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.