# A VERITAS/Breakthrough Listen Search for Optical Technosignatures

A. Acharyya Department of Physics and Astronomy, University of Alabama,
Tuscaloosa, AL 35487, USA  C. B. Adams Physics Department, Columbia
University, New York, NY 10027, USA  A. Archer  Department of Physics and
Astronomy, DePauw University, Greencastle, IN 46135-0037, USA  P. Bangale
Department of Physics and Astronomy and the Bartol Research Institute,
University of Delaware, Newark, DE 19716, USA  P. Batista  DESY, Platanenallee
6, 15738 Zeuthen, Germany  W. Benbow Center for Astrophysics $|$ Harvard &
Smithsonian, Cambridge, MA 02138, USA  A. Brill N.A.S.A./Goddard Space-Flight
Center, Code 661, Greenbelt, MD 20771, USA  M. Capasso  Department of Physics
and Astronomy, Barnard College, Columbia University, NY 10027, USA  M. Errando
Department of Physics, Washington University, St. Louis, MO 63130, USA  A.
Falcone Department of Astronomy and Astrophysics, 525 Davey Lab, Pennsylvania
State University, University Park, PA 16802, USA  Q. Feng Center for
Astrophysics $|$ Harvard & Smithsonian, Cambridge, MA 02138, USA  J. P. Finley
Department of Physics and Astronomy, Purdue University, West Lafayette, IN
47907, USA  G. M. Foote Department of Physics and Astronomy and the Bartol
Research Institute, University of Delaware, Newark, DE 19716, USA  L. Fortson
School of Physics and Astronomy, University of Minnesota, Minneapolis, MN
55455, USA  A. Furniss Department of Physics, California State University -
East Bay, Hayward, CA 94542, USA  S. Griffin  WIPAC and Department of Physics,
University of Wisconsin-Madison, Madison, WI 53703, USA  W. Hanlon Center for
Astrophysics $|$ Harvard & Smithsonian, Cambridge, MA 02138, USA  D. Hanna
Physics Department, McGill University, Montreal, QC H3A 2T8, Canada  O. Hervet
Santa Cruz Institute for Particle Physics and Department of Physics,
University of California, Santa Cruz, CA 95064, USA  C. E. Hinrichs Center for
Astrophysics $|$ Harvard & Smithsonian, Cambridge, MA 02138, USA  Department
of Physics and Astronomy, Dartmouth College, 6127 Wilder Laboratory, Hanover,
NH 03755 USA  J. Hoang  Santa Cruz Institute for Particle Physics and
Department of Physics, University of California, Santa Cruz, CA 95064, USA  J.
Holder Department of Physics and Astronomy and the Bartol Research Institute,
University of Delaware, Newark, DE 19716, USA  T. B. Humensky Department of
Physics, University of Maryland, College Park, MD, USA  NASA GSFC, Greenbelt,
MD 20771, USA  W. Jin Department of Physics and Astronomy, University of
Alabama, Tuscaloosa, AL 35487, USA  P. Kaaret Department of Physics and
Astronomy, University of Iowa, Van Allen Hall, Iowa City, IA 52242, USA  M.
Kertzman  Department of Physics and Astronomy, DePauw University, Greencastle,
IN 46135-0037, USA  M. Kherlakian  DESY, Platanenallee 6, 15738 Zeuthen,
Germany  D. Kieda Department of Physics and Astronomy, University of Utah,
Salt Lake City, UT 84112, USA  T. K. Kleiner DESY, Platanenallee 6, 15738
Zeuthen, Germany  N. Korzoun Department of Physics and Astronomy and the
Bartol Research Institute, University of Delaware, Newark, DE 19716, USA  S.
Kumar Department of Physics, University of Maryland, College Park, MD, USA  M.
J. Lang School of Natural Sciences, University of Galway, University Road,
Galway, H91 TK33, Ireland  M. Lundy Physics Department, McGill University,
Montreal, QC H3A 2T8, Canada  G. Maier DESY, Platanenallee 6, 15738 Zeuthen,
Germany  C. E. McGrath  School of Physics, University College Dublin,
Belfield, Dublin 4, Ireland  M. J. Millard Department of Physics and
Astronomy, University of Iowa, Van Allen Hall, Iowa City, IA 52242, USA  H. R.
Miller  Santa Cruz Institute for Particle Physics and Department of Physics,
University of California, Santa Cruz, CA 95064, USA  J. Millis  Department of
Physics and Astronomy, Ball State University, Muncie, IN 47306, USA  C. L.
Mooney Department of Physics and Astronomy and the Bartol Research Institute,
University of Delaware, Newark, DE 19716, USA  P. Moriarty School of Natural
Sciences, University of Galway, University Road, Galway, H91 TK33, Ireland  R.
Mukherjee Department of Physics and Astronomy, Barnard College, Columbia
University, NY 10027, USA  S. O’Brien Physics Department, McGill University,
Montreal, QC H3A 2T8, Canada  Arthur B. McDonald Canadian Astroparticle
Physics Research Institute, 64 Bader Lane, Queen’s University, Kingston, ON
K7L 3N6, Canada  R. A. Ong Department of Physics and Astronomy, University of
California, Los Angeles, CA 90095, USA  M. Pohl Institute of Physics and
Astronomy, University of Potsdam, 14476 Potsdam-Golm, Germany  DESY,
Platanenallee 6, 15738 Zeuthen, Germany  E. Pueschel DESY, Platanenallee 6,
15738 Zeuthen, Germany  J. Quinn School of Physics, University College Dublin,
Belfield, Dublin 4, Ireland  K. Ragan Physics Department, McGill University,
Montreal, QC H3A 2T8, Canada  P. T. Reynolds  Department of Physical Sciences,
Munster Technological University, Bishopstown, Cork, T12 P928, Ireland  D.
Ribeiro School of Physics and Astronomy, University of Minnesota, Minneapolis,
MN 55455, USA  E. Roache  Center for Astrophysics $|$ Harvard & Smithsonian,
Cambridge, MA 02138, USA  J. L. Ryan Department of Physics and Astronomy,
University of California, Los Angeles, CA 90095, USA  I. Sadeh DESY,
Platanenallee 6, 15738 Zeuthen, Germany  L. Saha Center for Astrophysics $|$
Harvard & Smithsonian, Cambridge, MA 02138, USA  M. Santander  Department of
Physics and Astronomy, University of Alabama, Tuscaloosa, AL 35487, USA  G. H.
Sembroski  Department of Physics and Astronomy, Purdue University, West
Lafayette, IN 47907, USA  R. Shang Department of Physics and Astronomy,
Barnard College, Columbia University, NY 10027, USA  D. Tak  DESY,
Platanenallee 6, 15738 Zeuthen, Germany  A. K. Talluri  School of Physics and
Astronomy, University of Minnesota, Minneapolis, MN 55455, USA  J. V. Tucci
Department of Physics, Indiana University-Purdue University Indianapolis,
Indianapolis, IN 46202, USA  N. Vazquez  Santa Cruz Institute for Particle
Physics and Department of Physics, University of California, Santa Cruz, CA
95064, USA  D. A. Williams Santa Cruz Institute for Particle Physics and
Department of Physics, University of California, Santa Cruz, CA 95064, USA  S.
L. Wong Physics Department, McGill University, Montreal, QC H3A 2T8, Canada
J. Woo  Columbia Astrophysics Laboratory, Columbia University, New York, NY
10027, USA  D. DeBoer Breakthrough Listen, University of California Berkeley,
Berkeley, CA 94720, USA  H. Isaacson Breakthrough Listen, University of
California Berkeley, Berkeley, CA 94720, USA  University of Southern
Queensland, Toowoomba, QLD 4350, Australia  I. de Pater  Department of
Astronomy, University of California Berkeley, Berkeley, CA 94720, USA  D. C.
Price International Centre for Radio Astronomy Research, Curtin University,
Kent St, Bentley WA 6102, Australia  Radio Astronomy Laboratory, 501 Campbell
Hall, University of California, Berkeley, CA 94720, USA  A. Siemion
Breakthrough Listen, University of California Berkeley, Berkeley, CA 94720,
USA  SETI Institute, Mountain View, CA 94043, USA  Department of Physics and
Astronomy, University of Manchester, UK  University of Malta, Institute of
Space Sciences and Astronomy, Msida, MSD2080, Malta G. M. Foote

