Updated the Hartmann coefficients for the collimator piston movements. This is used to focus the spectrograph. Recall that there is a collimator (hitched to the spectrograph wall) in the optical path of light. Best focus is achieved by moving this collimator mirror using three pistons. Hartmann coefficients are used to map the piston movements to the shift in the Hartmann exposures. Hartmann exposures are nothing but arc exposures taken with one side of the light beam closed by a lid. It comes in pairs. Left side closed and Right side closed. The left and right Hartmann exposures are cross-correlated to find the shift. For the best focus, there will no shift between the left and right Hartmann exposures.
To measure the Hartmann coefficients, we took focus sweep data for a plate plugged on to a cart. Focus sweep is nothing but, a set of Hartmanns, normal arc, normal flat exposures are taken with changing piston movements. Here is the fit of Hartmann pixel shifts as function of the collimator piston A (COLLA) position and blue ring angles.
The change in the coefficients is well within 3 percentage (which is the change in coefficients between different cartridges). So, there is no need to update. Will leave it to MaNGA team to decide.
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