1. Balloon Experimental Twin Telescope for Infrared Interferometry
Exploration Goals and Methods
By: Charles Horvath
In association with the Precision Engineering Center;
Dr. Thomas Dow, Dr. Stephen Furst, Ken Garrard
Introduction
BETTII is an experimental telescope and spatial interferometer that will be launched on a
balloon. The NASA project is being developed at the Goddard Space Flight Center in
Maryland. Dr. Rinehart and his team aim to show the effectiveness of an infrared ‘double
fourier’ interferometer at spatially separating stellar objects and emissions. The telescope
will peer at high dense regions of star formation within the Milky Way and at other galaxies
with curiously active nuclei. As noted in Figure 1, the Precision Engineering Center at NC
State was contracted to design and fabricate the primary optical systems of the telescope.
This paper summarizes the stellar objects to be observed, methods of interferometry, and
niche BETTII fills among comparable telescopes.
Figure 1, The Twin Telescope Interferometer
Star Formation Observations
BETTII’s primary area of study involves understanding the earliest stages of star formation.
The telescope will observe dense clusters known to contain many stars that are in their
infancy at less than a million years old. For a reference, our adult Sun is thought to be 4.5
billion years old. Many of these stars will never achieve fusion and make it to adulthood.
Previous telescopes have shaped the general timeline of star formation but the chemical
evolution that occurs in a star’s infancy in especially high-mass regions has yet to be fully
realized.
Below is the example NASA’s Dr. Rinehart gives as a suitable cluster for observation by
BETTII. The actual flight date will determine the target; there are a number of suitable
clusters [1]. Figure 2(a) is a composite image of Perseus NGC 1333 that combines light from

2. multiple observations at different wavelengths. The image combines X-ray data from
NASA’s Chandra Observatory, optical/visible data from the Digitized Sky Survey/National
Optical Astronomy Observatory, and infrared data from the Spitzer Space Telescope [2].
Figure 2b contains only the infrared component; similar to the wavelength BETTII will
observe. As opposed to near infrared, which is ‘near’ the visible spectrum, this study is
concerned with a portion of the Far-IR spectrum (30-90 μm). These wavelengths are closer
to those of microwaves and provide some inherent benefits that will be discussed below.
Figure 2, (a) NGC 1333 composite (b) Spitzer Infrared component (c) FOV [2]
NGC 1333 resides relatively nearby within our Milky Way galaxy around 770 light years
away and is small component of the entire Perseus constellation, which can be seen with
the naked eye. It is considered a young dusty high mass region [3]. This type of cluster has
been shown by the Spitzer telescope to contain many of these Young Stellar Objects (YSOs
or protostars). The dust that envelops these objects obscures the YSOs and makes
understanding their chemical evolution a challenge. As previously noted, the BETTII
telescope, and others like it, observes infrared waves, which essentially shine through the
dusty elements and map the important bodies contained within. A hypothetical field of
view (FOV) of BETTII is noted in Figure 2. The telescope will focus on just a small portion of
the cluster in order to get a picture of the individual YSOs contained within the dust.
The true challenge with observing these young stellar objects has been that many reside in
especially dense regions and remain currently unresolved from their neighbors. Current
telescopes that observe these dusty clusters lack the angular resolution needed to isolate
these stars from the numerous stars that appear nearby from our perspective. Figure 2 is a
perfect example. The Spitzer telescope that took Figure 2b has an impressive field of view
but an angular resolution of considerably worse than the proposed resolution of BETTII.
Figure 2c appears fuzzy as a result and many of the brighter objects seen in that image may
actually be clusters of multiple YSOs. This is where BETTII’s unique capabilities will help
clear up the picture.
Exactly how the stars themselves grow and combine is not fully understood at this early
stage. As seen in Figure 3, the dusty envelope steadily flows into the protostar while it
accretes and gains mass. There is some evidence however, that this process is unsteady and
that the stars do much of their growing in short outbursts that heat up the envelope [4].
BETTII’s
Hypothetical FOV
of 2 arcmin.

