2.4 Detector Read-Out Modes and Dithering

The detectors in both channels offer full-frame and subarray readout capability; see Section 5.3 and Section 6.4.4 for UVIS, or Section 7.4.4 and Section 7.7 for IR. The UVIS channel also allows 2 × 2 and 3 × 3 on-chip binning of full-frame images (see Section 6.7.2).

Additionally, a variety of dithering patterns can be requested using the Astronomer’s Proposal Tool (APT) software. Predefined patterns have been established to address common requirements, such as sub-pixel dithering to improve PSF sampling or dithering to achieve large areal coverage while also sampling the inter-chip gap in UVIS, or avoid self-persistence or blob artificacts in IR (see Appendix C).

The post-observation pipeline software, calwf3, carries out appropriate calibration of data taken in all of these configurations, and offers the option of reconstructing dithered images with a "drizzling" algorithm. For more information regarding the reduction and calibration pipeline maintained by STScI, see Appendix E.2 of this handbook. Chapter 3 of the WFC3 Data Handbook also details how WFC3 data are calibrated.

Image drizzling can be optimized using functions and modules in the DrizzlePac software package. If the dither pattern incorporates non-integer pixel offsets, it can effectively improve the sampling of the point-spread function (PSF). The software can also handle mosaicked images according to a set of rules or associations, rectifying them onto a Cartesian pixel coordinate system.