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Routine Focus check - Transmission Mode

Assuming the the camera focus is fine in reflection mode (see section ??) then the transmission focus entails a possible shift in slit focus relative to that. Firstly set the grism to the correct angle - it is required to be at 5.2 tex2html_wrap_inline283 to the collimator axis. I believe the grating encoding reads negative for all used angles, both in reflection and transmission, so the grating angle should be set to -5200 millidegrees. To check, the grating cell should be rotated clockwise viewed from above.

Illuminate the fibres with a line comparison source, and take exposures with firstly the right hartmann shutter closed, then with the left closed. measure the Hartmann shift in the centre of the chip, and adjust the slit focus to get the difference (right - left) to be  0.5. Note that a change of +1000 in slit focus implies a change of -0.4 in hartmann shift.