Field Scanner Design for MUSTANG of the Green Bank Telescope

TL;DR

This paper analyzes various optical configurations for the MUSTANG bolometer camera on the GBT, identifying a paraboloid mirror pair as optimal for minimizing aberrations and achieving the required field of view for effective atmospheric noise cancellation.

Contribution

It introduces a ray tracing analysis of different scanning mirror arrangements, proposing a paraboloid mirror pair as the best solution for the GBT system.

Findings

Paraboloid mirror pair achieves near diffraction-limited spot size within 3 arcmin.

Other configurations suffer from larger off-axis aberrations.

The optimized system meets the field of view requirements for MUSTANG.

Abstract

MUSTANG is a bolometer camera for the Green Bank Telescope (GBT) working at a frequency of 90 GHz. The detector has a field of view of 40 arcseconds. To cancel out random emission change from atmosphere and other sources, requires a fast scanning reflecting system with a few arcminute ranges. In this paper, the aberrations of an off-axis system are reviewed. The condition for an optimized system is provided. In an optimized system, as additional image transfer mirrors are introduced, new aberrations of the off-axis system may be reintroduced, resulting in a limited field of view. In this paper, different scanning mirror arrangements for the GBT system are analyzed through the ray tracing analysis. These include using the subreflector as the scanning mirror, chopping a flat mirror and transferring image with an ellipse mirror, and chopping a flat mirror and transferring image with a pair of face-to-face paraboloid mirrors. The system analysis shows that chopping a flat mirror and using a well aligned pair of paraboloids can generate the required field of view for the MUSTUNG detector system, while other systems all suffer from larger off-axis aberrations added by the system modification. The spot diagrams of the well aligned pair of paraboloids produced is only about one Airy disk size within a scanning angle of about 3 arcmin.