3. Observations made by BETTII in the far-IR may add to the evidence that supports these
outbursts, which are not yet universally accepted [4]. Another aspect of star formation that
BETTII could help explain is the interactions between stellar systems [1]. The YSOs often
form in clusters or binary systems; the effects of which are not entirely understood.
Figure 3, Known features of a typical YSO [5]
In addition to the dusty envelope, a circumstellar disk surrounds a typical protostar. The
dusty envelope can be seen in both Figures 2 and 3 but the dust largely obscures the stars
and their disks in the visible spectrum. The protostar grows in size by accreting matter
from the surrounding envelope. The circumstellar disk also grows over time due to this
mass accretion. As shown in Figure 3, powerful bipolar outflows are generated
perpendicular to the disk and release energy during formation [5].
The data obtained is easily transformed to show the spectral energy distribution (SED) of
the individual object(s). This method will be described later on but essentially yields a
spectral fingerprint about the chemical makeup and energy distribution of an object. The
spectral fingerprint or interferogram of these disks can show signs of ice and other mineral
features surrounding the protostar. Similarly, the spatially resolved data can yield spectral
information about the envelopes and disks that surround these stars [4]. The circumstellar
disks are in the same plane where planets would eventually orbit the young star. Therefore,
the elements that makeup these disks would influence the landscape and atmospheres of
planets that may form millions of years down the line. In addition to shedding light on the
chemical evolution that occurs in the earliest of stages of star development, the spectral
makeup of the protostars and their disks would also add understanding to solar system and
planet formation.
Having the spectral energy distribution of these deeply embedded YSOs will add to the
incomplete picture and timeline of star formation. While other studies have fleshed out a
nearly comprehensive timeline for star formation, most knowledge about the youngest
stars has come from studies of single-stars forming in low mass regions [1]. On the other

4. hand, BETTII takes on a greater challenge by providing insight into the cloudy higher-mass
regions that tend to be more dynamic and energetic.
Active Galactic Nuclei Observations
In addition to the contributions BETTII will make to the timeline of stars forming within
our galaxy, the telescope will peer at other galaxies. A typical galaxy like our milky way
orbits a super massive black hole (SMBH). SMBHs are thought to co-evolve with galaxies as
they affect star formation, energy distribution and other galactic parameters. Some galaxies
are said to contain active galactic nuclei (AGN). These galaxies are characterized by
extremely luminous bursts of energy emitting from the nucleus ranging from X-ray to the
far infrared [6]. The Milky Way does not fall into this class of galaxy but it is important to
note the classification of AGN encompasses many subclasses of galaxies including quasars
and blazars; the brightest objects in our universe [6].
While many observations have been made of these curiously bright galaxies, the physical
mechanism that fuels the AGN emissions has not yet been fully explained [7]. The current
understanding is that the black hole grows by accreting matter. The SMBH is surrounded
by this accretion disk, which is a few light days across [6]. Stars and other superheated
nuclear matter spin furiously around the SMBH. As matter falls into the immense source of
gravity some of that energy gets converted to the bright emissions typical of AGN. This
process is remarkably similar to the mass accretion process discussed above for a
protostar. Beyond the basics however, the mechanics of this process are very complex and
must be studied at many different wavelengths. So far deep radio, IR, optical and X-ray
observations of AGNs have provided a wealth of information about the accretion of SMBHs
and the chemical evolution of AGN emissions [8]. Various objects and interactions in this
region emit light at different wavelengths. BETTII will gather infrared data from these AGN
emissions but also from the accretion disk that surrounds the SMBH and the stars forming
regions nearby. Just as in the star formation section, FIR emissions can give us spatial and
spectral information about young stars.
The Herschel and ISO telescopes have studied the FIR coming from these AGN regions. The
ISO long wavelength spectrometer observed NGC 1068. Pictured in Figure 4, the seyfert
galaxy is located around 50 million light years from Earth and contains a SMBH about twice
as massive as the one in the center of the Milky Way [2]. ISO was able to probe the bulk of
the emission from this AGN but was not able to spatially differentiate the AGN emissions
from bursts of star formation (starbursts) that occur near the center [1]. With its
unprecedented angular resolution, BETTII will help separate the emission from the AGN
and starbursts. It is important to understand how stars form in such an active environment
so that they may be compared to the relatively tame environments discussed above.
Starbursts have been shown to positively correlate with AGN luminosity in Seyfert type
galaxies [7]. This implies an interaction between these processes that is not yet
understood.

5. Figure 4, Composite image of NGC 1068 and AGN [2]
Figure 4 contains X-ray data from the Chandra X-ray Observatory shown in red, optical data
from the Hubble Space Telescope in green and radio data from the Very Large Array in blue [2]
With its larger FOV of 2 arcmin, BETTII will gather FIR emissions of NGC 1068 or a similarly
situated galaxy. More importantly than FOV, BETTII has spatial resolution of 1 arcsec; the units
of arcsec will be explained below. This resolution will enable BETTII to obtain the FIR
emissions of the AGN component and spatially resolve them from the starburst region. A
spectral energy distribution can then be obtained for the star forming regions near the nucleus of
the galaxy [1].
Angular Measurements and Rayleigh’s Criterion
Angular measurements must be understood because they can describe the apparent size of
an object in space as well as the apparent distances between multiple objects [9].
Arcminutes and arcseconds are both small angular units. An arcmin is 1/60th of a degree
while an arcsec is 1/60th of an arcmin or 1/3600th of a degree of the night sky. This in itself
is a difficult concept that can be better understood visually.
Figure 5, (a) Angular Size or FOV (b) Angular Distance or Resolution [9]

