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Note that due to the long optical path and high magnification in the periscope optics, only stars of about magnitude 12 or brighter can be seen there. This also depends strongly on the seeing, and the moon phase. For fainter, and therefore invisible objects, either a blind off--set can be done from a nearby brighter object, to which the off--set is accurately known, or the CE mark has to be relied upon. Once centred using the centre position, fine centring can be done by stepping in small steps and ``peaking--up'' the signal in the count-rate box on the PP screen. The signal goes down sharply when the object hits the edge of the diaphragm, count steps to the other side, and put in the centre by taking half the number of steps in the opposite direction. Do this in both RA and Dec. When StanCam is running, it can be used for centring by calibrating the centre of the diaphragms onto the CCD pixels and using a CCD image to determine an accurate off--set. StanCam is the CCD standby camera of the NOT which is always mounted and available when requested in the NIRF. This method has not been used to date.

Average polarisation degree and angle values from the accumulated observations of the object can be viewed at any time by clicking on the REDUCE menu, Averages option. The associated errors are also displayed. Another useful item here is the ``View Listing'' option, which shows the list of data obtained up to that point, including instrumental magnitudes.

The error message 'Retarder found in wrong position' may appear, if the stepper motor has lost one or more steps. The effect in the measured position angle is usually entirely negligible (a small fraction of a degree) and the message can be neglected. Remove it from the screen with a mouse click. The retarder is automatically put in the correct position at the start of each measurement cycle (set of 8 sub--integrations).

The error message 'TMP Channel 2 timeout' is seen immediately after starting the PP-POLAR program if the Polarimeter head power is not ON, or the chopper does not rotate. Cure this by switching ON the power, or the chopper OFF and ON again.

CAUTION ! Do not switch on the fluorescent lights in the control or electronics room during an integration, as bursts of error counts may result.

If you STOP an integration in the Polarimetric mode, it is recommended to make a SKY integration before restarting. The retarder position is initialised when you click INT after the STOP command.

PAUSE is a toggle switch and will not interrupt the measurement sequence, except for halting the integration, and the flip mirror is turned so that the centring of the star in the diaphragm can be checked with pe-t, if deemed necessary.

next up previous contents
Next: Circular polarimetry Up: Polarimetric observations Previous: Linear polarimetry

Lars Freyhammer
Wed Oct 27 18:02:24 ACT 1999