For uniform surface brightness sources, an aperture with an area of 1 square arcsec is used.
The filter wheel includes a 4x magnifier that results in 0.12 arcsec pixels for near diffraction limited imaging.
However, with a count rate of counts s -1 arcsec -2 the dark count rate of the detector is generally negligible.
An example would be a delivered image quality of 0.45 arcsec when 0.3 arcsec was requested.
The specification for the former can be estimated using the 0.25 arcsec slit width.