1. We present new post-processing routines which are used to detect
very fast optical and near-infrared pulsed signals using the latest
NIROSETI (Near-Infrared Optical Search for Extraterrestrial
Intelligence) instrument. NIROSETI was commissioned in 2015 at Lick
Observatory (Fig.1) and searches for near-infrared (0.95µm to 1.65µm)
nanosecond pulsed laser signals transmitted by distant civilizations.
Traditional optical SETI searches rely on analysis of coincidences that
occur between multiple detectors at a fixed time resolution. We
present a multi-time resolution data analysis that extends our search
from the 1ns to 1ms range. This new feature greatly improves the
versatility of the instrument and its search parameters for near-
infrared SETI.
We aim to use these algorithms to assist us in our search for signals
that have varying duty cycles and pulse widths. We tested the fidelity
and robustness of our algorithms using both synthetic embedded
pulsed signals, as well as data from a near-infrared pulsed laser
installed on the instrument. Applications of NIROSETI are widespread
in time domain astrophysics, especially for high time resolution
transients, and astronomical objects that emit short-duration high-
energy pulses such as pulsars.
The NIROSETI instrument shown below (Fig.2) was designed to lower
the number of false positive signals, which are generated by noise in
the Avalanche Photodiodes (APDS), electronics, and spurious signals.
The captured light is divided into two photon streams, delivering each
photon stream to a separate near-Infrared detector. The two detectors
are placed at equal lengths from the beam splitter so that only photons,
arriving simultaneously trigger an alarm. Waveforms are recorded using
a 4GHz Agilent Oscilloscope with a time resolution of 0.25 ns. The
instrument also contains a near-Infrared laser pulsating with a duty
cycle of 4.6 MHz and 10ns width used to calibrate the instrument.
The primary goal of the NIROSETI program is to search for near-
infrared pulsed laser signals (< nanosecond) or transient signals that
may originate from an extraterrestrial intelligence origin. NIROSETI
can also characterize transients, variable stars and pulsar emissions
with unequaled time precision in the near-infrared.
The data we analyze is gathered from an ongoing NIROSETI
campaign, which includes 1288 FGKM stars within 50 parsec from
the Earth.These targets were selected to optimize the sensitivity of
our instrument and because of the possibility for containing
exoplanets.
1. Laurie Hatch, http://www.lauriehatch.com/
2. Jérôme Maire, Shelley A. Wright, Dan Werthimer, Richard R. Treffers, Geoffrey W. Marcy, Remington P. S.
Stone, Frank Drake, Andrew Siemion, “A near-infrared SETI experiment: probability distribution of false
coincidences”, Proceedings of the SPIE, Volume 9147, id. 91474K 11 pp. (2014)
References
Abstract
Below we plot the calculated signal to noise ratio vs. boxcar length
(Fig.6). We find that the optimum boxcar length for a 10ns pulse-
width signal emitted at 4.6Mhz is 43 * (.25ns), which is
approximately 10ns.
Instrument Overview
The prime challenge of this program is developing analysis tools
that can detect pulse signals at varying pulse widths, amplitudes,
and periodicities. To address this, we have started investigating
analysis tools for digital signal processing using smoothing techniques
to identify whether we can detect varying pulse signal properties.
Boxcar smoothing is a technique used with time series data to smooth
out short-term fluctuations and highlight longer-term trends or cycles.
The technique involves averaging over a fixed boxcar length, starting
with the first element, and then shifting the boxcar forward by one
element each time. This process is then repeated over the entire data
series. We begin by testing our methods using the calibration laser
attached to the instrument.
Which boxcar length should we use to optimize our signal to noise
ratio and minimize the number of false positives detected in our
signal? To address this question we first generate a synthetic signal
that mimics the amplitude (~6mV) and pulse width (10ns) of the
calibration laser. We add noise taken from our darks to the synthetic
signal. Next, we smooth the synthetic signal using the same boxcar
length we used for the calibration laser.
Digital Signal Processing Methods
Digital Signal Processing Results
Fig.1 Home of
NIROSETI, Lick
Observatory
Picture credit:
Laurie Hatch
Photography
Fig.2 NIROSETI instrument setup
Picture credit: Laurie Hatch Photography
Scientific Goals
We evaluate for the signal to noise ratio by dividing the number of
true positives by the number of false positives.
Our analysis shows that boxcar smoothing is great at minimizing the
number of false positives detected in our signal, even at various time
resolutions. To continue testing our algorithms, we plan on boxcar
smoothing signals of various pulse widths and amplitudes by adjusting
the settings on the calibration laser. To further reduce the number of
false positives, we plan on advanc these digital signal processing
methods using varying boxcar smoothing widths on both detectors in
coincidence.
Conclusion & Future Work
phd[synth sig (4mV)]
phd[laser sig (4mV) ]- phd[synth sig (4mV)]
# true positives
# false positives
SNR = =
Fig.4 Measured waveforms (top and middle) and simulated
ones (bottom) after smoothing with a boxcar length 3 and 43.
Fig.6 SNR vs. boxcar
Fig. 5 pulse height distributions for boxcar length 3 (left) and
boxcar 43 (right)
Telescope
Opening
Source
Unit
Dichroic
Optical Guider
Camera
near-Infrared
detectors 1 & 2
50/50 Beam
Splitter
A near-Infrared SETI Experiment:
A multi-time resolution data analysis
Melisa Tallis1, Jerome Maire2, Shelley A. Wright1, Andres Duenas1, Frank Drake3,Geoffrey Marcy4, Andrew Siemion4, Remington Stone5,
Richard Treffers6, Dan Werthimer4
1. University of California, San Diego, 2. Dunlap Institute for Astronomy and Astrophysics, 3. SETI Institute; 4. University of California,
Berkeley, 5. Lick Observatory, University of California, Santa Cruz, 6. Starman System, LLC
Notice that the pulsed signal is visually stronger and has reduced noise
(Fig.4) when smoothed with boxcar length 43.
Next, we plot together the pulse-height distributions of both the
smoothed laser and synthetic signals (Fig.5). To calculate their pulse
height distributions, we count the number of pulses detected above
various threshold voltages.
laser signal
synthetic signal
dark signal
laser signal
synthetic signal
noise signal
laser signal
synthetic signal
noise signal
1. Shelley A. Wright, Dan Werthimer, Richard R. Treffers, Jérôme Maire, Geoffrey W. Marcy,Remington P.S.
Stone, Frank Drake, Elliot Meyer, Patrick Dorval, Andrew Siemion, “A near-infrared SETI experiment:
instrument overview”, Proceedings of the SPIE, Volume 9147, id. 91470J 10 pp. (2014)
2.3.
Fig.3 Boxcar length of 3