For imaging programs, STScI generally recommends that observers employ dithering patterns. Dithering refers to the procedure of moving the telescope by pre-determined amounts between individual exposures on a target. The resulting images are subsequently combined via post-observation processing techniques using software such as
MultiDrizzle (see the
MultiDrizzle Handbook).
Use of dithering can provide improved sampling of the point spread function (PSF) and better correction of undesirable artifacts in the images (e.g., hot pixels, cosmic rays, the UVIS channel’s inter-chip gap, and the UVIS “droplets”). Cosmic ray removal is more effective if more than 2 images are obtained, using CR-SPLIT exposures and/or dithers, especially for exposure times greater than 1000s. A sequence of offsets of a few pixels plus a fractional pixel in each coordinate is generally used to simultaneously remove hot pixels and cosmic rays and to sample the PSF. A larger offset along the image Y axis is needed to fill in the interchip gap in full-frame images (the WFC3-UVIS-MOS-DITH-LINE pattern uses a conservative step size of 2.4 arcsec). To ensure the best accuracy consider dithering to compensate for droplets (
Section 6.9.6).
Larger offsets, up to sizes approaching the detector’s field of view, can also be used to create mosaics. However, as a result of geometric distortion (
Appendix B), some objects shift by an integer number of rows (or columns), while others shift by an integer plus some fraction of a pixel. The PSF is not resampled in that dimension in the former case, but is resampled in the latter case. Where the exposures overlap, the PSF is thus better sampled for some objects than for others. If PSF sampling is important, a combination of mosaic steps and small dither steps should therefore be used. Note that, in practice, mosaic steps 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 for all exposures. The rms pointing repeatability is significantly less accurate if different guide stars are used for some exposures (see Section 4.1 of the
MultiDrizzle Handbook).
The set of Pattern Parameters in the observing proposal provides a convenient means for specifying the desired dither pattern of offsets. The pre-defined mosaic and dither patterns that have been implemented in APT to meet many of the needs outlined above are described in detail in the Phase II Proposal Instructions. The WFC3 patterns in effect in APT at the time of publication of this Handbook are summarized in
Appendix C. Observers can define their own patterns to tailor them to the amount of allocated observing time and the desired science goals of the program. Alternatively, they can use POS TARGs to implement dither steps (
Section 6.4.3), but the exposures will then not be automatically associated and combined in the OPUS pipeline
While the design of WFC3 precludes the simultaneous use of both the UVIS and IR channel, it is possible to use one or more of the other
HST instruments in parallel with WFC3. Since each instrument covers a different location in the
HST focal plane (see
Figure 2.2), parallel observations typically sample an area of sky several arc minutes away from the WFC3 target. For extended targets such as nearby galaxies, parallel observations may be able to sample adjacent regions of the primary target. In other cases, the parallel observations may look at essentially random areas of sky.
For processing and scheduling purposes, HST parallel observations are divided into two groups: coordinated and pure.
A coordinated parallel is an observation directly related to (i.e., coordinated with) a specific primary observation, such as in the extended galaxy example above. A
pure parallel is an observation typically unrelated to the primary observation, for example, parallel imaging scheduled during long spectroscopic observations. The primary restriction on parallel observations, both coordinated and pure, is that they must not interfere with the primary observations: they may not cause the primary observations to be shortened; and they must not cause the stored-command capacity and data-volume limits to be exceeded. The proposal software (APT) enforces these rules and notifies the observer when a specified parallel is not permitted.
In order to prolong the life of the HST transmitters, the number of parallels acquired during each proposal cycle is limited. Proposers must provide clear and strong justification in order to be granted parallel observing time. Please refer to the
HST Call for Proposals for current policies and procedures concerning parallels.
The observing technique of spatial scanning can be used to turn stars into well-defined streaks on the detector or to spread a stellar spectrum perpendicular to its dispersion. Although the technique can be used to good effect in imaging mode with either the UVIS or the IR detector (see
Section 7.10.4), order overlap may limit the useful length of UVIS spectral scans or the effective free spectral range of the scanned UVIS spectra.
