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Positional accuracy and spatial resolution


The positional accuracy available from a WFC observation depends on both the significance of the detection and the systematic uncertainty of the WFC coordinate system, mainly determined by the accuracy of the attitude solution derived from the WFC star trackers.

For an on-axis source (off-axis angle tex2html_wrap_inline3222 ), the estimated positional uncertainties (90% confidence radii, tex2html_wrap_inline3224 ) are: strong source (signal-to-noise tex2html_wrap_inline3226 :)           tex2html_wrap_inline3228

weak source (signal-to-noise tex2html_wrap_inline3230 :)          tex2html_wrap_inline3232 In the ``strong source'' case, where the positional uncertainty is dominated by the accuracy of the attitude reconstruction, the tex2html_wrap_inline3224 value is expected to improve with in-orbit experience.

The spatial resolution for the WFC is shown in Figures 5.4 gif and 5.5gif. Note that the HEW is roughly constant over the central tex2html_wrap_inline3236 of the field of view, but then increases rapidly towards the edge. For studies of structure in extended sources, the FWHM of the instrument is more relevant than the HEW. The behaviour of the FWHM as a function of off-axis angle is shown in Figure 12.13 gif. The degradation of spatial resolution at large off-axis angles should be taken into account for studies of extended sources, and for position determinations of off-axis point sources.

For studies of very extended objects, the detailed behaviour of the WFC background on large angular scales is important. Although initial in-orbit experience is encouraging, the capability of the WFC to measure extended low-surface brightness emission is still under evaluation.

 fig12-1 figure1728

 fig12-2 figure1734

 fig12-3 figure1740

 fig12-4 figure1745

 fig12-5 figure1750

 fig12-6a figure1755

 fig12-6b figure1760

 fig12-7 figure1765

 fig12-8a figure1770

 fig12-8b figure1775

 fig12-8c figure1780

 fig12-9 figure1785

 fig12-10 figure1790

Michael Arida
Tue Jun 11 16:18:41 EDT 1996