###### Abstract

The Breakthrough Listen Initiative is conducting a program using multiple
telescopes around the world to search for “technosignatures”: artificial
transmitters of extraterrestrial origin from beyond our solar system. The
VERITAS Collaboration joined this program in 2018, and provides the capability
to search for one particular technosignature: optical pulses of a few
nanoseconds duration detectable over interstellar distances. We report here on
the analysis and results of dedicated VERITAS observations of Breakthrough
Listen targets conducted in 2019 and 2020 and of archival VERITAS data
collected since 2012. Thirty hours of dedicated observations of 136 targets
and 249 archival observations of 140 targets were analyzed and did not reveal
any signals consistent with a technosignature. The results are used to place
limits on the fraction of stars hosting transmitting civilizations. We also
discuss the minimum-pulse sensitivity of our observations and present VERITAS
observations of CALIOP: a space-based pulsed laser onboard the CALIPSO
satellite. The detection of these pulses with VERITAS, using the analysis
techniques developed for our technosignature search, allows a test of our
analysis efficiency and serves as an important proof-of-principle.

††software: ROOT (Antcheva et al., 2009), Astropy (Astropy Collaboration et
al., 2022), Astroquery (Ginsburg et al., 2019), BeautifulSoup, GeoPy,
Matplotlib (Hunter, 2007), Mechanize, NetworkX (Hagberg et al., 2008), Numpy
(Harris et al., 2020), Pandas (Wes McKinney, 2010), PyTeVCat, Seaborn (Waskom,
2021), Skyfield (Rhodes, 2020)

##  1 Introduction

The search for extraterrestrial intelligence (SETI) can be defined as the
“theory and practice of searching for extraterrestrial technology or
technosignatures” (Wright et al., 2018b). Technosignatures are
extraterrestrial signals whose only explanation is that they were produced
artificially. Examples of potential technosignatures include interstellar
radio-based communications (Cocconi & Morrison, 1959), interstellar laser-
based communications (Schwartz & Townes, 1961; Tellis & Marcy, 2017; Zuckerman
et al., 2023), radio and optical leakage from technological civilizations
(Sullivan et al., 1978; Schneider et al., 2010), infrared emission from Dyson
spheres (Dyson, 1960), spectral evidence for industrial pollutants in
exoplanet atmospheres (Wright, 2018), and physical artifacts deposited within
our solar system (Bracewell, 1960). Since the founding of the field in the
1950s, there have been numerous searches for these technosignatures using
radio, optical, and infrared telescopes, but the fraction of the total
parameter space which has been searched remains extremely low (Wright et al.,
2018a).

This paper presents the results of a partnership between the Very Energetic
Radiation Imaging Telescope Array System (VERITAS) Collaboration and the
Breakthrough Listen Initiative in a search for pulsed optical laser-based
communications. The Breakthrough Listen
Initiative111https://breakthroughinitiatives.org/ is currently the foremost
technosignature search campaign (Worden et al., 2017; Isaacson et al., 2017a).
It began searching for radio technosignatures in 2016 through a partnership
with the Green Bank Telescope and the Parkes Observatory, subsequently adding
MEERKAT in 2018. Similarly in the optical band, a partnership with the
Automated Planet Finder in 2016 at the Lick Observatory and the Keck
Observatory enabled a spectral search for laser-based communication (Tellis &
Marcy, 2017; Isaacson et al., 2019; Lipman et al., 2019). More recently,
Breakthrough has partnered with the exoplanet-hunting Transiting Exoplanet
Survey Satellite (TESS) to search for anomalous stellar lightcurves, and to
search targets of interest from the TESS catalog with radio telescopes (Traas
et al., 2021; Franz et al., 2022). Taken together, these partnerships
constitute the most comprehensive search for technosignatures thus far (Gajjar
et al., 2019).

Each search for a specific technosignature has benefits and drawbacks,
justifying the approach of performing many such searches concurrently. For
example, radio-leakage technosignatures emit continuously in every direction,
but the inverse-square law and the expected low radio intensity lead to a
requirement for radio telescopes which are still in the planning and
construction phases, with the full Square Kilometer Array being a notable
example (Siemion et al., 2015).

