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Hartmann unit

The Hartmann unit can be inserted into the spectrograph beam just in front of the collimator, in order to focus the spectrograph. The unit contains a mask which obscures half of the collimated beam; by rotating the mask it is possible to select which half of the beam is obscured (top, bottom, left or right). The spectrograph can then be focussed using a Hartmann test as described in sec 35.