Designing WFPC2 pointing patterns
Some WFPC2 "convenience" patterns with pre-defined default values are readily available within the APT tool: WFPC2-LINE and WFPC2-BOX. But many more one-dimensional patterns can be defined with the generic LINE pattern form, and two-dimensional patterns can be defined with the generic BOX pattern form. The main physical limitation here is the ~100 arcsec limit for finding guide stars that can be used to execute your pattern. Very large patterns will simply require additional pointings (visits) with the acquisition of additional guide star pairs, and their related overhead times.
Since designing optimal pointing patterns and expressing them in the POS TARG reference frame can be a little tricky, we provide here a suite of carefully-designed dither and mosaic pointing patterns for WFPC2 users to simply choose from. It is always advisable to review your pointing strategy using Aladdin within APT, to ensure that your target is well-placed within your pattern.
Things to consider when choosing a pointing strategy
- Small-scale dithers for artifact rejection and/or sub-sampling.
- Large-scale mosaics to increase the field-of-view for large targets.
- Combining Primary and Secondary patterns, and modifications to improve overall pointing efficiency or "orbit packing" in APT.
- Optimizing for desired chip usage: all 4 chips? all 3 WF chips? just the PC chip? just the WF2 and WF3 chips? (See next bullet.)
- WF4 has a recurrent anomaly which will slightly increase noise and photometric uncertainties.
- Large long-term programs have special issues.
- Send your pointing questions to the STScI Help Desk (firstname.lastname@example.org).
Figure 1: WFPC2 chips, and the POS TARG axes (X,Y) used to define pointing patterns. The CCD readout direction for each chip is indicated by the arrows on each chip, near the center. HST's U2 and U3 axes are indicated.