For pulsed optical laser-based communication, the benefit lies in
concentrating all of the emitting power into a small angular diameter over
nanosecond timescales. These laser pulses could, in principle, be produced
with today’s technology, and could be easily distinguished from the emitter’s
host star, without significant dispersion losses. A $3\mathrm{\ ns}$,
$3.7\mathrm{\ MJ}$ optical laser pulse, collimated at the source using a
$10\mathrm{\ m}$ reflector and observed from a distance of 1000 light years,
would appear approximately $10^{4}$ times as bright as its host star (Horowitz
et al., 2001; Howard et al., 2004). Constructing an interstellar communication
system based on this technology is not only theoretically possible, but is
currently feasible. While these pulses could be bright when observed from
within the beam’s solid angle, they would occur only over very short
timescales. The optical receiver therefore requires a large-aperture mirror
with fast photon detectors and associated instrumentation. These requirements
are the same as those for atmospheric Cherenkov telescopes (ACTs), which are
used to measure nanosecond-timescale Cherenkov emission from cosmic-ray- and
gamma-ray-initiated particle showers in the Earth’s atmosphere. These
telescopes can therefore be used to search for optical laser pulse
technosignatures (Covault, 2001; Eichler & Beskin, 2001; Holder et al., 2005;
Armada et al., 2005).

Nanosecond pulsed SETI searches in the blue/UV region of the electromagnetic
spectrum specifically are well-motivated, if the background due to cosmic ray
events can be removed. The study of cosmic rays has been ongoing for more than
a century and is closely tied to the development of modern physics. Cosmic
rays are an important and ubiquitous constituent of the Galaxy — their energy
density is similar to that of starlight, Galactic magnetic fields or the
cosmic microwave background radiation. Any developing technological
civilization would almost certainly study cosmic rays and, if located on a
planet with a transparent atmosphere, would very likely use the atmospheric
Cherenkov effect to do so. The key technology required for this —
photomultiplier tubes — has been widely available to our civilization since
the late 1930s. Furthermore, Cherenkov telescopes are (by far) the largest
optical telescopes in the world. The H.E.S.S. II telescope, currently
operating, has a remarkable $28\mathrm{\ m}$-aperture and the field as a whole
has been operating 10-meter-class telescopes since the late 1960s. We argue,
therefore, that nanosecond pulsed emission in the blue/UV (at the peak of the
spectrum of Cherenkov light) represents a preferred search region for SETI,
similar to the famous “water hole” in the radio band. Other wavelengths, such
as near infra-red, might also be a natural choice since they experience less
extinction due to dust. However, we also argue that any advanced civilization
attempting to communicate with an emerging technological civilization would be
aware that Cherenkov telescopes are one of the earliest methods capable of
easily detecting signals over interstellar distances and that these
observations will occur naturally as a side-project of fundamental physical
investigations.

One of the first such searches was conducted using the Solar Tower Atmospheric
Cherenkov Effect Experiment (STACEE), which re-purposed a New Mexico solar
energy research facility for nighttime operations as a wavefront-sampling ACT
(Gingrich et al., 2005). STACEE consisted of a field of 64 steerable
heliostats, each with $37\mathrm{\ m^{2}}$ mirror area. Light received at the
heliostats was reflected onto two sets of secondary mirrors at the top of a
tower before being focused onto a set of 64 photomultiplier tubes (PMTs). This
system had a $10-15\mathrm{\ photon}\mathrm{\ m^{-2}}$ sensitivity between 400
and 500$\mathrm{\ nm}$, peaking at 420$\mathrm{\ nm}$. The STACEE
Collaboration conducted dedicated observations of 187 targets from the HabCat
catalog (Turnbull & Tarter, 2003) for 10 minutes each, between January and May
2007, and did not find any evidence for technosignature signals during their
observations (Hanna et al., 2009).

Imaging atmospheric Cherenkov telescopes are also designed to detect
atmospheric Cherenkov radiation, but differ from wavefront-samplers such as
STACEE in that the telescopes are equipped with photomultiplier tube cameras
which allow recording of an image of the Cherenkov light flash. The potential
use of such imaging ACTs (IACTs) for SETI for technosignature searches was
first discussed in the early 2000s (Tarter, 2003), and an analysis methodology
and a single test observation using the Whipple $10\mathrm{\ m}$ IACT was
performed in 2005 (Holder et al., 2005). The importance of the imaging
technique is that it provides efficient discrimination between point-like
pulsed optical technosignatures and the enormous background of Cherenkov
flashes generated by cosmic ray particle showers in the Earth’s atmosphere.

The power of IACTs for optical technosignature searches is dramatically
improved when multiple telescopes are combined together in an array. An array
of physically separated telescopes provides an additional coincidence
requirement for the pulses detected by each telescope, combined with the
ability to measure parallax. This approach was first developed using the
VERITAS array, in an archival search for pulsed optical technosignatures from
KIC 8462852 (Abeysekara et al., 2016). The analysis allowed efficient
identification of laser-like events over the background of cosmic ray images.

The VERITAS Collaboration has since partnered with the Breakthrough Listen
Initiative to continue the research started with the study of KIC 8462852.
This partnership has led to 30 hours of dedicated observations of Breakthrough
Listen targets with VERITAS and an analysis of 110 hours of observations from
the VERITAS archive of sky regions containing Breakthrough Listen targets. The
analysis and first results of this program are reported here.

##  2 VERITAS

VERITAS is an IACT array, designed to detect Cherenkov radiation from particle
showers in the Earth’s atmosphere and to identify those initiated by gamma-ray
photons over the background of those due to cosmic rays. A description of the
VERITAS telescopes can be found in Holder et al. (2006), and the methods of
ground-based gamma-ray astronomy are summarized in e.g. Holder (2021). Here we
briefly describe those technical aspects of VERITAS most relevant to the
search for optical technosignatures.

