We are a group
based at the Massachusetts Institute of Technology Kavli
Institute for Astrophysics and Space Research.

What
better way to show our love for science than to try our hand
at being universe cartographers! Mapping our
universe in 3D by imaging the redshifted 21 cm line from
neutral hydrogen has the potential to overtake the cosmic
microwave background as our most powerful cosmological probe,
because it can map a much larger volume of our Universe,
shedding new light on the epoch of reionization, inflation,
dark matter, dark energy, and neutrino masses. We report on
MITEoR, a pathfinder low-frequency radio interferometer whose
goal is to test technologies that greatly reduce the cost of
such 3D mapping for a given sensitivity. MITEoR accomplishes
this by using massive baseline redundancy both to enable
automated precision calibration and to cut the correlator cost
scaling from N^2 to N log N, where N is the number of
antennas. The success of MITEoR with its 64 dual-polarization
elements bodes well for the more ambitious HERA project, which
would incorporate many identical or similar technologies using
an order of magnitude more antennas, each with dramatically
larger collecting area.

The actual
Omniscope instrument has several components as illustrated
below.

Hover over a component to see what it looks like!

This shows data
flow in a very large omniscope. A hierarchical grid of
dual-polarization antennas converts the sky signal into volts,
which get amplified and filtered by the analog chain,
transported to a central location, and converted into bits every
few nanoseconds. These high-volume digital signals (thick lines)
get processed by field-programmable gate arrays (FPGAs) which
perform a temporal Fourier transform (using polyphase filter
banks). The FPGAs (or GPUs) then multiply by complex-valued
calibration coefficients that depend on antenna, polarization
and frequency, then spatially Fourier transform, square and
accumulate the results, recording integrated sky snapshots every
few seconds and thus reducing the data rate by a factor 10^9.
They also cross-correlate a small fraction of all antenna pairs,
allowing the redundant baseline calibration software to update
the calibration coefficients in real time. Finally, software
running on regular computers combine all snapshots of
sufficient quality into a 3D sky ball representing the sky
brightness as a function of angle and frequency in Stokes
(I,Q,U,V), and subsequent software deals with foregrounds and
measures power spectra and other cosmological observables.

**As with any project this large, one of the most
important components is our awesome team!
**

Victor Buza |
Joshua S. Dillon |
Hrant Gharibyan |
Jack Hickish |
Eben Kunz |
||

Adrian Liu |
Jon Losh |
Andrew Lutomirski |
Scott Morrison |
Sruthi Narayanan |
Ashley Perko |
Devon Rosner |

Nevada Sanchez |
Katelin Schutz |
Shana M. Tribiano |
Matias Zaldarriaga |
Kristian Zarb Adami |
Ioana Zelko |
Kevin Zheng |

Richard Armstrong |
Richard F. Bradley |
Matthew R. Dexter |
Aaron Ewall-Wice |
Alessio Magro |
Michael Matejek |
Edward Morgan |

Abraham R. Neben |
Qinxuan Pan |
Courtney M. Peterson |
Meng Su |
Joel Villasenor |
Christopher L. Williams |
Hung-I "Eric" Yang |

Yan Zhu |
Bob Penna |
Michael Valdez |

This was the first expedition. The antennas are arranged on the roof of the Kavli Institute at MIT.

The second expedition took place in Greenbank, West Virginia. Pictured above is the arrangement of the 16 antennas.

On the third expedition we started with the Roach system. We had 8 antennas and this was the first expedition in West Forks, Maine.

The fourth expedition was actually done in the snow in West Forks, Maine. It was again using 8 antennas instead of 16.

We reverted back to 16 antennas on the fifth expedition, which was in West Forks, Maine in the summer time.

For the last expedition we improved on the calibration as well as making it possible to use 64 antennas! This took place this past July in West Forks, Maine.

Let's walk through all the components of the
Omniscope.

The Analog
System consists of all the actual hardware and circuitry that we
use to obtain signals before sending them to computers to be
parsed together.

