Q: How accurate are the ACS aperture overlays in the VTT?

A: The VTT uses the same HST aperture file to overlay apertures on sky
images as is used by the telescope during scheduling and
execution. The positions of apertures such as WFC, WFC1, WFC2,
WFCENTER, etc. should therefore be astrometrically accurate. However,
two things should be noted. First, the absolute pointing accuracy of
HST is only of order 1 arcsec. Second, the aperture and detector areas
are drawn in VTT as quadrilaterals. This is only an approximation to
the true shape because of the geometric distortion in ACS. In reality,
the aperture and detector areas are bounded by curved lines. These
might deviate from the lines drawn in the VTT by as much as 1 arcsec.