Wide-field-of-view millimeter-wave telescope design with ultra-low cross-polarization
2012
As millimeter-wave arrays become available, off-axis imaging performance of the fore optics increases in importance
due to the relatively large physical extent of the arrays. Typically, simple optical telescope designs are adapted to
millimeter-wave imaging but single-mirror spherical or classic conic designs cannot deliver adequate image quality
except near the optical axis. Since millimeter-wave designs are quasi-optical, optical ray tracing and commercial design
software can be used to optimize designs to improve off-axis imaging as well as minimize cross-polarization. Methods
that obey the Dragone-Mizuguchi condition for the design of reflective millimeter-wave telescopes with low cross-polarization
also provide additional degrees of freedom that offer larger fields of view than possible with single-reflector
designs. Dragone's graphical design method does not lend itself readily to computer-based optical design approaches,
but subsequent authors expanded on Dragone's geometric design approach with analytic expressions that describe the
location, shape, off-axis height and tilt of the telescope elements that satisfy Dragone's design rules and can be used as a
first-order design for subsequent computer-based design and optimization. We investigate two design variants that obey
the Dragone-Mizuguchi conditions that exhibit ultra-low cross-polarization and a large diffraction-limited field of view
well suited to millimeter-wave imaging arrays.
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