Figure 1: An elevated view of the VERITAS array located at the base of Mount
Hopkins near Tuscon, AZ. Pictured are the four individual telescopes, which
are roughly $100\mathrm{\ m}$ apart, the Fred Lawrence Whipple Observatory
visitor center, and the VERITAS control building (with the white roof). Image
from Abeysekara et al. (2016)

VERITAS consists of four IACTs located at the Fred Lawrence Whipple
Observatory in southern Arizona (Figure 1). Each telescope has a 12-m-diameter
tessellated reflector mounted on a steerable alt-azimuth platform. The
reflector dish comprises 345 hexagonal mirror segments (Roache et al., 2008)
arranged in a Davies-Cotton design (Davies & Cotton, 1957), giving a total
mirror area of $\sim 110\mathrm{\ m^{2}}$. Alignment of the individual mirror
segments is performed and regularly verified using the method described by
McCann et al. (2010), resulting in an on-axis optical point-spread function of
less than $0.1\degree$ (68% containment radius). The focal length of the
optical system is $12\mathrm{\ m}$, giving a focal ratio of 1.0. The focal
plane is instrumented with a close-packed array of 499 Hamamatsu R10560 super-
bialkali photomultiplier tubes (PMTs), covering an approximately circular
$3.5\degree$-diameter field-of-view (FOV) with a pixel spacing of
$0.15\degree$. CCD cameras installed on the telescope structure monitor the
position of the PMT camera with respect to the sky, and provide pointing
corrections with an absolute positional accuracy of $\sim 50\arcsec$. The
camera PMT pixels are sensitive over a wide spectral range, with a peak
detection efficiency around $400\mathrm{\ nm}$. Dead space between the
circular entrance windows of the PMTs is reduced by the addition of truncated
Winston cones to the PMT front faces. These cones are shaped such that the
entrance is hexagonal and the exit is circular, allowing them to effectively
tile the FOV (Nagai et al., 2008).

All PMT signals are digitized using 2-ns-sampling, 8-bit flash analog-to-
digital converters (FADCs). The FADC read-out is initiated by a three-level
trigger system, which requires a signal at the individual pixel level, the
telescope camera level, and over the full array. The individual pixel trigger
condition is determined by a constant fraction discriminator, while the
telescope camera trigger requires at least three adjacent PMT pixel triggers
within a coincidence time window of $\sim 5\mathrm{\ ns}$. The array trigger
requires at least two telescope camera triggers within a $50\mathrm{\ ns}$
coincidence window, after the application of hardware timing delays to correct
for path-length differences between telescopes. Since the optical
technosignature images are expected to resemble the telescope optical point-
spread function (which can be smaller than the angular size of a single PMT
pixel) the impact of the 3-pixel camera-level trigger requirement is
particularly important for technosignature searches. We discuss this issue in
more detail in section 4 of this paper.

The recorded FADC pulses are calibrated, integrated and used to create a
499-pixel image for all four telescopes in the array. These images (or
“events”) are recorded at a rate of typically $300\mathrm{\ Hz}$, the vast
majority of which are due to Cherenkov emission from cosmic-ray-initiated
particle cascades in the atmosphere (Kieda, 2013). Subsequent analysis of
these images allows identification of the small fraction (typically
$<10^{-4}$) that are due to gamma-ray-initiated showers or, as described in
the following section, to search for images which resemble a distant optical
laser pulse.

##  3 OSETI analysis with VERITAS

Figure 2: A schematic illustration of the optical SETI (OSETI) technique with
IACT arrays such as VERITAS. Particle air showers, initiated by cosmic-ray
particles or gamma-ray photons, produce extended images with parallax shifts
when viewed from separated telescopes (left). A distant laser pulse produces
identical point-like images, located at the same position in the field of view
in each telescope (right).

The analysis applied in this work is similar to that used in the original
search for optical technosignatures from KIC 8462852 with VERITAS (Abeysekara
et al., 2016). The data are first reduced using the standard VERITAS analysis
packages (Maier & Holder, 2017; Cogan, 2008), which calibrate and parameterize
the recorded images using a moment analysis. Cuts on the resulting image
parameters (the image width, length, etc. — usually referred to as Hillas
parameters (Hillas, 1985)) are then used to filter almost all events due to
Cherenkov emission from cosmic ray air showers from the data. While more
sophisticated machine learning approaches are under investigation for this
analysis, and are already in use for VERITAS gamma-ray analyses (Krause et
al., 2017), simple image parameter cut selections are computationally cheap
and robust, and have proven to be extremely effective.

The key characteristics of a potential optical technosignature are: (i) that
the emission is point-like (i.e. indistinguishable from the telescope optical
point-spread function); (ii) that it originates from infinity (i.e. shows no
parallax shift and has uniform intensity, when viewed from different locations
on the ground); and (iii) that it originates from the position of a target
star. This is in contrast to the Cherenkov radiation images of particle
cascades produced locally in the Earth’s atmosphere, which can have large
angular extent (up to a few degrees), are uniformly distributed over the FOV,
and which display significant parallax and non-uniform intensity, when viewed
by separated telescopes. Although not used in our work, pulse timing
differences may also be used to identify technosignature candidates (Wright et
al., 2018c). These features are illustrated schematically in Figure 2.

The choice of image parameter cuts follows logically from these differences.
In the analysis of KIC 8462852, the cuts required that at least three of the
four telescope images must contain light, the centroid coordinates of the
images in every telescope must be separated from each other by less than
$0.15\degree$, and the length and width of all images must be less than
$0.1125\degree$. After the removal of a few examples of easily identified
meteor and satellite tracks, only 28 of the initial 7036970 events passed
these selection cuts. There were no events retained in which the average
position of the images was within $0.15\degree$ of the location of the target
star for that study, KIC 8462852.

In this work, we are analyzing a much larger dataset, testing a catalog of
many targets, and have a less homogeneous set of observations. These factors
motivate the application of stricter cuts to further improve the background
rejection. The most important of these is a modification to the length and
width cuts. First, we reduce the cut values to $length<0.09\degree$ and
$width<0.07\degree$, this matches the optical point-spread function of the
telescopes better than before. Second, we apply these cuts to the telescope
with the third-smallest measured length or width in each event. The motivation
behind this is to reduce the impact of PMT afterpulses in the data.
Afterpulses are a well-known phenomenon caused mainly by residual positive
ions in the PMTs. They appear in the telescope cameras as a single, relatively
bright pixel, randomly located in the field of view. This can distort the
Hillas parameters of the image in the affected telescope. However,
afterpulsing typically affects at most one telescope image in a given event.
Applying the length and width cuts to the image with the third-smallest values
of these parameters allows an event with one afterpulse-contaminated image to
be retained. The telescope image that exceeds the length and width cuts is
then also removed from consideration for the other selection criteria. An
additional modification is to remove any events that include images
potentially truncated by the edge of the camera. This is implemented using the
loss parameter, defined as the fraction of the total light in the image
contained in pixels that lie on the edge of the camera (we require $loss=0$).
As a final check, we visually inspect any remaining candidate events (and
their associated ancillary data) to ensure that the telescope cameras were
functioning correctly, and that each telescope contributed to the event as
expected. For example, if a bright pulse was recorded in only three of the
four telescopes, this would exclude it as a candidate — except if the missing
telescope had an inoperative PMT pixel at that location in its FOV. At any
time, typically a few percent of the PMTs in the telescopes’ cameras are
malfunctioning, or are temporarily disabled to avoid damage due to bright
stars.

