Polarimetry

 

A cornerstone of EST is its ability to perform precise and sensitive polarimetry. The science requirements of EST list specifications for both the polarimetric sensitivity and accuracy.

The polarimetric sensitivity is defined as the ability to detect a signal above the noise and without crosstalk between the Stokes parameters. EST is required to achieve a sensitivity of 3×10-5 of the continuum intensity.  Polarimetric accuracy can be defined as the error in establishing the zero polarisation level. EST will achieve a polarimetric accuracy of 5×10−4 of the continuum intensity.

 

Baseline configuration

 

An important aim of the optical design of EST is to produce time-independent instrumental polarisation to avoid the need for modelling which is required for accurate polarisation measurements. Variations in time must be kept to a minimum, since calibration quickly becomes a time-demanding (and thus expensive) task and reduces the calibration accuracy.

In the baseline optical design, the primary mirror (M1) and secondary mirror (M2) are arranged in a Gregorian configuration, maintaining rotational symmetry. The relative orientation between M1 and M2 remains fixed throughout telescope operations.

The space surrounding F2 is an optimal location for the PCA assembly. The PCA assembly contains a suite of optical elements for polarimetric calibration. These elements can be inserted, rotated and removed from the light path as needed during telescope operations, allowing for effective characterisation of the system's instrumental polarisation starting from this point downstream the optical path.

Following the optical path, a set of mirrors (M3 - M6) directs the light down to the POP, where the beam is divided into VIS (380 - 680 nm) and IR (680 - 2300 nm) spectral arms before being sent to the CLD system and subsequently to each instrument. The azimuth and elevation axes of the telescope are offset from the optical axis, to allow matching these mirrors with incidence-reflection planes perpendicular to each other to create a polarimetrically compensated configuration.

As mentioned above, these mirrors control the pointing of the telescope, so slight changes in the angle of incidence (AOI) on mirrors M3 through M6 occur during the observation, while the following conditions are maintained:

  • M3 and M4 remain fixed relative to the telescope (M1 + M2).
  • M1 + M2 + M3 + M4 rotate together around the elevation axis.
  • M5 + M6 maintain their relative orientation.
  • M1 through M6 rotate together around the azimuth axis.

Therefore, this configuration effectively minimises temporal variations in instrumental polarisation.

A significant effort will be required to ensure sufficient compensation between these mirrors, which imposes very stringent requirements. These include careful selection of mirror coatings to ensure homogeneity between pairs, as well as adherence to strict positioning tolerances.