Figure 5.
GPI (center) installed
on the up-looking
port of Gemini South.
FLAMINGOS-2 is at
top, and GMOS-S is
at bottom.
GPI and Telescope Vibration
In late 2013, early commissioning tests
of the Gemini Planet Imager (GPI) on the
Gemini South telescope (Figure 5) revealed
a strong oscillation in the corrected wavefront, similar to defocus. The 60 Hz oscillation frequency pointed to the GPI Stirling
cycle cryocoolers (which run at 60 Hz) as the
cause. But we did not understand the mechanism that disturbed the optical wavefront.
After fitting the telescope optics with accelerometers, a team of Gemini scientists and
engineers detected the oscillations in the
primary mirror (M1). The center of M1 was
Figure 6.
Close-up of GPI
attached to the
instrument support
cube (white, at top).
Vibrations from the
instrument’s cold heads
coupled very efficiently
into the cube and then
into the mirror cell.
18
GeminiFocus
vibrating relative to the outer edge with a
peak-to-peak amplitude of 840 nanometers
(nm) — sufficient to cause a focus-like shift
of about 1 millimeter at the GPI focus. The
vibration completely disappeared when we
turned off the GPI cryocoolers.
To improve the delivered wavefront, the GPI
team first developed a software filter to measure the 60 Hz focus oscillations. They then
applied a correction signal to GPI’s adaptive
optics. The filter improved GPI’s performance
to a satisfactory level, but the 60 Hz vibrations remained in M1, potentially affecting
other science instruments. Therefore, in mid2015, Gemini upgraded the GPI cryocooler
controller to a new model
— one with an active damping system that measures the
cryocooler’s acceleration and
applies a counteracting one to
dampen the vibrations at their
source (Figure 6). Measurements with GPI indicate that
the new system reduces the 60
Hz defocus residual wavefront
errors from about 50 nm rootmean-square (rms) to as low
as 1 nm rms — a factor of 50
reduction!
April 2016