6. Figure 5 visualizes how both Field of View (FOV) and resolution can be referred to using
these angular units. Figure 5a shows the size of an object from our perspective. This same
concept applies to the FOV of BETTII: the angular size of the image. On the other hand,
Figure 5b visualizes the angular separation of two objects. BETTII will observe both nearby
young stars (~800 light years) and far away galaxies (~50 million light years). While much
further away, the galaxies are also much larger. The angular units inherently account for
this provide more insight into the effectiveness of a telescope than units of pure distance
and size. This is why we describe the size of objects, FOV, and resolution in angular units
rather than traditional spatial units.
The FOV is dependent on the size of the detectors contained within the interferometer.
BETTII’s 8x8 pixel detector arrays offer a field of view of around 2 arcmin (1/30th of a
degree). The high contrast between FOV and resolution will allow BETTII to produce
extremely sharp data and interference patterns.
Spatial/angular resolution refers to how well a telescope can distinguish between two
objects in space, which are separated by a small angular distance [9]. Rayleigh’s Criterion
dictates that for a traditional telescope with one aperture, the diameter of that optic
determines the angular resolution [10]. This criterion is written mathematically in
equation (1) below.
𝚯 𝐚𝐧𝐠𝐮𝐥𝐚𝐫 = 𝟏. 𝟐𝟐
𝛌
𝑫
radians (1)
A lower value for angular resolution (Θ) results in a clearer picture. Rayleigh’s criterion
also dictates that resolution is closely related to diffraction patterns. A single aperture acts
as a single slit for the light to pass through which is not ideal for peering at multiple objects.
When two objects are closely angularly spaced in the sky, their diffraction patterns
interfere and overlap. This overlap causes the two objects to appear fuzzy or as one object.
Instead of a single aperture, BETTII utilizes two optical systems to form a spatial
interferometer where the resolution is no longer dependent on the individual telescopes
diameter but rather on the distance between the two telescopes, called the baseline vector
b [11]. The immediate benefit from this method from an engineering perspective it is much
simpler to place two telescopes 8 meters apart than make a single aperture that would
need to be over 8 meters in diameter to achieve similar resolution. Rather than a single slit,
the interferometer collects lights with two, which creates an interference pattern rather
than a traditional image. This makes it a ‘double fourier’ interferometer ratherthan a
traditional fourier transform spectrometer. The interference pattern contains spatial and
spectral information of the object or objects under observation.
𝚯 𝐚𝐧𝐠𝐮𝐥𝐚𝐫 =
𝛌
𝟐𝒃
radians (2)
The 8-meter separation between BETTII’s telescopes produces an impressive angular
resolution of 1 arcsec (1/3600° or 4.85e-6 radians) in the FIR band. BETTII’s ability to
isolate or resolve two YSOs or AGN emissions will be dependent on this angular resolution.
The Light Path

7. Figure 6 highlights an important phenomenon must be understood to recognize several
functions of the telescope. Anyone who has peered at the stars long enough will notice that
the night sky rotates above our heads. The rotation of the earth about its axis causes the
view of the night sky to rotate in a similar manner. Figure 6 captured this phenomenon
through frame stacking multiple long exposure shots and can be thought of as a time-lapse.
This phenomenon is largely accounted for using siderostat mirrors. As seen in Figure 1,
these mirrors are the first mirrors to contact the light and redirect it towards the beam
compressors. They are attached to motor controls that follow the object under observation
while the night sky rotates.
Figure 6, Phenomenon of the rotating nights sky*
Beam Compressors and NCSU’s Role
Depending on what is being observed (star formation or AGN), the infrared light will travel
several hundred years or perhaps several million years before striking BETTII’s two
siderostats. These mirrors redirect the infrared towards the beam compressors systems,
which utilize 4 mirrors to focus the original collimated beam into one that is much skinner.
As seen in Figure 7, the primary mirror is an off-axis parabola that has been tilted 13°,
which compresses the light towards another flat. The turning flat directs the light towards
the ‘2nd mirror’ that is an off-axis hyperbola. Finally the third mirror is another concave off-
axis parabola that outputs the desired result of one of these telescopes: a compressed
version of the original beam of light. This new beam still contains the vital information
about the sources but has been scaled down fit into the onboard light detectors and
interferometer.

