How to observe with Turpol - Nordic Optical Telescope
How to observe with Turpol
Point the telescope to your object (in the CAT menu on the TCS
select target and press numerical key 4 to go there). Wait until
tracking. If you have set auto-pos-full at the beginning of the
night, a guide-star will be automatically found and autoguiding started
upon each pointing to a new target.
If the star box is in the centre of the guide TV screen, then for the
first preset of the night, select a bright star. After the automatic
guiding has started, type on the TCS ag-off and select the move
mode with numerical key '5' to be telescope. Set the step
size to for instance 2 arc seconds typing o-s 2 2, and step the
telescope to have the guide star close to the marked position for Turpol
on the guide tv. When you see your stars in the Turpol diaphragms, this
is the correct location of the guide star. Then change the move mode to
be star box and move the box to the star. Then use ag-rel
to start guiding.
On subsequent pointings the star box will not be moved, and the pointing
offset needed for Turpol will be corrected for. The target should land
inside the Turpol diaphragms, or at least very close. (NB! we do not use
the permanent pointing offset command, as we earlier did.)
- Check the centering of the object using the ST-4 CCD
diaphragm viewing . You will probably have to make fine adjustments
to the centering. If you do not see the star, it might be just outside
your diaphragm. Select the open diaphragm, position #3, to check. In order
to center the star, type on the TCS ag-off to stop autoguiding,
set the step size by for example o-s 0.5 0.5 and move both the
telescope and guide probe simultanously in steps using the
Alt Function plus ARROW KEYs until the star is centred in
NB! Make sure TCS move mode is set to Telescope/Guide Probe.
Scroll through move modes with the numerical key 5. Start
relaxed autoguiding by typing ag-rel. This ensures that the
telescope is not moved while the box is centred on the guide star.
- If you do not see your object with the ST-4 camera, and it does not
help to adjust the ST-4 integration time, try to step with
some larger step size, such as o-s 10 10. If still no luck, and
if your object is bright enough for the guide TV, then check that it
lands approximately on the centre spot (marked by staff) or place it
there. To do this:
- type g-p-c to have the guide probe in the centre
- type tv-foc 600 and possibly fine-adjust further
- if the object is faint, type tv-fil o to open the TV filter
- type o-s 1 1 and move the telescope (NB!) to put star on the
- then type get-guide-star on the TCS and possibly adjust
TV-fil and TV-foc. Check if your object is now in the diaphragm.
- change move mode to star-box and move the box to the star
- start relaxed guiding by typing ag-rel
- Give the name of your object in the PP program as a star number under
the menu StarNumber.
- Check the brightness of your object to see if you need to use a
neutral density filter .
- After having the star perfectly centred, type ag-off on the TCS to
stop autoguiding, then type o-s 60 60 and make one offset step
with the telescope for sky measurement. (Make sure you select Telescope
move mode, scroll through move modes with the numerical key 5 on the TCS.)
Take a sky measurement with Turpol by clicking on SKY on the PP
display. Step the telescope back and start autoguiding with the command
ag-on (NB! This command moves the telescope until the star is back
in the box and therefore ensures that the centering in the diaphragms is
maintained.) Wait until it says "autoguiding" on the TCS.
- Make your polarimetric measurement by clicking on INT in the PP
program. Now we assume that you have selected your set-up according to
your choice of Linear polarimetry ,
Circular polarimetry , or
- Check average polarisation degree and angle from the accumulated
observations of the object at any time by clicking on the REDUCE
menu, option: averages. The display will also show the associated
- It is recommended to take sky measurements before and after each object
observation cycle (i.e. normally about every 15 minutes). Also, if you
STOP an integration in the polarimetric mode, it is recommended to
make a SKY integration before restarting.
- PAUSE is a toggle switch which halts the integration, but does not
interrupt the measurement sequence. The flip mirror is turned so that
centering in the diaphragm can be checked - if necessary.
- For an overview of the data obtained so far you may click View Listing.
- Note that due to flexure of the TV-camera, the position of the diaphragms
may change on the TV screen. Make sure you know where they really are
when centering a new target, and don't rely on old markings.