Summer Fun with Optics Stephen Muchovej UC Berkeley CARA - REU Program

Overview South Pole 8 meter Telescope Optics Design Bolometer Construction and Calibration Testing of Fourier Transform Spectrometer

Telescope Optics When it comes to telescopes, bigger is better; or at least more detectors is better. Current Telescopes/Receivers at the South Pole are limited in size and complexity, ~10 detectors. Next step: large focal planes. Think CCDs for mm wavelengths

1000 element Array for South Pole Telescope Large focal plane presents significant challenges

Telescope Optics Design ZEMAX (Optics Design Program) Spot Diagrams at Focus Field of View Stops (Important for detector Sensitivity) Sample spot diagram for a single field Geometrical Ray Tracing

Original Design The original design I developed was fairly simple. Simple design with a total of 4 mirrors and 2 lenses.

Ray Fans and Spot Diagrams Ray Fan Plot Full-Field Spot Diagram

Limitations of simple design Vaccum window D<6 inches. Lens outside Dewar Lenses are emissive; warm lens reduces detector sensitivity. Need to have cold (~4K) Stop to truncate stray radiation. Solution: Design using cooled mirrors (in progress…)

Bolometers: Sensitive Detector of Radiation. Semiconductor thermistor measures temperature change. Sensitivity increases with decreasing temperature Sensitivity limited by statistical arrival of background photons Heat Sink: 1.2K   Absorber Weak Thermal Link

Bolometer Construction Bismuth Coated Saphire Wafer on Kevlar wires NTD-4 (Neutron Transmutation Doped) chip. Precise Semiconductor 0.5 mm copper wires going to the leads Lots of microscope work (aka “Summer Camp Arts and Crafts”)

Cooling the Bolometer NTD-4 chip works at temperatures below 2K Dewar is first filled with liquid Nitrogen Liquid N 2 is then drained and filled with Liquid He Vacuum pump is used to cool the He to 1.2K Putting Nitrogen into the Dewar Pumping the Helium

Millimeter wavelength Spectroscopy Bolometers are broadband radiation detectors. Filters must be used to limit spectral response. This is especially true for ground based millimeter wavelength observations where atmosphere can be opaque (hot). Fourier Transform Spectroscopy (FTS) used to determine the spectral response of bolometer and filter system. Measured ACBAR bands plotted on top of Atmospheric Transmission at the South Pole.

Fourier Transform Spectrometer: FTS Michelson interferometer with moveable mirror Computer Controlled translation stage and DAQ Single frequency signal results in delta function Fourier Transform. We used Eco-sorb foam and liquid Nitrogen (close approximation to a blackbody).

Results and Conclusions Bolometer has been characterized. Hope to run bolometer on FTS (successfully) this week. Characterize response and compare with theoretical model from beam-splitter. Bolometer/FTS: Optics Design: The telescope using a single lens produced desired results. Next step is to design one without the aid of lenses outside the dewar. Harder than it looks! Still in Progress