8. Figure 7, (a) One of Two Beam Compressors [12] (b) Optical Path of Compression
As seen in Figure 7, the beam compressor optics (primary and turning flat) and siderostat
optics were fabricated out of aluminum by the Precision Engineering Center at NCSU. They
were cut using a diamond turning machine with a maximum shape error of 300 nm and a
desired surface finish of 100 nm. The team here also designed and machined the
lightweight aluminum mount that grips the mirrors in place with proprietary clamps. As
BETTII ascends into the high atmosphere, it will undergo temperature changes of 250 F°.
As a solution to this temperature change, the beam compressors have been uniformly
fabricated out of Aluminum 6061-T6 so that the optics will uniformly expand and contract
at the same rate [12].
Interferometry
Once the light beams have been compressed they are directed into the onboard interferometer,
which utilizes the double fourier technique. A fourier transform spectrometer is similar to a
Michelson interferometer but includes a variable path difference between the two arms
[13]. This is achieved by sending the light from one of the two telescopes through a delay
line that can be adjusted during flight. The rotation seen in Figure 6 causes the length of the
light path going to each telescope to change constantly relative to one another. The
movable delay line compensates for these changing distances and resulting changes in
phase between the wavefronts reaching the two telescopes [10]. As seen in Figure 8, the
optical delay line will ensure both beams entering the interferometer are in phase with one
other [14].
Primary Mirror
Turning
Flat
Lightweight
Mount

9. Figure 8, The ‘double fourier’ Interferometer and Delay Line [14]
In other words, micro adjustments must be made to one of the light paths so that the
interference pattern can be properly studied. The entire crystostat seen in Figure 8 is
actually cooled to 1.5° K [1]. This is done so all of the internal optics and instruments can
be delicately controlled under the same conditions. Additionally the detectors are cooled to
just above absolute zero at 300 mK.
The ‘double fourier’ interferometer is able to resolve multiple objects by taking three
different baseline observations spanning a 60° angle across the sky and comparing the
fringe patterns [1]. The telescope will take these multiple observations of a star system
while the sky rotates 30° between each as seen in Figure 6. The first observation will be
made at time 1 at 0°, second observation at time 2 at 30°, and the third observation at time
3 at 60 degrees. Since the observations are not made at the North Pole, the times between
these observations will not be evenly spaced due to the declination of the target.
The key aspect of this method is the comparison of the wave fronts from the three baseline
observations. As in Figure 9, if a binary star system were being observed then each of the
three baseline observations (black fringes) would contain information about both stars
(Source 1 and 2). These interferograms contain both spectral and spatial information about
the objects [14]. Because the observations are taken at 30° increments, they are essentially
different viewpoints of the same system. The top baseline in the left panel of Figure 9 is
especially vulnerable to projection effects; this is where the wave front of two sources are
closely spaced and have overlapping fringe patterns. By taking the other two baseline
observations, the projection effects can be greatly mitigated [1]. The individual fringe
pattern of each star can then be resolved by comparing the 3 wave fronts and
understanding which source is responsible for the various peaks and troughs. As seen in
the right two panels of Figure 9, information about two individual sources can then be
extracted. This is where the exceptional angular resolution of BETTII becomes so essential.
A lower resolution telescope would be unable to effectively spatially resolve these two
fringe patterns into two separate entities.
Once two objects are resolved, NASA is able to transform the data to show the spectra from
the individual sources. The right two panels of 98 show the resolved fringe patterns are
individually transformed into spectral energy distributions and therefore the chemical
makeup of each source can be understood. The SED of two resolved sources will be critical

10. for understanding star formation and AGN. Dr. Rinehart’s explanation within Figure 9
clarifies this intricate method [1].
Conclusions and BETTII’s Niche
Ultimately, Dr. Rinehart and his team will demonstrate the effectiveness of an airborne
‘double fourier’ interferometer so that a spaceborne twin telescope can achieve
comparable angular resolution. BETTII and its successor will complement observations
made by other infrared telescopes such as Herschel and James Webb Space Telescope
(JWST). Considered Hubble’s successor, JWST will probe the most distant regions of the
observable universe to understand the first galaxies and the time just after the big bang
[15]. Figure 10 compares BETTII to other telescopes with similar goals by their angular
resolutions and wavelength bands of observation. The lines are slanted in accordance with
equations (1) and (2). Yielding a 6.5 m-diameter primary mirror, JWST’s angular resolution
is clearly unmatched in the NIR [15].
Figure 10 shows the important understanding that no one telescope can fill every niche.
The many different techniques provide complementary understandings of how the
Universe forms and interacts. With its unprecedented angular resolution and therefore
resolving power in the far infrared band, BETTII will resolve previously obscured deeply
embedded objects and emissions.
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*Photo taken and permission granted by Matt Quinn. More can be seen at mattquinn.ca