Cookbook for simple 2-3 point dithers 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 a simple 2 or 3 point dither line pattern was used, or a small mosaic pattern (with shifts less than 100 arcseconds).
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 target falls on just one of the WF chips, for example WF3, you can set multidrizzle.group=3 to process only that chip. Similarly, if you wish to exclude the data from any chips (e.g. the PC data, or the WF4 data if it suffers too badly from the bias anomaly), you can mask out entire chips by flagging every pixel in the appropriate group (chip number) of the data quality file, for example:
> imreplace u*.c1h 2048 # mask out all the PC1 chips > imreplace u62p030ar.c1h 2048 # mask out a WF4 chip with bad bias anomaly
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 parametersThe 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' # a convenient filenaming convention multidrizzle.group = '' # leave blank to include all chips, or provide a chip number multidrizzle.ra = 115.480667 # decimal NED coords to center the target in the output multidrizzle.dec = -18.208472 # decimal NED coords to center the target in the output multidrizzle.build = no # produce 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 = 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 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_wht_type = 'EXP' multidrizzle.final_outnx = 2048 multidrizzle.final_outny = 2048 multidrizzle.final_kernel = 'square' multidrizzle.final_wt_scl = 'exptime' multidrizzle.final_scale = 0.0996 # output pixel = input pixel scale multidrizzle.final_pixfrac = 1.0 # don't shrink the drop size multidrizzle.final_rot = 0.0 # rotate north up multidrizzle.final_fillval = 0.0 multidrizzle.final_bits = 0* There is some potential for confusion in that drizzle.scale is a unitless ratio (output/input scale), but in PyDrizzle and MultiDrizzle the units became absolute arcsec a few years ago. So if input WF pixels are 0.10 arcsec, then 60% of that is 0.06 arcsec.
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).
Send any questions or concerns to the STScI Help Desk (firstname.lastname@example.org).