1 mm Polarization Science with CARMA
Chat HullCollaborators: Dick Plambeck, Greg Engargiola, & all the CARMA staff
19 August 2011
URSI General Assembly XXXIstanbul, Turkey
• Science goals– Molecular emission
in galaxies– Galaxy clusters– Protoplanetary
debris disks– Dust polarization in
star-forming regions
CARMACombine 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)
Class 0 Dust Polarization
Credit: NASA, ESA, STScI, J. Hester and P. Scowen (Arizona State University)
Credit: Bill Saxton, Harvard-Smithsonian Center for
Astrophysics
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
First Target: NGC 1333 IRAS 4A
• We want to search for “hourglass” shape of the B-field structure in the circumbinary envelope
Girart+ 2006
CARMA observations
SMA observations
How do we make it work?Grad
student
Waveguide
circular polarizer
Orthomode transducer
SIS mixers
WBA13 I.F.
amplifiers (1-9 GHz)
1 mm Dual-polarization Receivers
1 inch
Turnstile-junction OMT
Navarrini & Plambeck 2006, IEEE-MTT, 54, 272-277
1 cm
OMT construction
Testing OMTs using 300 GHz network analyzer at Agilent
Some OMTs have resonances…
Unequal sidearm lengths in OMT can
cause resonances (simulation)
OMT tests at 4K(passbands, LO = 210-255 GHz in 1 GHz steps)
OMT10 (bad) OMT15 (good)
Polarizer construction
Aluminummandrel
Copper electroplate
d onto mandrel
Machined Soldered into waveguide
flange1 inch
Final design: 2-section polarizer
λ/2 retarder at 15°
λ/4 retarder at 74.5°
0.047’’ diameter facets 0.006’’
deep
XY RL
Sky
Receiver
Sample polarizer test data(mandrel machining errors)
Fra
cti
on
of
lin
ear
rad
iati
on
con
vert
ed
to
RC
P a
nd
LC
P
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
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!
XY phase calibration
AMBIENT LOAD(300 K)
SKY (~60
K)
FEED HORN
WIRE-GRID POLARIZER
XY phase fit results
1 mm signal path
Blo
ckdow
nco
nver
ters
8-w
ay
split
ters
RF = 210 – 270 GHzIF = 1 – 9 GHzBaseband = 0.5 – 1.0 GHz
Feed
horn
Pola
rize
r
OMT
RCP 1 – 9 GHz
5 – 9 GHz
1 – 5 GHz
RF
mix
er
Base
band
mix
er
10
GH
z m
ixer
LO 2
1.75 – 4.25 GHz
LCP
0.5 – 1.0 GHz
NOISE
Correlated Noise
Source
All
phase
s flat
To digitizer, filter,
& correlator
FILTER
FILT
ER
FILTE
R 1 – 5 GHz
τ4
τ3,hi
τ3,lo
τ2
τ1
1 – 9 GHz
5 – 9 GHz
1 – 5 GHz
0.5 – 1.0 GHz
FILTER
FILT
ER
FILTE
R 1 – 5 GHz
To digitizer, filter,
& correlator
Status/conclusions
• Dual-polarization receivers are installed on all 15 6- 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