Hubble Legacy Archive FITS files - why three images?

[dwormuth]

hst_05962_01_wfpc2_f814w_wf_drz_CTX
   
   file-info                       
      file-path                     hst_05962_01_wfpc2_f814w_wf_drz.fits
      file-format                    FITS
   

When I open this file, I get three image files with the root name and "CTX" "WHT" and "SCI" appended.

Any idea what the initials stand for? Google searching didn't help.

[pfile]

you've got multiextention fits files that were output by the multidrizzle tool -

from:

http://hla.stsci.edu/hla_help.html

The Images view offers a few additional options for data access. The image can be downloaded either as a simple FITS file with only the science image data (named FITS-Science) or the full multiextension FITS file (named FITS-MEF) created by MultiDrizzle that contains the science image in extension [1], the weight array in extension [2], and the context image in extension [3]. When source lists are available, they can also be added to the cart. There are two types of source lists; DAOPHOT (primarily for point-like objects) and SExtractor (for both point-like and extended targets). There is also a plot link that pops up a window showing some properties of the catalogs.

from:

http://www.stsci.edu/hst/acs/analysis/multidrizzle/

The combined images produced by MultiDrizzle in the ACS pipeline have a pixel scale that depends on which camera (WFC, HRC or SBC) was used (0.05 arcsec/pixel for the WFC, 0.025 arcsec/pixel for the HRC and SBC). The orientation for all these images is in the default unrotated detector frame. The drizzled images contain three extensions: [sci] being the drizzled science image, [wht] being the weight image, and [ctx] being the context image, showing which exposures contributed to each output pixel. The background sky value is calculated by MultiDrizzle and stored in the image headers of each individual flatfielded file ("*_flt.fits"). The actual science pixel values in the flatfielded files are not modified by MultiDrizzle. However, pixels that have been identified as cosmic rays are flagged in the data quality array using the bit value 4096. The MultiDrizzle image product drz.fits is generally sky-subtracted, with the exception of narrow-band filters and UV filters that are expected to have a dark sky background.

rob