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About ALFOSC Polarimetry and the use of filters


General information about filters


Filters can be mounted in two different positions when using ALFOSC. ALFOSC itself includes the ALFOSC filter wheel in which seven filters can be mounted. Additionally, filters can be mounted to the two FASU filter wheels FASU A and FASU B, six positions are available in each of these filter wheels. However, not all filters can be mounted in both wheels due to different sizes of the holders, please check the NOT filter page for information about general availability of filters.

What to consider when choosing filters for your polarimetry observations


In general, for polarimetric observations it is not advised to use filters that are located before the Calcite and retarder plate in the lightpath as reflections inside the filter might introduce instrumental polarization. Filters that are located after the Calcite and retarder in the lightpath do not have any influences on the polarization measurement.

For ALFOSC the order of the optical elements inside the lightpath is the following:

telescope -> retarder plate -> FASU filter wheels -> ALFOSC aperture wheel -> ALFOSC filter wheel -> ALFOSC grism wheel -> CCD

Thus, if possible, filters for polarimetry should be mounted to the ALFOSC filter wheel. This is also valid for second order blocking filters for spectropolarimetry.

In case the used filters can not be mounted to the ALFOSC filter wheel, both the zero-polarization standards and the high-polarization standards should be observed with the used filter in the beam to correct the influence of the filter on instrumental polarization.


Useful links


  • General information on NOT filters

  • A list of all available filters

  • ALFOSC second-order blockers

  • The ALFOSC instrument layout

    If you have any questions, contact NOT staff!


  • Back to top Last modified: 11-Feb-2009