Sets of exposures with offsets executed using patterns or POS TARGs are associated and combined automatically during calwf3 pipeline processing, as long as the same guide stars have been used for all exposures. Pointings must be contained within a diameter ~130 arcsec or less (depending on the availability of guide stars in the region) to use the same guide stars. Note that the rms pointing repeatability is significantly less accurate if different guide stars are used for some exposures. (See Appendix B of the
DrizzlePac Handbook.)
The names and purposes of the patterns in effect in APT at the time of publication are given in Table C.1. (The initially adopted names of patterns have been preserved for continuity, although they do not always correspond to the distinction between dither steps and mosaic steps outlined above.) The small BOX dither patterns are designed to optimally sample the PSF when 4 steps are used. Since time constraints do not always permit visits to be broken into multiples of 4 steps, LINE dither patterns that optimally sample the PSF in 2 or 3 steps are also given. The BOX and LINE dither patterns are illustrated in
WFC3 ISR 2010-09. For a full discussion and illustrations of patterns that optimally sample the PSF for different numbers of steps, see Section C.2 of the
DrizzlePac Handbook.) Note that PSF sampling generally produces a more significant improvement for IR images than for UVIS images (see
Section 6.6.1 and
Section 7.6.1) The remainder of the patterns in
Table C.1 are special-purpose mosaic patterns that are expected to be commonly needed. We have not defined patterns to deal with specific features in flats—notably, the circular dead spot on the IR detector (
WFC3 ISR 2008-08) and the UVIS “droplets” (
WFC3 ISR 2008-10). However, patterns that can be used to mitigate the effects of these artifacts are discussed in
WFC3 ISR 2010-09.
The default specifications of the patterns are summarized in Table C.2. The equivalent POS TARG moves are summarized in
Table C.3, along with the approximate number of pixels corresponding to these moves. The number of pixels was computed using only the linear distortion terms with coefficients measured at the center of each detector. This is an excellent approximation for small moves and for objects that remain in the central region of the detector. (See
Figure B.1 and
B.3 in
Appendix B.)
where a11 ~ 0.1355 arcsec/pixel and
b10 ~ 0.1211 arcsec/pixel near the center of the detector. For the UVIS detector, there is a cross-term that takes into account the fact that the projected axes are not perpendicular:
where a11 ~ 0.0396 arcsec/pixel,
b11 ~ 0.0027 arcsec/pixel, and
b10 ~ 0.0395 arcsec/ pixel near the center of the detector. This relationship is illustrated in
Figure C.1.
The values of these coefficients were derived using optical models and apply to the centers of the detectors. On-orbit geometric distortion solutions give marginally different coefficients (WFC3 ISR 2010-09). The corresponding changes in pixel steps in small dithers are insignificant. The corresponding changes in pixel steps in large dithers or mosaic steps are inconsequential, since non-linear distortion makes the step size in pixels variable over the detector.