##  4 Analysis verification using the CALIOP instrument on the CALIPSO
satellite

The probability of the VERITAS array triggering on and recording an optical
pulse, as well as the efficiency of the subsequent analysis, is difficult to
test under realistic conditions. Monte Carlo simulations provide one approach
and are commonly used to estimate the effective detection area and to define
the analysis and event selection cuts for gamma-ray astronomy. In this case,
the simulated gamma-ray events can be compared with a known bright source of
astrophysical gamma-rays, such as the Crab Nebula, and the telescope model
parameters tuned until a satisfactory match is achieved. For the analysis,
however, no natural standard signal exists with which to compare simulations,
or to verify the analysis. Furthermore, the precise properties of the pulse to
be simulated (risetime, pulse width, wavelength, etc.) are not known.

An ideal test signal would be a distant laser which flashes the telescope
cameras from a known location, as this matches the technosignature we are
looking for. Pulsed light sources have been used for calibration of IACTs for
many years. From 2005, nightly calibrations of VERITAS were performed using a
laser with a 337 nm wavelength and a pulse duration of 4 ns at a distance of
roughly 5 meters from the camera (Hanna, 2008) before switching to a similar
LED-based calibration system in 2010 (Hanna et al., 2010). However, these
measurements are designed to illuminate the entire FOV uniformly and do not
serve as a useful analog to a distant point source. Another calibration
technique once used by VERITAS involves firing a laser pulse upwards from the
ground and observing the Rayleigh-scattered laser light with the telescopes
(Shepherd et al., 2005; Hanna, 2008). Again, the observed image is not point-
like, but corresponds to an illuminated column in the atmosphere.

The Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) instrument
onboard the polar orbiter Cloud-Aerosol Lidar and Infrared Pathfinder
Satellite Observation (CALIPSO) satellite is a space-based backscattering
lidar, designed to provide high-resolution vertical profiles of aerosols and
clouds, which emits $110\mathrm{\ mJ}$, $20\mathrm{\ ns}$-duration laser
pulses at a repetition rate of $20.16\mathrm{\ Hz}$, at both $532\mathrm{\
nm}$ and $1064\mathrm{\ nm}$ (Winker et al., 2009). This provides an excellent
technosignature verification source for VERITAS. The camera PMTs are sensitive
at $532\mathrm{\ nm}$, with a quantum efficiency of approximately 12%. At an
orbital height of $700\mathrm{\ km}$, the laser is effectively a point source
at infinity relative to the size of VERITAS. The lidar is directed 3° from
geodetic nadir in the forward along-track direction of the satellite’s orbit,
and the laser footprint on the ground is predicted to be less than 100 m in
diameter (Winker et al., 2009), making a coincidental overlap with the VERITAS
telescopes extremely unlikely. However, observations by the TAIGA-HiScore
collaboration of the CALIPSO laser (Porelli & Taiga Collaboration, 2022)
demonstrated that the actual footprint extends far beyond the nominal
distance, out to at least tens of kilometers, for reasons which are not
entirely clear. This motivated both new observations with VERITAS, and a
search of the VERITAS archive for serendipitous passages of the satellite
through the field of view.

Examples of these CALIPSO observations are shown in Figure 3. The top image
illustrates a passage from a dedicated observation on May 17, 2021, which
occurred at an elevation of 74°. The pulse intensity observed by VERITAS was
approximately 2000 photo-electrons at each telescope, corresponding to
150$\mathrm{\ photons}$$\mathrm{\ m^{-2}}$ at ground level. During the
transit, 69% of the pulses emitted by CALIPSO triggered VERITAS and 55% passed
the optical SETI analysis cuts without accounting for the loss parameter. The
efficiency without the loss cut applied is most relevant for comparison with
our analysis, since we select target locations which are not close to the
camera edge (section 5). For these very high intensity pulses, the missing
triggered events are largely explained by the deadtime of the telescope data
acquisition (which was 9% for this observation) and by the existence of a
large patch of inoperative PMTs in one telescope along the track of the
satellite, as can be seen in the figure.

A second transit is shown on the bottom of Figure 3. This was a serendipitous
passage which occurred on November 11, 2013 at an elevation of 54°, when the
center of the laser footprint was $\sim 400\mathrm{\ km}$ distant from the
location of VERITAS. The measured pulse intensity is more than an order of
magnitude lower in this case, corresponding to approximately 10$\mathrm{\
photons}$$\mathrm{\ m^{-2}}$. 21% of the pulses emitted by CALIPSO triggered
VERITAS and 20% passed our analysis without accounting for loss. The
relatively low trigger efficiency during this transit is likely the result of
the extremely non-uniform sensitivity of the VERITAS trigger system to point-
like pulses, as we discuss in more detail in section 6.

Figure 3: The tracks of the CALIPSO transits (in camera coordinates) on May
17, 2021 (top) and November 11, 2013 (bottom) with the location of the average
of the image centers in an event shown as blue points. The hexagons correspond
to VERITAS pixels. Grayed out pixels were non-functional during the transit
for VERITAS telescope 1.

##  5 Observations and Results

The VERITAS/Breakthrough Listen search we have conducted comprises two
datasets, which we describe below. The first of these is a program of
dedicated VERITAS observations of Breakthrough Listen targets, while the
second is an analysis of serendipitous archival observations.

###  5.1 Dedicated VERITAS Observations

Between March 2019 and March 2020, VERITAS spent 30 hours observing objects
selected from the Breakthrough Listen target catalog (Isaacson et al., 2017b).
This catalog lists targets originally identified for Breakthrough Listen
observations with the Green Bank Telescope, Parkes Telescope, and the
Automated Planet Finder. It includes: the 60 nearest stars; 1649 stars within
a distance of $50\mathrm{\ pc}$ sampling a range of masses, ages and elemental
abundances; 123 nearby galaxies; and several exotic objects, including white
dwarfs, brown dwarfs, neutron stars and black holes.

