Progress report on development of the COsmic Foreground Explorer (COFE)

Philip Lubin, Principal Investigator

January 29, 2007

# Summary 

![](media/image1.jpeg){width="3.5in" height="2.625in"}COFE is a balloon
borne centimeter wave survey instrument being built at the University of
California at Santa Barbara. The system consists of multiple identical
off axis telescopes mounted to a single balloon platform with cryogenic
detector arrays operating at 10 15 and 20 GHz. The primary goal of COFE
is a (nearly) all sky measurement of the polarized foregrounds for
measurements of the CMB polarization. This will likely be a rich data
set, of particular interest will be a strong ability to identify and
characterize (or set limits on) possible 'spinning dust'. A rendering of
the current system design is shown below. For scale note that the major
axis of the primary mirrors is 2.2 meters.

Work in the past year has focused on 3 main elements: detailed testing
of the modulator concept and systematics with a realistic full scale
prototype at 40 Ghz (Bmachine); Development and structural analysis of
telescope and gondola frames; Development of final design and research
into possible manufacturing techniques for primary and secondary
reflectors as well as feeds.

# Bmachine Testing

![](media/image2.jpeg){width="3.625in" height="2.71875in"}The Bmachine
prototype is shown here during optical testing at UCSB. The system uses
the baseline optical configuration of COFE, which employs a reflection
half wave plate at the confocal point of the telescope to modulate the
polarization of the telescope at 120 Hz. The COFE mirrors will be laid
up on the same mold as the Bmachine mirrors pictured. Primary goals for
Bmachine prototype testing this year have been to develop robust
wiregrids of high enough quality for the modulator, and to validate the
optical performance with the polarization modulator in the complete
optical system. Plots below show some of the principal results to date:

## 1/f noise reduction 

![](media/image3.png){width="4.625196850393701in"
height="2.3929133858267715in"}The optical modulator/demodulation
effectively removes the significant 1/f noise of the HEMT radiometer.

![](media/image4.png){width="5.125196850393701in"
height="2.8681102362204722in"}

## ![](media/image5.png){width="4.375196850393701in" height="2.2015748031496063in"}Scan synchronous artifacts

## Optical testing

Optical tests of the Bmachine prototype are crucial for validating the
COFE optical designa and modulation technique.

![](media/image6.png){width="5.875196850393701in"
height="2.764566929133858in"}![](media/image7.png){width="5.869291338582677in"
height="2.7625984251968503in"}

## Modulator Validation

Extensive testing has been done on many wire grid candidates to validate
the operation and efficiency of the polarization modulator. Two
representative plots below demonstrate efficiency of at least 95% and
the orthogonality of the demodulated phase states.

![](media/image8.png){width="6.874409448818898in"
height="3.0507874015748033in"}![](media/image9.png){width="6.721653543307086in"
height="1.9484251968503936in"}

# Gondola and Telescope structural design

Two different optical layouts are being built, one based on the
waveplate modulator, one based on internal switches. COFE may include
both designs, split by frequency, which determines the performance
available in cryogenic switching. Renderings and pictures below depict
the designs.

## ![](media/image10.jpeg){width="3.9799212598425195in" height="2.75in"}![](media/image11.jpeg){width="3.7456692913385825in" height="2.58740157480315in"}Optical Layout

## ![](media/image12.jpeg){width="4.120866141732283in" height="2.886220472440945in"}![](media/image13.jpeg){width="4.120866141732283in" height="2.886220472440945in"}Frame Design

## ![](media/image14.jpeg){width="3.5in" height="2.6251968503937007in"}Metal

# Reflectors and feeds

## Reflectors

![](media/image15.jpeg){width="3.05in"
height="2.396456692913386in"}![](media/image16.jpeg){width="1.9070866141732283in"
height="3.6346456692913387in"}![](media/image17.jpeg){width="2.2736220472440944in"
height="3.594488188976378in"}COFE will have 4 independent telescopes as
a baseline. We are using our existing molds, built to make the BEAST
reflectors in 1998-1999, to make the reflectors for COFE. A contract for
the first two sets is already underway with ATK (formerly Composite
Optics Incorporated) who made the original mirrors. ATK will use
standard CFRP layup techniques to reproduce our mirrors. We are also
pursuing more forward looking techniques to produce mirrors at hopefully
1/5 the cost of the traditional ones. Our strategy is to complete two
new sets with ATK, which will give us a total of 4 full mirror systems,
and then to do a demonstration set for validation with a different
company, A2ZFX, where the goal will be to produce mirrors with
reflective coatings already applied for around \$5K per set. This would
be a major advance for mmwave suborbital and flight systems, and would
provide us with our backup optics.

# Schedule

-   2006-Funding begins- Finalize optical frame, gondola and
    Polarization Modulator Prototype

-   2007-Assemble first telescope system and perform ground tests

-   2008-Two Ft Sumner flights (25 h nominal)

-   2009-Analysis of flight data and preparation for Southern Hemisphere
    flights.