... HST . ... Fundamental characteristics of the targetable areas of each Science Instrument is kept in the Science Instrument Aperture File (SIAF) in the project database. ... The Instrument Apertures tables contain parameters relating the instrument coordinates, pixels or deflection offsets, to the telescope coordinate frame. ... The x,y coordinate system relates to each instrument aperture and the angles Beta x and Beta y are measured anti-clockwise from the V3 axis to the x and y axes. ...