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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 both diaphragms. NB! Make sure TCS move mode is set to Telescope/Guide Probe. Scroll through move modes with the numerical key 5. Start now 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:

    1. type g-p-c to have the guide probe in the centre
    2. type tv-foc 600 and possibly fine-adjust further
    3. if the object is faint, type tv-fil o to open the TV filter
    4. type o-s 1 1 and move the telescope (NB!) to put star on the mark
    5. 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.
    6. change move mode to star-box and move the box to the star
    7. 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 Photometry .

  • 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 errors.

  • 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.

Back to top Last modified: May 31 2023