We start with
the antennas. MITEoR contains 64 dual-polarization antennas,
giving 128 signal channels in total. The 64 antennas are
arranged in 8 rows of 8. They are responsible for receiving the
signals from the universe. The signal picked
up by the antennas is first amplified by two orders of
magnitude by the low noise amplifiers (LNAs) built-in to the
antennas. Each antenna then has two channels which we
connect to a line driver island.

This past
expedition we tested connecting some antennas to swappers and
then to line drivers, while the others did not use the swappers
at all. The signals, after being amplified in the antennas, are
then phase switched in the swapper system, which greatly reduces
cross-talk downstream. Following this, the signal is amplified
again by about five orders of magnitude in our line-drivers
before being sent over 50 meter RG6 cables to the receivers.

The receiver boards are collected and stored in the racks that contain part of the digital system. All the wires from the antennas and amplifiers connect to these boards. The receivers perform IQ demodulation on a desired 50 MHz band selected between 100 MHz and 200 MHz, producing two channels with adjacent 25 MHz bands, and sends the resulting signals into our digitization boards containing 256 analog-to-digital converters (ADCs) sampling at 50 MHz.

From there we
move on to the digital system.

We designed
MITEoR's digital system to be highly compact and portable, with
the entire system occupying 3 shock-mounted equipment racks on
wheels, each measuring about 5 ft by 5 ft by 3 ft. It takes in
data from 256 ADC channels, Fourier transforms the data into the
frequency domain, reconstructs IQ demodulated channels back to
128 corresponding antenna channels, computes the
cross-correlations of all pairs of the 128 antenna channels with
8 bit precision (rather than 4 bits seen in most correlators of
similar specifications), and then time-averages these
cross-correlations. The digital hardware is capable of
processing an instantaneous bandwidth of 12:5 MHz with frequency
bin size 5 kHz, and the time interval for averaging and
subsequent output to our data server is usually configured to
be either 2.6 or 5.3 seconds.

On our last
expedition we had some fun with time-lapse photography. This is a movie of the building of two of our antennas!

http://www.cfa.harvard.edu/~loeb/StarBorn/StarBorn.pdf
-- Time Magazine article on high redshift observations.

http://www.cfa.harvard.edu/~loeb/sciam.pdf
-- Scientific American article on 21-cm tomography science with a focus on the
EOR.

http://www.mit.edu/~acliu/omnipop.pdf
-- Science magazine article on the Omniscope.

http://www.mit.edu/~acliu/thesis_proposal.pdf
-- M. Eng. thesis proposal written by Nevada Sanchez. Great summary of the Omniscope project
from an engineering perspsective.

http://www.mit.edu/~acliu/perko_thesis_revised.pdf
-- Ashley Perko’s undergraduate thesis.
The appendices are particularly useful for people who want to use the
actual instrumentation of the Omniscope.

http://www.mit.edu/~acliu/nevsan_thesis.pdf
-- Nevada Sanchez’s M. Eng. thesis.
An excellent introduction to the entire Omniscope.

http://www.mit.edu/~acliu/luto_thesis_final.pdf
-- Andy Lutomirski’s
Ph.D. thesis.

http://www.mit.edu/~acliu/EbenThesis.pdf -- Eben Kunz’s M. Eng. Thesis. Description of the Omniscope analog
chain.

http://arxiv.org/pdf/0902.3011v1 --
Astrophysics with the 21-cm line.

http://arxiv.org/abs/0902.3259 --
Cosmology with the 21-cm line.

http://arxiv.org/pdf/astro-ph/0608450v1
-- Big picture view of reionization.

http://arxiv.org/pdf/astro-ph/0608032v2
-- THE reference for anything in 21-cm tomography up until 2006.

http://arxiv.org/pdf/0910.3010v1 --
An update of the previous paper written by some different authors, focusing on
the recent interest in low redshift baryon acoustic oscillation measurements.

http://www.annualreviews.org/doi/abs/10.1146/annurev-astro-081309-130936
-- A more compact (and therefore more readable) review with a lot of nice
diagrams.

http://arxiv.org/abs/1109.6012 -- A
more theoretically-oriented review.

