to conduct our first on-sky propagation at
zenith on August 31st. This was conducted
remotely from the Hilo Base Facility. We
continue to prepare for commissioning.
GMOS-S Bubbles Eliminated!
A long-standing problem affecting the
Gemini Multi-Object Spectrograph (GMOS)
performance has been resolved recently. In
GMOS a special optical oil is used between
the different lenses to minimize interface
surface effects, particularly loss of through-
put by partial reflection, and degradation of
image quality. Over time, minuscule leaks
cause bubbles to appear in the interfaces
between the lenses. Many of these bubbles
can be filled again with small amounts of
optical oil, as has been done on both GMOS
North and South on several occasions. How-
ever, the lenses in the collimator assembly
are embedded within the instrument, not
allowing access to the filling ports — un-
less the instrument is disassembled, some-
thing which had never been done before.
Yet this was the task before us.
study and characterization of the problem,
the team spent a lot of time rehearsing the
alignment techniques, until they felt confi-
dent enough to dismantle GMOS.
The mask mechanism, on-axis wavefront
sensor probe, and the collimator were then
taken out of the instrument. With the colli-
mator now on the bench, the first step was to
modify the system to allow for future filling
without taking the instrument apart again.
After that, we used a special set-up (com-
bining a small vacuum pump to extract the
air, and a filling system to inject new optical
oil) to fill the bubbles. We then reassembled
the instrument, confirming at every step the
alignment and mechanical repeatability.
The results as measured with the detector
all fell well within specifications (the goal
was to be within a 10 pixel difference, and
there was a 4-pixel difference with respect
To access the filling ports, a complete disas-
sembly was required. We first designed and
built an opto-mechanical alignment set-up,
with a combination of lasers and detector
read-out and alignment telescopes used to
reference the collimator in its original posi-
tion. After several months and a thorough
January 2019 / 2018 Year in Review
Figure 7 (above left).
Gemini Optical Engineer
Tom Schneider prepares
insulation for GNEST (laser
housing designed for our
TOPTICA laser head and
optical bench).
Figure 8 (above center).
Gemini senior optical
technician Clayton Ah
Hee (left, in foreground)
and Gemini summit
crew member John
Randrup prepare GNEST
for installation onto
telescope.
Figure 9 (above right).
Verifying laser alignment
through the optical path
to the Beam Transfer
Optics Optical Bench on
the secondary.
Figure 10.
Installing GNEST onto
telescope truss below
top ring.
Credit: All photos this
page by Jeff Donahue
GeminiFocus
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