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Influence of mount equilibrium / tube flexion / seeing on resolution

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Added 20-06-2010: to decrease the criticality of equilibrium adjustment, I have turned my telescope tube. Now the CCD is on the side of the tube that is facing the mount. My setup is thus equivalent to the one described by Laurent Bernasconi.

Adjusting the mount equilibrium (position of counterweights) is a bit tricky, as the CCD cable is quite heavy and tends to pull the telescope tube (the cross section of the cables must be large for the Peltier cooler...). Ideally, the equilibrium must be checked for every new orientation of the tube and CCD camera. To do it accurately, I used a method with two elastics to mark the extreme positions of the counterweights where the equilibrium starts to be lost. Then I put the counterweight right in the middle between the two elastics.



Below 3 images of M101 taken on successive nights (19, 20 and 23 May 2010) with different methods of equilibrium adjustment.

picture 1: equilibrium was set only at the beginning of the night
picture 2: equilibrium was checked for every new orientation of tube / CCD camera / cables before starting CCD acquisitions
picture 3: as picture 2, with counterweights position being accurately measured with the two elastics method.

The effect seen on the images is clearly due to mount equilibrium, though it's still possible that the cable weight also induced some slight tube flexion, or that seeing was different during the successive nights (though I could not see any obvious difference).