Text Size:
Smaller Text Normal Text Larger Text


Follow us on Twitter

"Wedged DoubleWollaston" prism for one-shot polarimetric observations with ALFOSC

By arrangement with Asiago Observatory, Padova, Italy, a "Wedged Double Wollaston" prism (Oliva (1997) A&AS, 123, 589) for one-shot polarimetric observations with ALFOSC will be on loan to the NOT for a three-year period, starting April 1, 2010 (i.e. in Period 41).

The capabilities of the prism are described in Padova/Asiago Technical Report no. 18 (local copy) or from the Asiago Observatory.
The Asiago prism has been tested earlier at the NOT, and actual performance here is expected to be very close to that described in the Asiago report.

WeDoWo polarimetry supposes the one shot imaging procedure, very easy and quick to perform if a good flat field and a good zero polarization standard has been obtained. It is useful in the case you are interested to observe a star object with only one shoot.


With the 10 arc-second slit the FOV is 10 arcsec times the length of the slit.

Set the CCD Window (afternoon by the observer):

  • select 10 arc sec horizontal slit
  • select WeDoWo (in grism wheel)
  • make sure all the other wheel positions are open and no other optical elements in the beam
  • turn on the ThAr calibration lamp
  • take a 0.3 sec image and note the ysize and ybeg and possibly xsize and xbeg for the polarimetry window about ysize=400 and ybeg=820
  • if you keep the full xsize the benefit is having pre/overscn in the frame


  • Day time
    • take a dome flat fields (10 images for example) with slit/polarimetric mask, filter and polarimeter inserted with rot-pos 0 deg and
      second set with rot-pos 90 deg
      • put the halogen "Dome FF Lamp" under the telescope
      • for blue or narrow filters open sideports, note only when the weather allows (no rain, humidity less than 90%)
      • power-on the telescope
      • set the rotator to 0 degrees (TCS rot-pos 0)
      • open mirror covers
      • set the telescope to altitude 45 or so, (TCS alt 45)
      • make sure the guide probe is in the "Park"-position (out of any light path)
      • take the data
      • change the filter and/or set the new rotator position and take the data etc.
  • night time
    • low polarization standards in order to estimate instrumental polarization
    • high polarization standards in order to estimate the position angle zero point
Note that it is recommended to take the polarization standards with the same field-rotation angle as your science frames.

Telescope focus:

The best focus with the "WeDoWo+ no filter" is about +40 TCS focus units greater ('tcs.foc-del +40') than the one with "no filter".

Target acquisition:

  • take a target acquisition image with the required filter in the beam
  • offset the telescope
  • take a second image with the WeDoWo in the beam for fine tuning the position and estimating the needed integration time.
    For target acquisition you can use "alfosc.acquisition" script e.g.
    tessa$ alfosc.acquisition -q Slit_10.0 2
    tessa$ alfosc.resetxy
    tessa$ alfosc.ysize 400
    tessa$ alfosc.ybeg 820
    tessa$ alfosc.wheels -g WeDoWo
    tessa$ alfosc.expose 10
  • If you want to have two targets on the slit use "alfosc.slitrot"

Imaging Setup (by staff):

  • 10 arc sec horizontal slit in the aperture wheel
  • WeDoWo in the grism wheel, in such way that the image is split vertically (see image)
  • use broad band filters either in FASU or filter wheel.

The WeDoWo should be installed in the grism wheel in such a way that it splits the beam vertically. When inserting it in the slot, the element's dividing line should be horizontal. Install the 10 arc-second wide slit horizontally in the aperture wheel. In editsetup select WeDoWo with orientation Horizontal and and Slit_10.0" with orientation Horizontal.

Back to top Last modified: 05-Apr-2018