Not all of these targets are suitable for observations with VERITAS. We
removed all galaxies, on the assumption that optical emission is unlikely to
be detectable over such large (i.e. extragalactic) distances (Hippke, 2018).
We also limited targets to the declination band between $\delta=-10\degree$
and $\delta=+70\degree$, to ensure that the object culminates above $\sim
40\degree$. High elevation observations are preferred, as they provide greater
discrimination power between a pulsed point source at infinity and the
background of Cherenkov events generated in the Earth’s atmosphere. This is
due to the fact that Cherenkov flashes observed at low elevation occur at a
larger distance from the telescopes, reducing their parallax angle, image
intensity and angular size and making them appear more point-like.

We also removed all targets with a B-band magnitude of less than 7, and all
targets within $0.15\degree$ of an object (excluding the target itself) with a
B or V magnitude of less than 8. Bright stars generate a large amount of
background photon noise in the VERITAS PMTs, as well as high currents, which
accelerate wear. If the current on any individual PMT exceeds a preset
threshold, the high voltage supplied to that channel is automatically turned
off. This has little impact on the observation of Cherenkov events with large
angular extent, but reduces or completely removes the sensitivity to point-
like optical pulses from the star’s location.

Finally, we removed any targets which had been previously observed by VERITAS,
either intentionally or serendipitously, in the FOV of other observations.
Observations of these targets are included in the archival search described in
section 5.2. This resulted in a list of 506 targets, which were then ranked
according to the inverse square of their distance and their optical
brightness, with nearby, optically faint targets being preferred. Targets
lying close to the ecliptic (which could host civilizations that view the
Earth as a transiting exoplanet (Heller & Pudritz, 2016; Sheikh et al.,
2020)), or hosting known exoplanets, or located close to another target, were
also favored, but with lower weight than the two main criteria of brightness
and distance.

Candidates from this target list were selected for observation based on their
ranking and on observatory scheduling constraints. Observations were conducted
in typically 15 minute exposures taken within 90 minutes of culmination, with
the primary target offset from the center of the FOV by 1.25°. This offset was
chosen to improve the probability of triggering on a faint pulse, as discussed
in more detail in section 6.1. All data were taken under clear skies, at new
or crescent Moon phases, and with all four telescopes in the array operating
correctly. The final dataset comprises 127 observing runs of 108 non-
overlapping target fields, with a total exposure of 30.16 hours. Some target
fields contain multiple targets, allowing us to study a total of 136 targets
with this dataset. Most targets were observed only once, while 25 were
observed twice, and three were observed three times.

The locations of the observed stellar targets are shown in figure 4, with
their spectral class and distance indicated. Figure 5 shows the Hertzsprung-
Russell diagram for all of the stellar Breakthrough Listen targets, with those
targets observed by VERITAS indicated. The VERITAS selection covers a broad
range of spectral classes along the main sequence, from B to M, as well as a
few giant branch stars.

Figure 4: The stellar target locations, in equatorial coordinates, for both
the dedicated and archival VERITAS observations. Distance and spectral type
are also indicated, as described in the figure legend. Figure 5: The coverage
across an H-R Diagram for the stellar targets used in both dedicated
observations and archival analysis. It is similar to the coverage found in the
original Isaacson et al. (2017b) Catalog.

###  5.2 Archival Search

VERITAS has been fully operational since 2007, and records typically $\sim
1000\mathrm{\ hours}$ of observations each year. Thanks to the large VERITAS
FOV ($9.6\mathrm{\ deg^{2}}$), these observations provide coverage of over 20%
of the sky, with exposures cumulatively ranging from a few minutes to hundreds
of hours. A complete search of this extensive archive for optical pulses is a
worthwhile subject for future work, but will require further development of
the analysis tools – in particular, to deal with observations recorded at low
elevation and to overcome the increased background from examining the entire
FOV as opposed to just the region around a set of pre-defined locations.

For this work, we selected a reduced set of archival VERITAS observations to
analyze. To create this set, we required that the observations were recorded
at high elevations ($>40\degree$), with at least three telescopes operating,
and with excellent weather conditions. Data taken prior to summer 2012 were
not considered, as this was when the VERITAS photomultiplier tube cameras were
upgraded, improving the photon detection efficiency by $\sim 30\%$ (Kieda,
2013). Unlike the dedicated observations, the radial distance of the target
from the center of the field of view for archival observations could not be
fixed at $1.25\degree$; We instead set the maximum radial distance to be
$1.5\degree$. We also set the maximum exposure to be analyzed on any single
target to be $1\mathrm{\ hour}$. If any target exceeded this threshold, we
analyzed the first hour of good-quality data and left the remainder for the
future full archival analysis. With these conditions, we selected 249 archival
observations with an average length of 28 minutes which altogether represents
110 hours of observations containing 140 individual Breakthrough Listen
targets and 119 non-overlapping fields. The list of Breakthrough Listen
targets in this archival dataset includes 25 galaxies, which were
serendipitously inside the studied fields and are included here for
completeness. This dataset constitutes all of the Breakthrough Listen targets
for which we have good quality, high elevation data taken between September,
2012 and March, 2019. We note that the entire VERITAS archive comprises almost
$20,000\mathrm{\ hours}$ of data, including additional observations of the
Breakthrough targets analyzed here. The full analysis of this dataset will be
presented in future work. Figure 4 shows the location, spectral type,
distance, and originating dataset of all analyzed targets. Figure 5 shows the
same targets, but instead within the H-R diagram, showing the color and
magnitude of the targets. As the figures show, the analyzed targets selected
occupy a significant portion of the parameter space — locations, distances and
spectral properties — of the Isaacson et al. (2017b) catalog.

###  5.3 Results

Table 1 shows the results of each stage of the analysis pipeline for both the
dedicated observations and for the archival dataset. For the dedicated
observations, only one event survived the pre-defined selection cuts (target
HIP 83043). For the larger archival dataset, three events survived (targets
HIP 51317, HIP 93871 and NGC 4551) and were subjected to visual inspection.
For three of these four events, two of the four telescopes in the array
triggered and three of the four telescopes registered an image. The fourth
telescope did not, despite being operational and having no disabled PMTs at
the pulse location. These events therefore fail our requirement for uniform
intensity, and are rejected. The remaining event shows three OSETI-like
images, thereby satisfying the third-smallest width and length criterion, but
the fourth telescope image contains a bright, extended flash, with an angular
(parallactic) displacement from the other images. This clearly identifies the
event as being due to a cosmic ray air shower in the atmosphere, and so it is
also rejected. We therefore have zero candidate events remaining after the
full analysis.

