What is a Hartmann mask?
A Hartmann mask blocks one half of the beam at a time. In ESPaDOnS, the mask can block the UP part or the BOTTOM part of the beam of light. This is used to focus the spectrograph. If the spectrograph is not focussed, each position of the Hartmann mask will produce spectral lines at different locations on the detector. If the spectrograph is focussed, the spectral lines are superimposed. This is a figure to explain how the Hartmann mask is used.
