The Hubble Helix

ACS data retrieval and pipeline reduction

After the best calibration reference files became available, the data was re-retrieved from the archive, such that on-the-fly (OTFR) pipeline processing automatically applied the latest calibrations. In particular, the "best" dark reference file corrects and flags the hot pixels present on the date of our observations. An improved filter-specific distortion correction solution was also available at this later time (which was applied during the re-drizzling described below). The remainder of this report describes post-pipeline data reduction, although a future version of MultiDrizzle may someday be incorporated into the ACS pipeline.

Combining the ACS mosaic with drizzle

The primary pipeline product we utilized was the flat-fielded images (*flt.fits) - our input files for drizzling. We used MultiDrizzle to apply the distortion and registration corrections, combine the exposures into the full mosaic, and reject cosmic rays in any overlap regions. We combined all the exposures for each filter using the following parameters (F658N scale 2 shown here). We provide explanations for some of the non-default parameters used below:

--> lpar multidrizzle     

    output = f658n_s2          Rootname for output drizzled products
   (suffix = )                 Suffix of the input files
 (filelist = list_f658n_all) 
 (refimage = )               
  (runfile = multidrizzle.run) File for logging the script commands
  (restart = )               
   (coeffs = header)           Use header-based distortion coefficients?
  (context = no)               Create context image during final drizzle?
    (clean = no)               Remove temporary files?
  (section = )               
     (bits = 8578)             Integer mask bit values considered good
       (ra = 337.389)          right ascension output frame center
      (dec = -20.832)          declination output frame center
    (build = no)               Create multi-extension output file?
(shiftfile = shifts_delta_pix) Shiftfile name
                                
                               STEP 1: STATIC MASK

        (static = no)          Create static bad-pixel mask from the data?
    (staticfile = )            Name of (optional) input static bad-pixel mask
(static_goodval = 1.0)         Value of good pixels in the input static mask
                                
                               STEP 2: SKY SUBTRACTION

      (skysub = no)            Perform sky subtraction?
     (skytype = quadrants)     Type of sky subtraction
     (skyname = SKYSUM)        Header keyword containing sky value
    (skywidth = 50.0)          Interval width for sky statistics
     (skystat = median)        Sky correction statistics parameter
    (skylower = -50.0)         Lower limit of usable data for sky (DN)
    (skyupper = 200.0)         Upper limit of usable data for sky (DN)
                                
                               STEP 3: SEPARATE DRIZZLED IMAGES

   (driz_separate = yes)       Drizzle onto separate output images?
  (driz_sep_outnx = 6300.0)    Size of separate output frame's X-axis (pixels)
  (driz_sep_outny = 6900.0)    Size of separate output frame's Y-axis (pixels)
 (driz_sep_kernel = turbo)     Shape of kernel function
  (driz_sep_scale = 2.0)       Linear size of output pixels in terms of input
(driz_sep_pixfrac = 1.0)       Linear size of drop in input pixels
    (driz_sep_rot = 0.0)       Rotation to be applied (degrees anti-clockwise)
(driz_sep_fillval = 9999.9)    Value assigned to undefined output points
                                
                               STEP 4: CREATE MEDIAN IMAGE

         (median = yes)        Create a median image?
(median_newmasks = no)         Create new masks when doing the median?
   (combine_type = median)     Type of combine operation
 (combine_reject = minmax)     Type of rejection
 (combine_nsigma = 6 3)        Significance for accepting min instead of med
   (combine_nlow = 0)          minmax: Number of low pixels to reject
  (combine_nhigh = 17)         minmax: Number of high pixels to reject
(combine_lthresh = 0.0001)     Lower threshold for clipping input values
(combine_hthresh = 9998.8)     Upper threshold for clipping input values
   (combine_grow = 1.0)        Radius (pixels) for neighbor rejection
                                
                               STEP 5: BLOT BACK THE MEDIAN IMAGE

           (blot = yes)        Blot the median back to the input frame?
                                
