.group = ''  # include all chips, or provide a chip number
multidrizzle.ra = 115.480667   # center the target in the output image
multidrizzle.dec = -18.208472  # center the target in the output image
multidrizzle.build = no  # single-extension FITS output
multidrizzle.shiftfile = 'shifts.txt'  # apply delta-shifts, if relevant
multidrizzle.static = yes
multidrizzle.skysub = no
multidrizzle.driz_separate = yes
multidrizzle.driz_sep_outnx = 1600
multidrizzle.driz_sep_outny = 1600
multidrizzle.driz_sep_kernel = 'turbo'
multidrizzle.driz_sep_wt_scl = 'exptime'
multidrizzle.driz_sep_scale = 0.0996  # WF detector pixel scale in arcsec
multidrizzle.driz_sep_pixfrac = 1.0
multidrizzle.driz_sep_rot = INDEF  # don't rotate North up here
multidrizzle.driz_sep_fillval = -9.9  # arbitrary low value
multidrizzle.driz_sep_bits = 0  # exclude all flagged pixels
multidrizzle.median = yes
multidrizzle.combine_type = 'median'
multidrizzle.combine_lthresh = '-8.8'  # exclude fill values
multidrizzle.blot = yes
multidrizzle.driz_cr = yes
multidrizzle.driz_cr_snr = '4.0 3.5'
multidrizzle.driz_combine = yes
multidrizzle.final_wht_type = 'EXP'
multidrizzle.final_outnx = 2048
multidrizzle.final_outny = 2048
multidrizzle.final_kernel = 'square'
multidrizzle.final_wt_scl = 'exptime'
multidrizzle.final_scale = 0.06  # in arcsec; output pixels are smaller*
multidrizzle.final_pixfrac = 0.8 # shrink the drop size
multidrizzle.final_rot = 0.0  # rotate North up in final output
multidrizzle.final_fillval = 0.0 
multidrizzle.final_bits = 0 # exclude all flagged pixels
multidrizzle.crbit = 0
* There is some potential for confusion in that drizzle.scale is a unitless ratio (output/input scale), but in PyDrizzle and MultiDrizzle, the scale units became absolute arcsec a few years ago. For example, input WF pixels are 0.10 arcsec, so setting scale=0.06 means the output pixels will be 60% as large as the input pixels.

Inspect your output and iterate as needed

You might need to make your output dimensions (outnx, outny) larger to include all the pixels, or wish to make them smaller to truncate unneeded outer parts of the data (i.e. if your target is small).

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). Patterns in your weight map (drz_weight.fits) may indicate that you need to adjust the scale and pixfrac.

Send any questions or concerns to the STScI Help Desk (help@stsci.edu).