RF Optics Design for the QU Imaging Experiment

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RF Optics Design for the Q/U Imaging Experiment

• W. A. ImbrialeJet Propulsion LaboratoryCaliforni
  a Institute of TechnologyPasadena, CA 91109

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The Q/U Imaging ExperimenT (QUIET)

• Purpose
• Study anisotropy in the cosmic microwave
  background (CMB)
• Instrumentation
• Large Focal Plane detector arrays
• Each element simultaneously measures both Q and
  U, the linear Stokes parameters.
• Phase I
• 91 elements at 90 GHz and 19 elements at 40 GHz
  each installed in a one-meter telescope mounted
  on the Cosmic Background Imager (CBI) platform in
  Chajnantor, Chile
• Phase II
• Four additional arrays two 91-element 40 GHz
  arrays and two 397-element 90 GHz arrays.
• These large arrays will observe alternately on a
  set of three 2m telescopes mounted on the CBI
  platform and on the co-located 7m telescope.

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Figure 3 Left Photograph of the first QUIET 90
GHz receiver module. All of the parts associated
with one input are labeled. Upper Right
Photograph of body of an earlier prototype 90 GHz
module. The modules are 1.2500 1.1400. Lower
Right Photograph of the interior of a (200
200) 40 GHz module.
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Figure 4 Schematic view of a QUIET cryostat
shown with a 91 element array of W-band modules.
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The Q/U Imaging Experiment

• Requirements for the Optics
• Wide field of view (12 degrees)
• Excellent polarization characteristics
• Minimal beam distortion
• Minimal instrument polarization
• Freedom from systematic errors
• Minimal spillover
• Minimal sidelobes

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Front-Fed
Figure 5. Classical offset Dragonian antenna
The classical Dragonian antenna employs a
paraboloidal main reflector illuminated by a
concave hyperboloidal subreflector. The
relatively large offset distance and focal length
of the main reflector, and the avoidance of
caustics between the two reflector surfaces (a
consequence of the concave subreflector), yield
relatively flat reflectors with wide field of
view capability.
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Typical Radiation Patterns
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Feed Horn Requirements

• High-gain (gt26 dB)
• Excellent cross-polarization characteristics
• Nearly identical E- and H-plane patterns
• Excellent return loss over the 80 105 GHz
  frequency band
• 40 GHz horn is a scale model of 90 GHz horn

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Figure 1
Semi-flare angle 7.6
First 6 corrugations Optimized for return loss
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Figure 2 Module layout
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Figure 3. Horn Layout
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Figure 8 A cutaway view of the prototype seven
element W-band platelet array for QUIET. The
platelet array consists of 103 equally thick
platelets. Each platelet has a series of holes
machined into it. Seven of the holes are
counterbored to form a tooth and a groove. Other
holes are used as access/lightweighting holes.
The interface plate resides between the platelet
array and the OMTs and provides for, among other
things, the mechanical connection to the
cryostat. Also shown are the OMT, splitter and
module blocks. The overall length and mass of the
assembly is 17 cm and 2.7 kg, respectively.
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Frequency Band
Figure 4 Horn Return Loss
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Table 1 W-band Horn Parameters
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Parabola Main Reflector F
D X0 Y0
Z0 3.27737 1.00000 0.00000
-3.26808 0.00000 Hyperbola Subreflector
E DIRX DIAHYP X0HYP
Y0HYP Z0HYP 2.24370 1.69842
1.10000 0.00000 -0.60866 -4.23886

Main Reflector Coordinate System
Subreflector Coordinate System
All dimensions in meters and degrees
Subreflector Coordinate System in Main Reflector
Coordinates XS YS ZS
0.000
0.24831 -2.4441 ALPHAS BETAS
GAMMAS
180.000000 116.630029 180.000000
Feed in Subreflector Coordinate System XF
YF ZF
0.000 -0.60866 0.00 ALPHAF
BETAF GAMMAF
180.000000 153.369971 180.000000
Figure 6. One-meter telescope geometry
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-1.0 0.0
1.0
(degrees)
Figure 7. Principal and cross-polarization plots
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Subreflector
Main reflector
dB levels
dB levels
Figure 8. Illumination on main and subreflector
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Figure 9. Mirror illuminations for x 7 inches,
y 0.0 inches
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8.0
0.0
-8.0 -4.0 0.0 4.0 8.0
-8.0 -4.0 0.0 4.0 8.0
(degrees)
(degrees)
Figure 10 Beam plots for feed at x 7.0, y 0.0
inches
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- 100 dB range
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-80 dB range
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-50 dB range
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Figure 10 Large Scale Optics co- and cross-
polarization. Physical optics simulation of the
beam response for a) co- polar central feed b)
cross-polar central feed c) co-polar for x 0,
y 0 feed position on edge of array and d)
cross-polar for x 9.8, y 7.2 feed position.
The contours in the co-polar plots are -3, -10, -
20 and -40 dBi. The contours for the cross-polar
plots are -40, -50 and -60 dBi. For our science
goals, -45 dBi in the cross-polar response is
needed this remarkable level of performance can
only be achieved with corrugated feeds in the
focal plane.
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Crawford Hill Geometry
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Subreflector
Main Reflector
Reflector Illumination for center feed
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Principal Polarization on-axis beam
dB levels
-0.2
0.2
(degrees)
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Cross - polarization
dB Levels
0.2
-0.2
(degrees)
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Subreflector
Main Reflector
Reflector illumination for feed scanned 7 inches
in x-direction
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Scanned beam (same dB scale as previous plot,
different angular scale)
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Cross pol for scanned beam
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QUIET Unique Features
QUIET is the only experiment that makes an
instantaneous measurement of both the Q and U
stokes parameters at the same sky pixel. QUIET
is the only experiment that simultaneously
measures large and small angular scales. The
number of QUIET detectors is a factor of 50
increase over currently operating polarimeters
and a factor of 20 increase over those expected
to be deployed in the next year (QUaD and
BICEP). QUIETs frequency coverage will
complement QUaD, BICEP and all the other proposed
bolometric polarimeter experiments. This is
especially important for foreground
discrimination. QUIET makes extensive use of
existing infrastructure the CBI platform and
observing site, the existing 7m antenna, and the
large prototype arrays at W and Q-band under
development in order to reduce costs and quicken
deployment. QUIET will make use of MMIC
polarimeter modules which represent the enabling
technology that allows a large number of
detectors to be fabricated at low cost.
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