Dome/Sky Flat Fields -- SpectroPolarimetry


Spectropolarimetry Flat frames are obtained in a manner similar to Spectroscopic Flats. Observers should go through the Dome/Sky Flats - Spectroscopy page before continuing here. Only the differences are highlighted here.

Long Slit Spectroscopy versus SpectroPolarimetry

  • Filter Wheel is set to Free in Long slit spectroscopy while a Wollaston prism is in that position for SpectroPolarimetry
  • Many slit options are available for Long slit spectroscopy (Slit#?.?in an OB) while only one special SpectroPolarimetric mask/slit (WollMask= in an OB) is available
  • The minimum exposure time allowed by the template is 1 second for Long slit spectroscopy and 40 seconds for SpectroPolarimetry. Other than these two all the issues dealt with under simple spectroscopic flats are valid.

SpectroPolarimetry Issues

The Wollaston prism is mounted on the filter wheel (unlike the polarimetric imaging case when it is on the grism wheel). Note The 10" Wollaston prism cannot be mounted on the filter wheel. Thus only the 20" Wollaston Prism is available for spectropolarimetry.

The Wollaston mask in the case of spectropolarimetry consists of a 1.5" long slit masked by parallel strips slightly more than 20" wide and separated by 20". Since grisms are always aligned with dispersion along the y-axis the slit has to be along the x-axis and so the Wollaston prism has to be aligned so as to split the images along the x-axis - i.e. it has to aligned as for a perpendicular mask. The support astronomer will ensure that the orientation is correct but observers should ensure that they do not inconsistent elements in their set-up request.

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. To ensure uniformity of illumination the procedure requires a minimum exposure time of 40 seconds.

Refer to  Polarimetric optical elements.

jP2PP panel with typical parameters

jP2PP Panel with Typical Parameters

The above example shows an OB comprising a sequence of

  • 5  SpectroPolarimetric flat frames of 40,000 ADU with  20" Wollaston Prism , Grism #4Wollaston Mask (1.5" SpecPol Mask), normal readout and 2x2 binning
This is the only combination of Wollaston Prism and Mask available

Note : If more than one mode of Polarimetric observation is planned the observer should contact the support astronomer before making the OB. For eg.  Spectropolarimetry requires a perpendicular Wollaston mask (i.e. the 1.5" spectropolarimetric slit)  and possibly the observer might also want  to do polarimetric imaging with a perpendicular Wollaston (imaging) mask. However there is only one slot defined for a perpendicular mask in jP2PP. The solution is trivial in that one can load the second perpendicular mask in the slot and name it as a parallel mask and choose the same in jP2PP. However this must be discussed beforehand to ensure that jP2PP definitions and instrument settings are consistent with the correct masks loaded in the corresponding slot on the instrument.

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

As an example the following image is given, which shows a SpectroPolarimetric flat of 40,000 ADU with 20" Wollaston Prism, Grism #13, Wollaston Mask (1.5" SpecPol Mask), normal readout and 2x2 binning. Click on the image to get a larger version:

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