Space Telescope Science Institute   7.3.4 HRC ACQ Mode  7.5 A Road Map for Optimizing Observations

7.4 Patterns and Dithering


A number of different patterns are available for ACS to support dithered observations, i.e., observations where the pointing is shifted between frames. The size of the offsets can be very different depending on the purpose of offsetting the pointing between exposures. In particular, it is useful to distinguish between mosaicing and dithering. Mosaicing is done with the aim of increasing the area covered by a particular set of exposures, while providing a seamless joining of contiguous frames. Dithering is done for a variety of reasons, such as

Patterns have been defined to allow ACS users to easily carry out both mosaicing and dithering. Using patterns allows exposures to be automatically associated in calacs pipeline processing with the following restrictions: only pattern exposures obtained within a single visit, and those patterns where the cumulative offset is under the ~100 arcsecond guide star limitation, can be associated. For the latter, these patterns include the dither patterns for all three cameras, the HRC and SBC mosaic patterns, and the 2-point ACS-WFC-MOSAIC-LINE pattern. All patterns designed with POS TARGs will not be associated. These are described in detail on the ACS Dither Web page: http://www.stsci.edu/hst/acs/proposing/dither.

The plate scale for the WFC varies by about ±5%, so a one pixel dither near the center will be 0.95 or 1.05 pixels near the corners. For this reason, dither patterns should strike a balance between being large enough to reject detector artifacts, and being as compact as possible to maintain the integrity of the pattern over the entire field-of-view. Large displacements will have varying sub-pixel properties across the image.

In addition to the plate scale variation associated with the significant ACS geometric distortion, there can also be a temporal variation of overall image alignment. Some CR-SPLIT images taken during SMOV testing, in which the two components were separated by the scheduling system across orbital occultations (about a one hour gap), showed registration differences of about 0.5 pixels corner-to-corner. Thus, to combine multiple images to create oversampled images at the resolution ACS is capable of providing, the user may need to allow for the general problem of combining distorted, misregistered images. A variety of tools are being made available within STSDAS and pyraf to assist with these tasks including pydrizzle and multidrizzle. See the ACS Drizzle Web page and the ACS Data Handbook.

Additional information on dithering, and processing drizzled data using multidrizzle can be obtained from the Dither Handbook


 7.3.4 HRC ACQ Mode  7.5 A Road Map for Optimizing Observations
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