the recent testing success. Figure 3 hints at
some of GHOST’s capabilities.
The Cassegrain Acquisition Unit, also de-
signed and built by the AAO, was previously
shipped to Chile from Australia and tested in
advance of the upcoming arrival of the spec-
trograph. After the spectrograph, slit viewer,
and optical cable arrive in Chile, we expect
to have all sub-assemblies of the GHOST in-
strument fully integrated and functioning in
the second quarter of 2020 in preparation for
commissioning.
First Light with NGS2
The Canopus adaptive optics (AO) bench of
the Gemini Multi-conjugate adaptive optics
System at Gemini South recently received a
significant upgrade: its new Natural Guide
Star Wavefront Sensor, also known as the
Natural Guide Star Next Generation Sensor
(NGS2). The original system consisted of three
moving probes to pick up guide stars in the
field, channel the light into fibers, and proj-
ect it onto a quad-cell for tip-tilt detection.
This system worked but in practice was cum-
bersome to use, mainly due to each probe’s
tiny field of view and the large light losses
in the system. This implied large acquisition
times and a brightness limit for the stars that
significantly restricted sky coverage.
For the above reasons, a team from the Aus-
tralian National University spearheaded an
alternative approach, making use of novel
electron-multiplying CCD technology that
allows imaging the whole field of view. Up
to three guide stars can be selected on that
image. For tip-tilt wavefront sensing on each
of the stars, small windows centered on each
star are then read out at high speed, making
use of the extreme low noise characteristics
of the electron-multiplying CCD.
required removing the AO’s three large opti-
cal components and dismantling the original
NGS system. But it all worked out, thanks to
the careful preparations made by the NGS2
team.
Commissioning took place last October.
Apart from Gemini personnel, the team had
the great pleasure to work with François
Rigaut (Australian National University) and
Benoit Neichel (Laboratory of Astrophys-
ics of Marseille) during the commissioning
nights (Figure 5, next page). Collaboration
from the weather was a weak point, serious-
ly hampering progress. However, the team
tested the full system, and put it through its
paces.
The first results have been very positive.
AO performance under reasonable weather
conditions achieved an image quality of 83
milliarcseconds, indicating that the fully in-
tegrated system worked well (Figure 6). Ac-
quisition of the three natural guide stars was
Figure 3.
Blue and red GHOST
images of a mercury
lamp, with the spectral
orders labeled and
1.1 x free spectral
range in each order
highlighted. Continuous
wavelength coverage
from 359 nm to well
beyond 1 micron
(Requirement: 363 -
950 nm). Significant
wavelength overlap
between orders (with
overlapping orders
between arms).
Credit: Greg Burley
Figure 4.
The new NGS2 unit after
installation into the
Canopus optical bench.
The new NGS2 was incorporated into the
Canopus optical bench last September (Fig-
ure 4). This was no trivial exercise, because it
January 2020
GeminiFocus
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