                               STEP 6: REMOVE COSMIC RAYS WITH DERIV, DRIZ_CR

        (driz_cr = yes)        Perform CR rejection with deriv and driz_cr?
    (driz_cr_snr = 3.0 2.5)    Driz_cr.SNR parameter
  (driz_cr_scale = 1.2 0.7)    Driz_cr.scale parameter
                                
                               STEP 7: COMBINED DRIZZLED IMAGE

   (driz_combine = yes)        Perform final drizzle image combination?
    (final_outnx = 6300.0)     Size of FINAL output frame X-axis (pixels)
    (final_outny = 6900.0)     Size of FINAL output frame Y-axis (pixels)
   (final_kernel = square)     Shape of kernel function
    (final_scale = 2.0)        Linear size of output pixels in terms of input
  (final_pixfrac = 1.0)        Linear size of drop in input pixels
      (final_rot = 0.0)        Rotation to be applied (degrees anti-clockwise)
  (final_fillval = 0.0)        Value assigned to undefined output point  
           (mode = al)             
      

Refining the mosaic registration

In any case, there are not enough stars in this field for tweakshifts or similar registration tools (e.g. wcsfix or imwcs) to work. At most there are typically zero or one stars in any overlap region (a total of only five for the entire mosaic) between adjacent tiles. See the "road map" on the Observations page for the relative locations of the exposures and registrations stars. The positions of these artificial "double stars" was measured with imexam in each exposure where they are present. This measurement can be made from the individual pipeline-drizzled images (*drz.fits), or from the separate drizzled images produced by MultiDrizzle Step 3 (*single_sci.fits). The tile 5 images were defined to be (0,0) and all other delta shifts are relative to them, which are provided to MultiDrizzle via a shift file named shifts_delta_pix (which augments the WCS information, see Hack et al. 2003):

# units: pixels
# frame: input
# form: delta
j8kr01lkq_flt.fits       8.11     5.59
j8kr01lmq_flt.fits       8.11     5.59
j8kr02maq_flt.fits	 5.85     1.10
j8kr02meq_flt.fits	 5.85     1.10
j8kr03mzq_flt.fits       7.67     2.25
j8kr03n3q_flt.fits       7.67     2.25
j8kr14jdq_flt.fits       8.45     6.91
j8kr14jfq_flt.fits       9.00     9.00
j8kr05k3q_flt.fits       0.00     0.00
j8kr05kaq_flt.fits       0.00     0.00
j8kr06o0q_flt.fits	 9.49     3.40
j8kr06o9q_flt.fits	 9.49     3.40
j8kr07ktq_flt.fits       5.34     4.40
j8kr07l2q_flt.fits       4.89     4.76
j8kr08odq_flt.fits       4.60    11.29
j8kr08omq_flt.fits       4.60    11.29
j8kr09nlq_flt.fits       7.05     7.35
j8kr09nvq_flt.fits       7.05     7.35

Residual cosmic ray rejection

We suppressed the residual cosmic ray contamination with a simple cosmetic smoothing method. Cosmic rays were detected and then replaced using a median-smoothed image (a 5x5 box for scales 2 and 4, a 7x7 box for scale 1) of the mosaic itself. Any pixel which was ~50% brighter than it's local median was flagged (flag=1) and then replaced with that median, along with adjacent pixels which are likely to be a part of the same cosmic ray event. Stars were arbitrarily protected (flag=2) from this process so that they (nor any part of their complicated PSFs) would get smoothed. Some cosmic rays near stars may therefore have survived this process, and some faint unprotected stars may, in fact, have been smoothed or even completely rejected. The pre-cleaned mosaics, and the residual rejection masks are available for inspection for anyone concerned about the impact of this cosmetic cleaning step (or would like to try and improve upon it with their own method).

Preliminary analysis

Acknowledgements

References

Providing Updated Image Shifts to PyDrizzle (PDF),
Hack, Mack, & Hook, 2003, STScI Technical Instrument Report SSB 2003-01

MultiDrizzle: An Integrated PYRAF script for Registering, Cleaning, and Combining Images (PDF),
Koekemoer, Fruchter, Hook, Hack, 2002, HST Calibration Workshop Proceedings

ACS dither and mosaic pointing patterns (PDF),
Mutchler & Cox, 2001, STScI Instrument Science Report ACS 2001-07

Appendix A: Object catalog