2.4	Dithering Considerations

The ACS/WFPC2 Team recommends that observers dither (or offset) their observations to remove hot pixels, cosmetic defects, and cosmic rays in their combined images. Dithering allows optimal sampling of the point spread function and yields better images than are possible with the CR-SPLIT option, which does not remove hot pixels, permanent cosmetic defects (e.g., bad columns), or the gap between the WFC CCDs. Dithering is especially important in this cycle as the number of hot pixels has reached nearly 1.6% after many years in orbit.

Dithering can be performed in two ways:

1.	explicit positional offsets between exposures via POS-TARG instructions; or

2.	flexible predefined dither patterns which can be nested to implement different pixel subsampling strategies.

Both methods yield associations of images for data pipeline processing. Currently available predefined dither patterns and their recommended uses are described on the ACS Dither Web page,

http://www.stsci.edu/hst/acs/proposing/dither

The ACS/WFPC2 Team at STScI is available to help observers select dither patterns that best suit their science goals. Observers who choose not to dither their ACS/WFC exposures must justify not doing so in the Description of Observations section of their Phase I proposal.

Figure 2.1: HST total system throughputs as a function of wavelength.

The plotted quantities are end-to-end throughputs, including filter transmissions calculated at the pivot wavelength of each broad-band filter. Full lines represent instruments currently active on board HST. Dashed lines represent instruments that are no longer offered (HRC, NICMOS) and a previously flown instrument (WFPC2).

Figure 2.2: HST survey discovery efficiencies.

HST survey discovery efficiencies of the cameras, defined as the system throughput multiplied by the area of the field-of-view. Full lines represent instruments currently active on board HST. Dashed lines represent instruments that are no longer offered (HRC, NICMOS) and a previously flown instrument (WFPC2).