__An
Introduction to Modern Astrophysics__ by Carroll and Ostlie – Chapter 29 (of the 2^{nd}
edition) is an excellent introduction to cosmology for those familiar with
basic physics but perhaps not astrophysics or cosmology.

__Introduction
to Cosmology__ by
Ryden – fantastic book for the same audience, with lots of intuitive
explanations of the physics to go along with the math.

__Interferometry
and Synthesis in Radio Astronomy__ by Thompson, Moran, and Swenson – probably the
definitive reference on radio astronomy.

__An
Introduction to Radio Astronomy__ by Burke and Graham-Smith – chapters 5 and 6 the most
relevant.

__Tools
of Radio Astronomy__
by Wilson, Rohlfs, and Huttemeister --
a popular reference.

http://prd.aps.org/abstract/PRD/v79/i8/e083530--
the first Omniscope paper.

http://prd.aps.org/abstract/PRD/v82/i10/e103501--
the second Omniscope paper.

We've publicly released the key Omniscope data products and you can download them here.

- We've made the largest-area sky-map to date around 150 MHz and you can download it either
as a gzipped tarball,
as individual files or
from github. The data product consists of the following four ASCII files in HEALPIX nest format with nside=64, in equatorial coordinates:
- map_dirty_150mhz_kelvin_healpynest.txt: the raw map
- map_150mhz_kelvin_healpynest.txt: the Wiener-filtered map
- noise_150mhz_kelvin_healpynest.txt: the error bar map
- fwhm_150mhz_degree_healpynest.txt: the FWHM angular resolution map

- We've made a global sky model from 10MHz to 3 THz using other data and compared it with our MITEoR map in our paper arXiv:1605.04920. You can download this global sky model from github .
- For those of you who want to create your own maps or other analyses, we've released the raw visibilities for the MITEoR experiement in this gzipped tarball. The file format is explained here.

Our publications
on the Omniscope project are listed below (starting from most
recent):

An Improved Model of Diffuse Galactic Radio Emission from 10 MHz to 5 THz

Brute-Force Mapmaking with Compact Interferometers: A MITEoR Northern Sky Map from 128 MHz to 175 MHz

It's Always Darkest Before the Cosmic Dawn: Early Results from Novel Tools and Telescopes for 21 cm Cosmology

Mapmaking for precision 21 cm cosmology

MITEoR: A Scalable Interferometer for Precision 21 cm Cosmology

Global 21cm signal experiments: A designer's guide

A Fast Method for Power Spectrum and Foreground Analysis for 21 cm Cosmology

How well can we measure and understand foregrounds with 21 cm experiments?

A Method for 21cm Power Spectrum Estimation in the Presence of Foregrounds

Precision Calibration of Radio Interferometers Using Redundant Baselines

Solving the Corner-Turning Problem for Larger Interferometers

Omniscopes: Larger Area Telescope Arrays with only N log N Computational Cost

An Improved Method for 21cm Foreground Removal

Will point sources spoil 21 cm tomography?

The Fast Fourier Transform Telescope

How accurately can 21 cm tomography constrain cosmology?

Brute-Force Mapmaking with Compact Interferometers: A MITEoR Northern Sky Map from 128 MHz to 175 MHz

It's Always Darkest Before the Cosmic Dawn: Early Results from Novel Tools and Telescopes for 21 cm Cosmology

Mapmaking for precision 21 cm cosmology

MITEoR: A Scalable Interferometer for Precision 21 cm Cosmology

Global 21cm signal experiments: A designer's guide

A Fast Method for Power Spectrum and Foreground Analysis for 21 cm Cosmology

How well can we measure and understand foregrounds with 21 cm experiments?

A Method for 21cm Power Spectrum Estimation in the Presence of Foregrounds

Precision Calibration of Radio Interferometers Using Redundant Baselines

Solving the Corner-Turning Problem for Larger Interferometers

Omniscopes: Larger Area Telescope Arrays with only N log N Computational Cost

An Improved Method for 21cm Foreground Removal

Will point sources spoil 21 cm tomography?

The Fast Fourier Transform Telescope

How accurately can 21 cm tomography constrain cosmology?

This web page was designed by Sruthi A. Narayanan (sruthian@mit.edu).