4.2 Distortion Corrections and Image Combination

Due to the tilt of the WFC3 focal plane with respect to the incoming beam, the projected pixel area on the sky varies across the field of view in the raw (raw.fits) and calibrated (flt/flc.fits) images. As a result of the different projected pixel area on the sky, pixels in different regions of the detector collect a different amount of light i.e. observations of a constant surface brightness object would have count rates per pixel that vary over the detector even if every pixel had the identical sensitivity. In WFC3, the pixel area on the sky varies by about 7% along the diagonal in UVIS images and about 8% from top to bottom in IR images. 

In order to produce images that appear uniform for uniform illumination, the WFC3 flat fields include the effect of the variable pixel area across the field (see Section 5.4). However as a consequence of dividing by the flat field, two stars of equal brightness in an flt/flc.fits image falling on different portions of the detector would not have the same total counts. To perform point source photometry on calibrated ‘flt/flc.fits’ images, they must be multiplied by the effective pixel area map (see WFC3 ISR 2010-08). Alternatively, the pixel area effect is accounted for in the pipeline by drizzling, where the geometric distortion solution is used to correct all pixels to equal area on the sky in ‘drz/drc.fits’ data products. Because drizzling conserves flux, users should recover the aperture photometry from drz/drc images and from flt/flc images multiplied by the pixel area map.

4.2.1 AstroDrizzle in the Pipeline

WFC3 data obtained from MAST are corrected for geometric distortion with AstroDrizzle, which replaced MultiDrizzle in the OPUS pipeline for WFC3 data on June 7, 2012. During pipeline processing, calibrated data that belong to an association (e.g. as defined by the user in APT via a standard dither pattern, a REPEAT-OBS, or CR-SPLIT pair, see Table 2.2) are corrected for distortion and drizzle-combined with cosmic-ray rejection. If the associated images are dithered, they are aligned using the World Coordinate System (WCS) information in their headers before being combined. If there is no association table, each single-exposure WFC3 image is drizzled to correct for geometric distortion. Additional WFC3-specific rules define that images obtained with the same filter within a given visit are associated and thus combined.

AstroDrizzle uses the MDRIZTAB reference table to define a default set of parameter values that work well for most use cases. Each detector has its own MDRIZTAB, and each row provides the parameter settings specific to the number of input images per association. To correct for distortions, AstroDrizzle relies on the following reference files.

  • Image Distortion Correction Table (IDCTAB, high-order polynomial coefficients)
  • Detector to Image Distortion Correction (D2IMFILE,lithographic mask pattern correction for UVIS only)
  • Non-polynomial Filter-Dependent Distortion (NPOLFILE, 2-D look-up table for each calibrated filter)  

The names of the reference files used by AstroDrizzle are stored in the  WFC3 UVIS and IR primary header keywords IDCTAB, D2IMFILE, and NPOLFILE. Users interested in these files may obtain them from CRDS

During AstroDrizzle processing, the geometric distortion is extracted from these reference files and stored as Simple Image Polynomial (SIP) header keywords and as additional FITS extensions in the *_flt.fits/flc.fits images. Please refer to Table 2.4 or to Section 3.2.3 in the DrizzlePac Handbook for details.
The steps performed by AstroDrizzle include the following:

  • Correct the geometric distortion.
  • Align images using the header World Coordinate System.
  • Perform cosmic-ray rejection using the *flt/flc.fits files as input. Note: IR flt data are already corrected for cosmic rays, but the drizzling process does identify further detector artifacts and rejects them during the combination process.
  • Convert the UVIS data from units of electrons to electrons per second; IR data are already in e-/s.
  • Combine associated (e.g. dithered) observations into a single product.

    Pipeline drizzled products (drc/drz) retrieved from MAST are generally recommended to be used as “quick-look” products only. Manual reprocessing through AstroDrizzle, as described in the next Section, is highly recommended to achieve the most scientifically accurate data products.

4.2.2 Manual AstroDrizzle Reprocessing

Drizzled images combined in the pipeline were produced using a default set of parameters that are suitable for the widest range of scientific applications. These defaults, however, may not produce the optimum science data quality for many programs, and those images will require post-pipeline processing. Additionally, MAST creates drizzled products only for images taken in a single visit, so multiple visits can be drizzled together only by reprocessing. 

Four main areas for improvement may include: (1) image alignment, (2) sky subtraction, (3) cosmic ray rejection, and (4) final image resolution. While single visit data with small dithers (like the 4-point dither box) are usually aligned to better than 0.1 pixel, the drizzled products are created using the native detector plate scale, and with a drop size or PIXFRAC of 1.0. In these cases, the resolution of the drizzled products can be improved by fine-tuning the final sampling i.e. experimenting with the scale and pixfrac parameters.

Single-visit data with larger dithers (for example, to create mosaics) may have residual shifts of a few tenths of a pixel or more and residual rotations of a few thousandths of a degree. When combining data from different visits, tweaks to the image alignment are usually necessary since different sets of guide-star pairs may have been used. Offsets of the same target at different roll angles are typically ~0.3 to 0.5". See Appendix B of the DrizzlePac Handbook for more information.

Poor alignment can lead to improper cosmic-ray rejection, and inadvertent flagging of some astronomical sources as cosmic rays, compromising the photometric accuracy of the final data products. Additionally, a poor estimate of the sky background, for example in images where a bright target fills the frame, may also affect the accuracy of cosmic-ray rejection, and in turn, the resulting photometry.

4.2.3 AstroDrizzle Documentation

AstroDrizzle is available as part of the DrizzlePac software, which contains all the tools for manually reprocessing (aligning and combining) flc/flt  HST images. This software may be obtained from the DrizzlePac web page. This page also provides useful resources such as the DrizzlePac Handbook, a 'Quick Start Guide' to drizzling, a set of worked examples, and some basic video tutorials.