Change of the source position with Z-focus; RZ 27 Nov 2014

In the Lissajou maps performed to focus the telescope the source changes the position while changing the Z-focus. The positional (= pointing) drifts are (with sufficient accuracy) linear, i.e. they can be described as

observedSorcePosInEl = a*observedSorcePosInAz + b

The figure below shows the positional offsets of all usable focus Lissajou maps relative to the source position for the fitted Z-focus. The 1mm (in red) and the 2mm (in green, upper left) data are shifted against each other because of the ~0.4mm focus difference between both arrays. The positional drifts while changing the Z-focus show a "butterfly" distribution (upper and bottom left). This is because the linear dependance observedSorcePosInEl(observedSorcePosInAz) changes the slope with the elevation, i.e. it rotates in (az,el) with the elevation (bottom left). The two right figures show the positional drifts as function of the Z-focus changes relative to the fitted Z-focus correction.

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Three examples for different slopes of observedSorcePosInEl(observedSorcePosInAz). Here the coordinates are relative to the pointing correctons.