Figure 2.
Speckle image
reconstruction of Pluto
and Charon obtained
in visible light at 692
nanometers (red) with
the Gemini North
8-meter telescope using
the Differential Speckle
Survey Instrument (DSSI).
Resolution of the image is
about 20 milliarcseconds
average. This is the first
speckle reconstructed
image for Pluto and
Charon from which
astronomers obtained not
only the separation and
position angle for Charon,
but also the diameters of
the two bodies. North is
up, east is to the left, and
the image section shown
here is 1.39 arcseconds
across.
Credit: Gemini
Observatory/NSF/NASA/
AURA
14
objects that can
cause a star’s light
to dim (speckle can-
not see planets).
This is achieved by
employing statis-
tical
techniques
to assess whether
the observed dim-
ming is likely to be
a true transit by an
orbiting planet or a
“false positive.” Us-
ing this technique,
the DSSI observa-
tions at Gemini
North in 2012 helped confirm over a dozen
planet candidates, including the five-planet
system Kepler-67; DSSI would eventually
provide more than 2,100 observations of Ke-
pler planet candidate host stars.
Based on the success of DSSI, and the need
to validate and characterize the 4,000 exo-
planet candidates discovered to
date by NASA’s Kepler/K2 Space
Telescope and the Transiting Exo-
planet Survey Satellite (TESS), How-
ell initiated the design of two new
speckle instruments: ‘Alopeke and
Zorro, which our team went on
to build at NASA Ames Research
Center. The twin instruments
each use two electron-multiply-
ing CCDs and combinations of
narrow-band (40- to 50-namom-
eter-wide) filters to provide simultaneous
two-color diffraction-limited photometric
and astrometric information at optical wave-
lengths.
Each instrument can also identify back-
ground objects and companion stars — to
within < 0.1 to 1.2 arcseconds of, and up to
10 magnitudes fainter than, the exoplanet’s
host star — that can contaminate exoplanet
transit detections. For any detected com-
GeminiFocus
panion, speckle imaging provides the po-
sition and separation from the host star, as
well as color and contrast information that
greatly reduces the likelihood of false posi-
tives and improves the estimates of the exo-
planet size.
Zorro and ‘Alopeke:
Specifics for Users
‘Alopeke and Zorro add great new capa-
bilities, and having identical instruments
on both Gemini telescopes allows collect-
ing homogeneous datasets over the whole
sky. The speckle mode provides diffraction-
limited (0.016 arcsecond Full-Width at Half-
Maximum at 500 nm and 0.025" at 800 nm)
resolution imaging at optical wavelengths
over a narrow field of view (~6 arcseconds).
The wide-field mode provides high-sensitiv-
ity natural-seeing imaging with virtually no
readout delay in the standard Sloan broad-
band filters over a moderate field of view
(~60 arcseconds).
Both instruments are considered
"permanent resident" visiting
instruments, meaning they are
available throughout the semes-
ters for regular queue and Fast
Turnaround proposals. This makes
them great for programs that
need simultaneous photometry
in two filters, variability studies,
and rapid events like occultations,
which also benefit from the flexibility of
Gemini’s queue scheduling.
Differential Speckle Imaging
at Gemini
Some Science Highlights
Speckle imaging at Gemini Observatory is a
forefront technology allowing researchers to
push the limits of high-resolution imaging (Fig-
ure 2). The following science references pro-
January 2020 / 2019 Year in Review