Polarization effects due to the segmented primary mirror of the Thirty Meter Telescope
Abstract
The instrumental polarization (IP) and crosstalk (CT) due to the telescope optics affect the accurate polarimetric measurements. In the case of the Cassegrain focus telescopes, they are found to be negligible. The next-generation telescopes such as Thirty Meter Telescope (TMT) consists of segmented primary mirror and an inclined Nasmyth mirror. The effect of these two mirrors on the polarimetric measurements need to be estimated. Towards that, a polarization model for TMT has been already developed with primary mirror approximated as a monolith. The model estimates an IP of 1.2% and CT of 40% at 600nm for on axis rays. Here we present the results based on a more realistic approach considering the effect of individual mirror segments. A total of 492 hexagonal segments are divided into six sectors with 82 unique segments in each sector. We performed the polarization ray tracing in Zemax for this configuration and obtained the polarization pupil maps. The Mueller matrices are estimated which gives the IP and CT. The difference between the IP for the monolith and segmented mirror is in the order of 10^-4 and CT in the order of 10^-3 respectively. We have analysed the cases of missing segments and their effect on the Mueller matrices. We find that as the number of missing segments increases, though the I-> Q and U->V does not change considerably, the I->U, I->V and Q-> V components appear in the Mueller matrices. As a part of coating non-uniformity study, we generated six different coating recipes with 5-10% variation in the refractive index of silver. The Mueller matrices showed the changes in the IP and CT elements. Along with these, the cases of random segment tilts and piston error on the Mueller matrices have been estimated. The variation in the piston of the segments is found to have no effect on the polarization measurements where as tip-tilt causes changes in some of the Mueller matrix elements. As, none of the first-generation instruments for the telescope have a polarimetric capability, these estimations would help in the design aspects of a second-generation instrument with the polarimetric capability.
- Publication:
-
American Astronomical Society Meeting Abstracts #233
- Pub Date:
- January 2019
- Bibcode:
- 2019AAS...23343701A