4.2 Distortion Corrections and Image Combination

Because the WFC3 focal plane is tilted with respect to the incoming beam, the projected pixel area on the sky varies across the field of view in calibrated (flt/flc.fits) images. As a result, pixels in different regions of the detector collect a different amount of light simply due to their projected area on the sky. Observations of a constant surface brightness object would therefore have count rates per pixel that vary over the detector, even if every pixel had the same sensitivity.

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). 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 correct for this effect, the calibrated ‘flt/flc.fits’ images may be multiplied by the effective pixel area map available at http://www.stsci.edu/hst/instrumentation/wfc3/data-analysis/pixel-area-maps (see WFC3 ISR 2010-08). For the UVIS channel, this represents a 7% effect across a diagonal of the mosaicked image. For the IR channel, the area of the pixels varies by 8% from the bottom to the top of the image.

Alternatively, this 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. AstroDrizzle relies on the Image Distortion Correction Table (IDCTAB), the Detector to Image Distortion Correction (D2IMFILE, UVIS only), and the Non-polynomial Filter-Dependent Distortion (NPOLFILE) reference files for a description of the WFC3 distortions. Additional reference files to correct for filter-dependent residuals (NPOLFILE) are added once on-orbit measurements are made.

Users may obtain the latest IDCTAB, D2IMFILE, NPOLFILE and MDRIZTAB reference files from the following page:

Information about 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

In general, pipeline drizzled products retrieved from MAST are recommended to be used as “quick-look” products. Reprocessing through AstroDrizzle is highly recommended to achieve the most scientifically accurate data products. Four main areas for improvement may include: (1) image alignment, (2) sky subtraction, (3) cosmic ray rejection, and (4) final image resolution.

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.

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 by 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 rolls 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, of 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) calibrated HST data. 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.