1. 1 mm Polarization Science with CARMA
Chat Hull
Collaborators: Dick Plambeck, Greg Engargiola, & all the CARMA staff
19 August 2011
URSI General Assembly XXX
Istanbul, Turkey

2. CARMA Combine Array for Research in Millimeter-wave Astronomy Consortium: Berkeley, Caltech, Illinois, Maryland, Chicago
Attributes
6 ⨯10-m, 9 ⨯ 6-m, 8 ⨯ 3.5-m telescopes
Observations at 1mm, 3mm, and 1cm
Located in Cedar Flat, CA (near Bishop)
Science goals
Molecular emission in galaxies
Galaxy clusters
Protoplanetary debris disks
Dust polarization in star-forming regions

3. Class 0 Dust Polarization
Credit: NASA, ESA, STScI, J. Hester and P. Scowen (Arizona State University)

4. Credit: Bill Saxton, Harvard-Smithsonian Center for Astrophysics
-Strong or weak?
-See hourglass?
-Dust emission strong at 1 mm
Credit: Bill Saxton, Harvard-Smithsonian Center for Astrophysics

5. Why observe polarization?
B-fields play an important role in star formation
How important?
Are they strong (& ordered)?
Are they weak (& chaotic)?
B-fields  polarization
Dust grains align their spin axes with B-fields
Dust emission is strong at 1 mm

6. First Target: NGC 1333 IRAS 4A
CARMA observations
SMA observations
Girart+ 2006
SMA: 8*6m antennas = 226 m^2
CARMA: 6*10m, 9*6m = 726 m^2 (more than factor of 3 more area)
-Factor of two more dishes
-Equivalent T_sys?
-Factor of 2 more bandwidth
We want to search for “hourglass” shape of the B-field structure in the circumbinary envelope

7. How do we make it work?
Grad student

8. 1 mm Dual-polarization Receivers
Waveguide circular polarizer
Orthomode transducer
SIS mixers
WBA13
I.F. amplifiers (1-9 GHz)
1 mm Dual-polarization Receivers
1 inch

9. Turnstile-junction OMT
The OMT is based on a turnstile junction and 2 power combiners - this design was worked out by Alessandro Navarrini when he was a postdoc in Berkeley.
It looks pretty intimidating to make, but it’s not as bad as it looks . The waveguides are all split down the middle, so what you have do do is to machine 4 blocks of metal and bolt them together.
1 cm
Navarrini & Plambeck 2006, IEEE-MTT, 54,

10. OMT construction Here’s an assembly diagram.
Now, as you know, it’s not so hard to get great results out of an electromagnetic simulation of a device like this, but can you actually machine it reproducibly?

11. Testing OMTs using 300 GHz network analyzer at Agilent

12. Some OMTs have resonances…

13. Unequal sidearm lengths in OMT can cause resonances (simulation)

14. OMT tests at 4K (passbands, LO = 210-255 GHz in 1 GHz steps)
OMT10 (bad)
OMT15 (good)

15. Polarizer construction
Copper electroplated onto mandrel
Aluminum
mandrel
Machined
Soldered into waveguide flange
1 inch

16. Final design: 2-section polarizer
λ/2 retarder
at 15°
λ/4 retarder
at 74.5°
0.047’’ diameter facets 0.006’’ deep

17. XY  RL
Receiver
Sky

18. Sample polarizer test data (mandrel machining errors)
Fraction of linear radiation converted to RCP and LCP

19. Polarization calibration
Two main steps to calibrate a polarimeter:
XY phase
The absolute phase offset between the RCP and LCP receivers of an antenna
Leakage terms
The fraction of LCP radiation detected in the RCP receiver, and vice versa

20. XY phase calibration How do we find an antenna’s XY phase?
Observe a strongly polarized source with known position angle
These don’t exist at mm wavelengths
We create our own by observing broadband noise from the ambient load through a wire-grid polarizer!

21. XY phase calibration SKY (~60 K) WIRE-GRID POLARIZER AMBIENT LOAD
-Extremely high percentage polarization, thus…
-High-SNR correlation between LCP and RCP.
-Band dependent due to hardware
-Antenna dependent
-Forward gains of antennas: 100 Jy/K (10m), 150 Jy/K (6m)
-Sky temperature calculation: atmosphere temp*(1 – \e^{-\tau}), or (280 K)*(1 – e^(-0.2)) = ~50 K for typical weather
FEED HORN

22. XY phase fit results
Tau_2: overall slope across IF. Tau_3_hi,lo: intra-band slopes. Tau_4: jumps and equal-and-opposite slopes in higher and lower chunks.

23. Correlated Noise Source
1 mm signal path
RF = 210 – 270 GHz
IF = 1 – 9 GHz
Baseband = 0.5 – 1.0 GHz τ2
τ3,hi
10 GHz mixer τ1
Baseband mixer
RF mixer
5 – 9 GHz
1 – 5 GHz
FILTER
FILTER
To digitizer, filter,
& correlator
0.5 – 1.0 GHz
RCP
1 – 9 GHz τ4
FILTER
1 – 5 GHz
τ3,lo
Feed horn
Polarizer
All phases flat
OMT
NOISE
Correlated Noise Source
downconverters
Block
LO 2
1.75 – 4.25 GHz
8-way splitters
We fit these delays to Xyphase data at a variety of IF setups.
5 – 9 GHz
1 – 5 GHz
FILTER
FILTER
To digitizer, filter,
& correlator
0.5 – 1.0 GHz
LCP
1 – 9 GHz
FILTER
1 – 5 GHz

24. Status/conclusions
Dual-polarization receivers are installed on all and 10-m telescopes
Full-Stokes (LL, LR, RL, RR) system is working
Observed NGC 1333-IRAS 4A as a commissioning test
XY phase offset is well understood
Leakages need to be determined better
Will soon install new Berkeley-made polarizers
Full-Stokes commissioning time will be in Oct., 2011