Table 1: The number of events remaining after each stage of the analysis for serendipitous archival observations and for dedicated observations of Breakthrough Listen targets Cut description | Events remaining after cut
---|---
| Archival Data | Dedicated Observations
Before cuts | 127,346,295 | 34,917,340
At least 3 images | 80,910,174 | 23,088,334
Point-like images ($3^{\text{\tiny rd}}$ smallest $length<0.09\degree$ & $width<0.07\degree$) | 1,894,155 | 508,637
Image centers co-located (within $0.15\degree$) | 237 | 35
Near target (within $0.15\degree$) | 3 | 1
Images are not truncated ($loss=0$) | 3 | 1
Visual inspection | 0 | 0
Candidate Events | 0 | 0

##  6 Discussion

We have presented the analysis of observations of 272 Breakthrough Listen
targets with VERITAS (there are 4 targets in common between the target lists
of the dedicated observations and the archival search) and have found no
evidence for rapid optical pulses from any of these objects. Here we attempt
to summarize the sensitivity of our search, both in terms of the minimum
optical pulse intensity detectable by VERITAS and in the constraints our
survey allows us to place on the frequency of emitting civilizations.

###  6.1 Optical pulse sensitivity

Abeysekara et al. (2016) estimated the minimum optical pulse intensity
detectable by VERITAS to be $0.94\mathrm{\ photons}\mathrm{\ m^{-2}}$ for a
$12\mathrm{\ ns}$ integration window, while noting that such estimates are
challenging due to the various unknown pulse properties (location, wavelength,
duration, temporal profile, etc). The CALIPSO observations demonstrate
experimentally that pulses with an intensity of $10\mathrm{\ photons}\mathrm{\
m^{-2}}$ can be detected. Furthermore, the CALIPSO pulses are relatively long
duration, with a pulse width of $\sim 20\mathrm{\ ns}$. The single photo-
electron pulse width for VERITAS is $4\mathrm{\ ns}$, and the camera trigger
coincidence time is $\sim 5\mathrm{\ ns}$, implying that shorter pulses with a
substantially lower integrated photon intensity must also be detectable.
However, the CALIPSO results also highlight that the efficiency for pulse
detection with VERITAS is not 100%. We discuss one of the reasons for this in
more detail here.

As mentioned, the VERITAS telescope cameras are each composed of 499
photomultiplier tubes on a hexagonal grid, with a pixel to pixel spacing of
$0.15\degree$. For an individual telescope to trigger on an optical pulse,
signals on three adjacent PMT pixels must exceed a discriminator threshold
within a $\sim 5\mathrm{\ ns}$ coincidence window. A laser pulse generated at
large distance is point-like, and so this 3-adjacent trigger condition would
never be met, if the optical system were perfect. In reality, an image of a
point source has the same shape and structure as the telescope optical point-
spread function (PSF), which may overlap multiple pixels. The angular extent
of Cherenkov showers and the PMT pixels allows for cheaper mirrors with a
significantly reduced angular resolution/increased PSF compared to typical
optical telescopes. This means that IACTs like VERITAS can be much larger and
overall cheaper than their optical counterparts (Canestrari et al., 2010). The
PSF will also vary across the field of view due to comatic aberration. At the
center of the camera, it can be approximated by a bivariate Gaussian with a
$\sim 0.08\degree$ 68% containment diameter, increasing to $\sim 0.15\degree$
at an offset of $1.2\degree$, and degrading further towards the edge of the
camera at $1.75\degree$.

The probability of satisfying the 3-adjacent trigger condition therefore
depends very strongly upon the exact pulse location in the field of view.
Specifically, it is determined by the amount of light received by the PMT
which is third-most-distant from the image centroid — i.e. that which measures
the third-largest signal. In the most favorable case, the pulse centroid
location lies equidistant between three pixels, each of which receives
approximately one third of the light. In the least favorable case, the pulse
lands exactly in the center of a pixel, and adjacent pixels receive only a
small fraction of the light. The difference between these two cases is most
extreme close to the camera center, where the optical PSF is small, and least
extreme at the camera edge, where the optical PSF is more extended.

Figure 6 shows the results of a Monte Carlo simulation which illustrates these
effects and how the minimum pulse sensitivity varies with radial distance in
the camera. The optimum sensitivity is taken to be the same as that estimated
by Abeysekara et al. (2016). The green dotted line in the figure corresponds
to the worst case, where the pulse is centered on a pixel. The blue solid line
corresponds to the best case, where the pulse is equidistant between 3 pixels.
For a random pulse location on the camera, the most likely distance between
the pulse location and the pixel containing the third brightest signal is
$0.13\degree$. The orange dashed line indicates this typical case. 75% of
possible pulse locations in the camera provide a sensitivity equal to or
better than this typical case. The cross-hatched region indicates this, as
well as the outer region of the camera corresponding to 75% of the total area.
The black vertical line at a camera radius of $1.25\degree$ indicates the
position of the Breakthrough Listen targets in the field of view for the
dedicated VERITAS observations reported here. The typical sensitivity in this
case is $3\mathrm{\ ph}\mathrm{\ m^{-2}}$. Observations of the CALIPSO
satellite laser over a wide range of elevations (and hence pulse intensity and
pulse location in the cameras) are currently being taken by VERITAS and will
allow testing these sensitivity estimates more rigorously.

Figure 6: The sensitivity (minimum detectable pulse intensity) as a function
of radial distance from the center of the VERITAS telescope field of view. The
three curves correspond to a pulse located at the center of a PMT (worst
case), equidistant between 3 PMTs (best case) and at the most common location
(typical case). The cross-hatched region indicates the outer 75% of the camera
area, and the sensitivity of 75% of the possible pulse locations within this
area. See text for more details.

As a final point, we stress that the issue of non-uniform sensitivity across
the field of view is not intrinsic to the technique; rather, it is a result of
the VERITAS trigger system design, which is optimized for gamma-ray astronomy.
A dedicated trigger for point-like optical pulses, requiring the same single
pixel to cross a trigger threshold on multiple separated telescopes, would
completely remove this limitation. This could be implemented in a relatively
straightforward manner on existing or future facilities and operate in
parallel with the existing Cherenkov trigger system.

