oai:arXiv.org:2407.08912
sciences: astrophysics
2024
7/17/2024
Front-end polarization modulation enables improved polarization measurement stability by modulating the targeted signal above the low-frequency $1/f$ drifts associated with atmospheric and instrumental instabilities and diminishes the impact of instrumental polarization.
In this work, we present the design and characterization of a new 60-cm diameter Reflective Half-Wave Plate (RHWP) polarization modulator for the 90 GHz band telescope of the Cosmology Large Angular Scale Surveyor (CLASS) project.
The RHWP consists of an array of parallel wires (diameter $50~\mathrm{\mu m}$, $175~\mathrm{\mu m}$ pitch) positioned $0.88~\mathrm{mm}$ from an aluminum mirror.
In lab tests, it
was confirmed that the wire resonance frequency ($f_\mathrm{res}$) profile is
consistent with the target, $139~\mathrm{Hz} The mirror tilt relative to the rotating
axis was controlled to be $<15''$, corresponding to an increase in beam width
due to beam smearing of $<0.6''$, negligible compared to the beam's full-width
half-maximum of $36'$. The median and 16/84th percentile of the wire--mirror
separation residual was $0.048^{+0.013}_{-0.014}~\mathrm{mm}$ in the optically
active region, achieving a modulation efficiency
$\epsilon=96.2_{+0.5}^{-0.4}\%$ with an estimated bandpass of 34 GHz. The
angular velocity of the RHWP was maintained to an accuracy of within $0.005\%$
at the nominal rotation frequency ($2.5~\mathrm{Hz}$). The RHWP has been
successfully integrated into the CLASS 90 GHz telescope and started taking data
in June 2024, replacing the previous modulator that has been in operation since
June 2018. ;Comment: 23 pages, 12 figures, 1 table, to appear in Proc. SPIE Astronomical
Telescopes and Instrumentation 2024
Shi, Rui,Brewer, Michael K.,Chan, Carol Yan Yan,Chuss, David T.,Couto, Jullianna Denes,Eimer, Joseph R.,Karakla, John,Shukawa, Koji,Valle, Deniz A. N.,Appel, John W.,Bennett, Charles L.,Dahal, Sumit,Essinger-Hileman, Thomas,Marriage, Tobias A.,Petroff, Matthew A.,Rostem, Karwan,Wollack, Edward J., 2024, Design and characterization of a 60-cm reflective half-wave plate for the CLASS 90 GHz band telescope