"Wedged DoubleWollaston" prism for one-shot polarimetric
observations with ALFOSC
By arrangement with Asiago Observatory, Padova, Italy, a "Wedged Double
Wollaston" prism 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.
Imaging
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=360 and ybeg=820
- if you keep the full xsize the benefit is having pre/overscn
in the frame
Calibrations:
- 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
- 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 360
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.
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