On startup of the PP programme, some default parameters are set: linear polarimetry, integration time 10s, number of cycles 4. These values are normally used for stellar observations and one complete cycle takes about 15 minutes. You can normally use the default set--up for linear polarimetry. Give a star number for identification of your object. For objects brighter than about 9th magnitude, filter no.1 (ND1) may have to be used, and for those brighter than 6th, filter no.2 (ND2). The list of available diaphragms and filters is given in Table 2.1. PP will warn and close down when too much light reaches the PMTs, but be careful with very bright stars.
Go to the dome and open the SHUTTER of the polarimeter head (see Fig. 1) by pushing it IN. Take great care with flashlights and keep the dome lights off when the shutter is opened. The SHUTTER must be closed (position=OUT) during the day for the safety of the photomultipliers.
Point the telescope to your first object. You will see that the guide probe moves to a position; this is given by the automatic guide star selection programme running on the lisa PC. When a beep is given and the TCS states that it is tracking, you should see the guide star. If not because the star is too faint, give the command: tv-fi o. When doing this for the first time during a run, fine tuning of the tv focus may be done, using the command tv-fo-po nnn. When the guide star is visible and in focus, type cc-ad-tv gu to adopt this guide configuration. Note that the guide TV has an automatic over--exposure detection system to protect the camera. When a star that is too bright is detected, the system puts in the gray filter, if still too bright the camera is closed.
Type cc-pe-tv to go to the periscope configuration, wait for it to
arrive, and you should see your object in the diaphragms. Fine tune
the centring by typing tm-os .5 .5 and using the arrow keys to
move the telescope. When well centred, type cc-gu-tv, move the arrow
on the auto guider screen onto the guide star, click on the middle
mouse button (star), and type ag-re-on to start auto-guiding with
automatic centring of the guide probe box on the star. Once the text
auto-guide has appeared on the TCS screen, type ag-of to
switch the auto-guider off, and do an off--set of, say, 60
by
typing tm-os 60 60 and giving one off--set step with an arrow key.
Start a SKY integration in the PP programme. Once this integration is
finished, do the opposite off--set step to bring the star back into the
auto-guider box, and type ag-on to start auto-guiding without
centring the box. This will bring the star back into the centre of the
diaphragm. Now you can start integrating by clicking INT on the PP
screen. If you did not see your object after going to the periscope
mode, it may be that it falls just outside the diaphragm. Either
``hunting around'' with small off--set steps or going to the centre
configuration with cc-ce-tv and putting the object on the mark on
the screen will get you your object.
Once one or more complete observing cycles have been done, with sky
samples in between, stop the auto-guider with ag-of, and finish
with another sky sample. A complete sky measurement is only one
sub--integration, and thus takes little time.
To go to another object, follow the instructions above. It is
recommended, especially when the sky is bright and rapidly varying, to
take a SKY measurement before and after each object observation
cycle. Sky background can be highly polarised too (this is cancelled
by the polarisation optics), but the varying intensity reduces the S/N
of the data. So frequent measurements are needed to allow good
interpolation of the sky brightness to be done.