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