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.
