Polarimetric Imaging Flat Fields

Contents


Differences between Simple and Polarimetric Imaging

Polarimetric Imaging mode is very similar to the simple imaging mode. Obervers should go through the  Dome/Sky Flats - Imaging page before continuing here. Further, the difference between the sky and dome flats is identical for the two groups.

  • Observations are made with the Half- or Quarter-Wave Plate, Wollaston Prism and the Wollaston mask in the optical path (in addition to the filter).
  • The minimum exposure time allowed by the procedure is 40 seconds.
Other than these two all the issues dealt with under simple imaging flats are valid.

Polarimetry Issues

The Wollaston prism is mounted on the grism wheel and the Wollaston mask on the aperture wheel. The elements have to be chosen accordingly while defining the OB using jP2PP.

Ensure that the width of the strips on the Wollaston mask corresponds to the throw of the image due to the Wollaston prism. Also ensure that the orientation of the mask and prism are appropriate. The support astronomer will ensure that everything has been mounted appropriately but please make sure that you do not request conflicting elements (e.g. a 10" mask with a 20" prism or a parallel mask with a perpendicular prism). It takes a while to rotate and align the Wollaston prisms in their holders accurately. Observers should not request a change in the orientation during the night.

A Half Wave Plate is introduced into the optical path and set into continuous rotation during Flat frames - this ensures that the light incident on the CCD is completely depolarised. The HWP completes a rotation in 2.3 seconds, however we have found that a minimum exposure time of 40 seconds is required to ensure uniformity of illumination.

Dome Flat - Polarimetric Imaging

jP2PP Panel with Typical Parameters   (click on link to see panel)

The above example shows an OB comprising a sequence of

  • 5 B-band Polarimetric dome flat frames with 20" Wollaston Prism and Perpendicular Wollaston Mask at 40,000 ADU level with normal readout and 2x2 binning followed by
  • 5 V-band Polarimetric dome flat frames with 20" Wollaston Prism and Parallel Wollaston Mask at 40,000 ADU level with normal readout and 2x2 binning and finally
  • 5 R-band Polarimetric dome flat frames with 10" Wollaston Prism and Parallel Wollaston Mask at 40,000 ADU level with normal readout and 2x2 binning.
These are the only 3 possible configurations of Wollaston Prisms and Masks possible. Refer to  Dome/Sky Flats - Imaging  for other possible choices of filter, binning etc

The FITS image output by the template is named  EFOSC_FlatPolIma.#.fits

Sky Flats - Polarimetric Imaging

jP2PP Panel with Typical Parameters   (click on link to see panel)

The above example shows an OB comprising a sequence of

  • U-band  Polarimetric sky flat  frames with  20" Wollaston Prism  and Perpendicular Wollaston Mask  at  40,000 ADU  level  with normal readout  and  2x2 binning
The other 2 combinations of Wollaston mask and prism shown under polarimetric dome flats are also possible options for the sky flats.

A polarimetric imaging sky flat requires twice as long an exposure as an equivalent simple Imaging sky flat. One can take only 2 or 3 sets of flats like the above during twilight.

The FITS image output by the template is named  EFOSC_FlatSkyPolIma.#.fits

As an example the following image is given, which shows a Polarimetric dome flat with 20" Wollaston Prism and Parallel Wollaston Mask at 40,000 ADU level with normal readout and 2x2 binning. Click on the image to get a larger version:

 
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