Preliminary design techniques to mitigate the polarization effects due to telescope optics of the Thirty Meter Telescope (TMT)

Abstract

In polarimetric observations of astronomical sources, instrumental polarization and crosstalk from the telescope optics affect the measurements. They arise due to the reflections from the mirror surfaces of the telescope. An analytical model was developed to estimate these values for the Thirty Meter Telescope. The model determines an instrumental polarization of 1.2% and crosstalk of 40% at a wavelength of 0.6 μm. The analyses indicated that the Nasmyth (tertiary) mirror of the telescope, (used to fold and steer the light to the various instrument ports) is the primary source of the high values of instrumental polarization and crosstalk. We present here, a possible optical layout to cancel out the effects introduced by the Nasmyth mirror. Different configurations are studied to reduce/cancel the effect of the IP and crosstalk before the light from the Nasmyth mirror, reaches the instrument. While the optical components introduced after the tertiary mirror reduce the telescope instrumental polarization and cross talk, it leads to a reduction in the intensity due to additional reflections.