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 in (az,el), i.e. they can be described as

obsSorcePosInEl = a*obsSorcePosInAz + b

The figure above shows the positional offsets relative to the source position for the fitted Z-focus of all usable focus Lissajou maps. The shift between the 1mm (in red) and the 2mm (in green, upper left) data is caused by 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 obsSorcePosInEl(obsSorcePosInAz) 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.

Three examples for different slopes of obsSorcePosInEl(obsSorcePosInAz). Here the coordinates are relative to the pointing corrections.

For comparison: the same analysis using GISMO 2mm data. The change of the source position is roughly 3 times smaller ! This demonstrates that the problem is not on the telescope side.

The consequences of this problem are quite significant:

- the 1mm and 2mm sources will appear shifted against each other

- in general the source position will be "unstable", i.e. will move depending on defocusing and elevation