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Multi-Pixel Optics Design for the Submillimeter Array(SMA). Yun- Chih Chou, Chao- Te Li, Ming-Tang Chen Institute of Astronomy and Astrophysics, Academia Sinica. Apr. 8, 2013 . Outline. SMA Introduction and Optics Multi-Pixel Upgrade Tryouts of Aperture Plane Array

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multi pixel optics design for the submillimeter array sma

Multi-Pixel Optics Design for the Submillimeter Array(SMA)

Yun-Chih Chou, Chao-Te Li, Ming-Tang Chen

Institute of Astronomy and Astrophysics, Academia Sinica

outline

Apr. 8, 2013

Outline
  • SMA Introduction and Optics
  • Multi-Pixel Upgrade
  • Tryouts of Aperture Plane Array
  • Tryouts of Focal Plane Array
  • Conclusion

The 24th International Symposium on Space Terahertz Technology, Netherlands

sma introduction
SMA Introduction
  • Full operation since 2005.
  • Antenna number: 8
  • Antenna focal ratio (f/#): 14 (main dish diameter: 6m)
  • Current operation frequencies: 176-256GHz, 250-350GHz, 330-430GHz.
  • Dual polarization observation: 345GHz.
  • Field Of View: ~30” at 345GHz.

Fig. SMA on Mauna Kea, Hawaii (elevation 4,045m)

receiver update
Receiver Update
  • Receiver: Wide IF upgrade of 200GHz from 4-8 GHz to 4-12GHz.
  • All 8 antennas have wide IF 200GHz receivers now.
  • Wide IF upgrade of 300GHz from 4-8GHz to 4-12GHz followed up, and 3 receivers have been installed.

Fig. Noise temperature (Trx) measurement of

300GHz SIS Junction tested in Dewar system.

sma optics
SMA Optics
  • Design purpose: Simple frequency-independent optics.
  • Frequency-independent illumination on secondary reflector: All bands have common “virtual feed” behind receiver lens.
  • Design techniques: Fresnel imaging techniques, multi-mode Gaussian beam methods.

Fig. SMA optics (top view).

sma optics 2
SMA Optics - 2
  • Relay optics: Two ellipsoidal mirrors. (beam waveguide M4 and M5)
  • Performance: Small coupling loss(M5 0.15% at 400GHz) and cross-polarization

(-38dB at 3dB beam).

  • LO: Injection before feedhorn.
  • Receiver : Scalar horn and lens.
  • Feed Max gain: Max gain is reached when Bessel field of corrugated feedhorn is 10dB on aperture edge.

Fig. Beam waveguide of SMA.

multi pixel upgrade1
Multi-Pixel Upgrade
  • Advantage: Increase FOV and mapping speed.
  • Largest FOV w/o cryostat window limit:
  • Observation frequency: 345 GHz, key frequency.
array element
Array Element
  • Array element: 7 (hexagonal).

Design goal:

  • Feature frequency: 345 GHz
  • Least modification from current optics.
  • Aperture Plane Array or Focal Plane Array.
  • Requirement: High beam efficiency, small spillover etc.
sma optics simulation
SMA Optics Simulation
  • Simulation software: GRASP 10.
  • Simple optics of SMA 300 band receiver was established.
  • Simulation method: Gaussian beam propagation and Physical Optics (PO) calculation.
  • Reciprocity: Horn transmitter=receiver.

Fig. SMA Optics simulated in GRASP.

grasp simulation result
GRASP Simulation Result
  • Beam efficiency: 84%, integrated to 12dB below peak.
  • Cross-polarization efficiency: 0.006%(-41dB)

Fig. 2,3. Far-field grid of optics

(Upper co-pol; right: Cx-pol.)

Fig. 1. Far-field cut of SMA Optics.

tryout 1
Tryout 1
  • Optics: RO unchanged.
  • Smaller lens due to aperture limit: Diameter reduced from 70mm to 25mm, so all beams are less truncated at cryostat window (diameter 76mm).
  • Lens radius 12.5mm corresponds to 0.9ω (Gaussian beam radius).
  • Feed spacing: 25mm.

Fig. Tryout 1 optics with off-axis Gaussian beam passing through the cryostat window.

i simulation result
I – Simulation Result

Fig. Tryout 1 far-field beam grid of 3dB and 6dB.

Lens truncation reduced beam efficiency by 30%.

Fig. Far-field cut of off-axis beam.

tryout ii
Tryout II
  • Goal: Make beams smaller to reduce lens truncation.
  • Feedhornand RO were changed to make beam radii smaller at lens and cryostat window positions.
  • Lens radius: 12.5mm, 1.25ω.

Fig. Relay Optics of original SMA optics (left) and tryout#2 optics (right). M5 has new geometry and position in this solution.

ii simulation result
II- Simulation Result
  • Beam efficiency of on-axis feed increased to over 70%, but off-axis fell dramatically due to spillover at subreflector.

Fig. Off-axis feed 25mm away

from propagation axis.

Fig. Tryout 2 far-field beam grid of 3dB and 6dB.

tryout iii
Tryout III
  • RO: Removed.
  • Focusing element: The original insert lens to focus feedhorn signal.
  • Focus matching: Place beam waist of feed-lens set at the Cassegrain Focal point.
  • Min. feeds spacing: 2.44 * f/# * λ = 29.7 mm.
iii feeds spacing
III - Feeds Spacing
  • Design feeds spacing: 45mm.
iii largest fov
III – Largest FOV
  • Increase Pixel #: From 7 to 19.
  • FOV: From 5.5’ to 9.2’.
  • Beam efficiency of outermost pixel becomes 65.9% and 65.3% at 12dB.
tryout iv
Tryout IV
  • RO: Two big identical lenses.
  • F of RO lenses: 780.8mm.
  • Lens diameter: 250mm.
  • Optical path: Same as original, but replace ellipsoidal mirrors with flat mirrors.
  • f/# unchanged.
  • Feeds spacing: 45mm.

Fig. Tryout 4 Optics.

i v simulation result
IV: Simulation Result
  • On- and Off-axis efficiency: 70.6% and 67.1%.
tryout v
Tryout V
  • RO: Two identical off-axis ellipsoidal mirrors
  • f 780.8mm, diameter 250mm. Incidence angle 14 deg.
  • f/# unchanged.
  • Cabin Restriction: Place two mirrors between M3 and M6.
  • Optical path: . Optical insert is removed to shorten optical path.
  • M6: Moved to right above receiver insert.

Fig. Tryout 5 Optics.

conclusion
Conclusion
  • 5 Tryouts of SMA 7-pixel upgrade were designed.
  • Possible solution is tryout 5. Distance from the second ellipsoidal mirror to insert lens is shortened.

* Original optics simulated in GRASP has beam efficiency 84%

at 12dB beam.

future task
Future Task
  • Receiver design: Using current SMA 300GHz rx as a single-pixel prototype. Putting together rx components in 45mm spacing.
  • New mixer block design: Change to couple LO after feedhorn.
  • Realization in current Cabin: Spare space and cryostat window issue etc.
please comment

Please Comment.

Thanks for your attention 

Yun-Chih Chou (Joy) 周允之

ycchou@asiaa.sinica.edu.tw