Summer Fun with Optics

1
Summer Fun with Optics

• Stephen Muchovej
• UC Berkeley
• CARA - REU Program

2
Overview

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

3
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

4
1000 element Array for South Pole Telescope
Large focal plane presents significant challenges
5
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
6
Original Design

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

7
Ray Fans and Spot Diagrams
Full-Field Spot Diagram
Ray Fan Plot
8
Limitations of simple design

• Vaccum window Dlt6 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)

9
Bolometers

• Sensitive Detector of Radiation.
• Semiconductor thermistor measures temperature
  change.
• Sensitivity increases with decreasing temperature
• Sensitivity limited by statistical arrival of
  background photons

g
g
Absorber
Weak Thermal Link
Heat Sink 1.2K
10
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)

11
Cooling the Bolometer

• NTD-4 chip works at temperatures below 2K
• Dewar is first filled with liquid Nitrogen
• Liquid N2 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
12
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.
13
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).

14
Results and Conclusions
Optics Design
Bolometer/FTS

• Bolometer has been characterized.
• Hope to run bolometer on FTS (successfully) this
  week.
• Characterize response and compare with
  theoretical model from beam-splitter.

• 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