###  6.2 Survey sensitivity

The sensitivity to optical pulses is an important instrumental metric.
Complementary to this, however, is the sensitivity of the search as a survey:
that is, how do our results constrain the parameter space of potential
emitters? There are many different ways to estimate this, usually discussed in
the context of searches for radio technosignatures (e.g. see Wright et al.
(2018a) and references therein). The most applicable prior works for our
purposes are those of Howard et al. (2004), Howard et al. (2007), and Mead
(2013) which discuss the search for nanosecond-duration, pulsed optical
emission using optical astronomical telescopes equipped with hybrid avalanche
photodetectors or with photomultiplier tubes. From 1998 to 2003, 11,600
targeted observations of 4730 stellar objects were made under good conditions
with the $1.5\mathrm{\ m}$-aperture Wyeth telescope at the Harvard/Smithsonian
Oak Ridge Observatory, with a total exposure of $1721\mathrm{\ hr}$.
Subsequently, the Harvard All-Sky Observatory utilized a custom optical setup
consisting of a $1.8\mathrm{\ m}$ telescope which focused a $1.6\degree\times
0.2\degree$ patch of the sky onto a beam splitter with matched arrays of 8
photomultiplier tubes down each path. From 2007 to 2012, it made 7320 hours of
observations over which it searched the entire northern sky four times.

Each of these campaigns used the same mathematical model, as explained in
Howard et al. (2004), to place an upper bound on the fraction of nearby stars
that host civilizations emitting optical pulses towards the Earth as a
function of $P$, the typical pulse repetition period, under the assumption
that any emitted pulse would exceed the minimum pulse sensitivity of the
instrument. The results are replicated in Figure 7. We emphasize, however,
that the minimum pulse sensitivity of the VERITAS observations ($\gtrsim
3\mathrm{\ ph}\mathrm{\ m^{-2}}$) is much better than that of the Harvard
experiment ($\gtrsim 100\mathrm{\ ph}\mathrm{\ m^{-2}}$).

We have applied a similar methodology to the sum of both VERITAS datasets
described in this paper, with an observed sample of 247 unique stellar
targets, and a total observation time of $140\mathrm{\ hr}$. Figure 7 also
demonstrates the potential survey sensitivity that can be achieved if we still
obtain no candidate events after removing the constraint that pulses must be
associated with a pre-defined location from the Breakthrough Listen target
list. This requires some additional analysis development, to further reduce
the remaining background, but is realistically achievable in the near future.
For this calculation, we assume a typical stellar density of $0.1\mathrm{\
stars}\mathrm{\ pc^{-3}}$ and a maximum range of $1\mathrm{\ kpc}$,
corresponding to $4\times 10^{8}$ stars over the whole sky — similar to the
values used for calculating the Harvard All-Sky limits (Mead, 2013). Using
only the $140\mathrm{\ hr}$ dataset considered in this paper, this search
would lower the upper-limit on the fraction of stars with transmitters by
roughly five orders of magnitude, corresponding to the ratio of the number of
stars searched between targeted and non-targeted techniques. Applying the same
approach to the entire VERITAS archive of 18,176 hours would further reduce
the minimum upper limit, and extend the search sensitivity to much longer
pulse transmission periods.

Figure 7: Upper limits on the fraction of stars with transmitting
civilizations as a function of the average period between pulses, using the
model from Howard et al. (2007) (building on Howard et al. (2004)) which
assumes no candidate pulses are found. From top to bottom the five lines
correspond to: all data from this paper (247 targets; upside-down triangles),
the Harvard targeted search (Howard et al., 2004)(4730 targets; circles), the
Harvard All-Sky untargeted survey (Howard, 2006; Mead, 2013) (7320 hours;
diamonds), a hypothetical non-targeted VERITAS survey using all of the data
from this paper (140 hours; right-pointing triangles), and a hypothetical non-
targeted survey using all data from the entire VERITAS archive (18,176 hours;
left-pointing triangles). The minimum detectable pulse is $\sim 30$ times
larger for Harvard than for VERITAS.

##  7 Conclusions and Prospects

VERITAS is not alone in searching for optical technosignatures. Table 1 in
Schuetz et al. (2016) summarized the capabilities of optical technosignature
searches at the time of the VERITAS analysis of KIC 462852. Since then, there
have been numerous developments in the field. The Near-InfraRed Optical SETI
instrumentation on a 1-meter telescope at the Lick Observatory has been used
to conduct a survey of 1280 celestial objects in the near-infrared
(950–$1650\mathrm{\ nm}$), sensitive to pulses with durations of $<50\mathrm{\
ns}$ (Maire et al., 2019). Tellis & Marcy (2017) conducted a survey of 5600
FGKM stars using the Keck $10\mathrm{\ m}$ telescope, searching for a spectral
(not temporal) signature of laser emission. An all-sky instrument called
PANOSETI is under development (Liu et al., 2020; Maire et al., 2022), and will
soon provide nightly all-sky coverage from two sites. Cherenkov telescopes
also have an important role to play in future developments. The TAIGA-HiSCORE
wide-aperture Cherenkov array, consisting of 100, $0.5\mathrm{\ m^{2}}$
telescopes spread over $1\mathrm{\ km^{2}}$ with a field of view of
$0.6\mathrm{\ sr}$, has searched for nanosecond optical transients and
detected pulsed emission from the CALIPSO satellite (Panov et al., 2021). In
the coming decade, the Cherenkov Telescope Array (CTA) will provide
unprecedented telescope light collecting area, exceeding the mirror area of
all of the world’s large optical telescopes combined (Cherenkov Telescope
Array Consortium et al., 2019). It will have the capability to conduct
nanosecond optical pulse searches similar to VERITAS, with much greater
sensitivity and stricter background rejection. As we have shown here,
verification and calibration of this capability with satellite-based lasers
will be an important component of this program, as will considerations of the
telescope optical performance and trigger system design.

This research is supported by grants from the U.S. Department of Energy Office
of Science, the U.S. National Science Foundation and the Smithsonian
Institution, by NSERC in Canada, and by the Helmholtz Association in Germany.
This research used resources provided by the Open Science Grid, which is
supported by the National Science Foundation and the U.S. Department of
Energy’s Office of Science, and resources of the National Energy Research
Scientific Computing Center (NERSC), a U.S. Department of Energy Office of
Science User Facility operated under Contract No. DE-AC02-05CH11231. We thank
the Breakthrough Prize Foundation and the University of California, Berkeley,
for their support. We also acknowledge the excellent work of the technical
support staff at the Fred Lawrence Whipple Observatory and at the
collaborating institutions in the construction and operation of the
instrument. Services: Celestrak, NASA Exoplanet Archive, TeVCat, the SIMBAD
database, the ViZieR catalog access tool

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