HUDF Paper

New Section 2.5:

2.5 Small Scale Pointing: Dithers

In addition to the large rotations between different phases of the
observations that were required for scheduling, smaller shifts in
telescope pointing were applied to different observations at the same 
position angle. These small changes in pointing between exposures, 
referred to as dithers, were introduced on two different scales.

Firstly, very small dithers were applied to each of the four exposures
within a two orbit visit. This dither pattern was intended to improve the
sampling of the final image by introducing half-pixel offsets and to ensure
that detector defects, such as bad columns, did not overlap on the combined image.
The ACS/WFC detector critically samples the effective point spread function
in the reddest bands but significantly undersamples at shorter wavelengths.
Such undersampling leads to loss of spatial information and aliasing artifacts.
The introduction of sub-pixel dithering improves the sampling and allows
the reconstruction of a higher-resolution final image and a reduction of
artifacts. In the case of ACS/WFC, half-pixel dithers, or small integer
numbers of pixels plus a half-pixel, in both X and Y directions provide
adequate sampling and this strategy was adopted. The integer pixel
components of the dithering were chosen to create the most compact dither
pattern that ensured that a bad row or column could not overlap in the
combined image because the pointings were always at least 1.5 pixels
away from other others in both X and Y. This final four-point dither,
suggested by Stefano Casertano, is shown in figure XXXX and tabulated
in table XXX. The most compact pattern was chosen because the exact
sub-pixel shifts will be different far from the center of the detectors
because of the very large non-linear component of the ACS/WFC optical distortion
and we wished to minimize this effect and ensure optimal sampling across
the field.

       Exposure within       Sky offset (arcsecs)    Pixel Offset
       Dither Pattern                                    

                 1            (0.0000,0.00000)        (0.0,0.0) 
                 2            (0.1488,0.08595)        (3.0,1.5)
                 3            (0.2232,0.24075)        (4.5,4.5)
                 4            (0.0744,0.15480)        (1.5,3.0)

The second dithers were introduced between visits and were approximately 3  
and 6 arcseconds in length in the direction perpendicular to the gap between
the two ACS/WFC chips. These offsets ensure that the regions of sky falling 
in the gap between the two ACS/WFC chips had at least two thirds of the exposure
of the rest of the field and hence minimized the lack of uniformity of
the final exposure map.