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.
[Quick Links: Dither Patterns | Mosaic 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.
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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. |