When he began consulting for the twin Gemini
Gemini telescope tips in elevation, the gravity vectors on
telescopes in 1994, the observatory was already
all of the telescope’s instruments change,” he remarked.
developing an ambitious LGS system for Altair on
“There was a lot of painstaking work.”
the Frederick C. Gillett Gemini North telescope. As a consultant, Ellerbroek’s role was to analyze various
Unfortunately, not every one of the initial Altair analyses
aspects of Altair to help the Gemini team make key
proved correct. One of the key design questions facing
design and construction decisions.
the instrument team involved understanding at what height to apply the corrections applied by the deformable
“From the beginning it was understood that Gemini
mirror. We now know that on average, the atmospheric
would want to have a laser-guide-star-operated AO
turbulence at Mauna Kea experiences is greatest closest
system,” said Ellerbroek. “And Altair was built with
to the ground.
that in mind.” “At the time it was thought that a dominant turbulent In August 1999, Brent Ellerbroek joined the team full
layer at Mauna Kea was at roughly six kilometers altitude.
time as the adaptive optics program manager. At that
And on that basis, Altair had a rather innovative design
point, the Altair design decisions were largely complete
with a deformable mirror conjugated to that altitude
and the instrument was under construction at the
instead of the ground level. That was an example of
Herzberg Institute of Astrophysics (HIA) near Victoria,
a fairly fundamental decision, which probably wasn’t
backed up adequately with the data available,” said Ellerbroek. “In retrospect, it would have been better if
“I was effectively the technical contract monitor for the
the Mauna Kea community in general had come up with
work being done at HIA,” said Ellerbroek. “They were
a better characterization of atmospheric turbulence.”
already under way in doing a good job. The basic concept
The problem has since been corrected with a field lens
had been developed, and at that point I was essentially
that images Altair’s deformable mirror back down to the
monitoring their work, making sure Gemini would be
ready to accept the instrument when it arrived.” When asked what he’s most proud of from his time at Construction went well under Ellerbroek’s watch. The
Gemini, Ellerbroek is quick to point to an instrument
instrument arrived at Gemini North in October 2002
that’s still nearing completion: the Gemini South Multi-
and was mounted onto the telescope less than a month
conjugate Adaptive Optics (MCAO) system.
later. The first half of 2003 was spent commissioning Altair, and Ellerbroek was heavily involved. But as with
If bigger is better when it comes to telescopes, the same
any instrument integration, there were bumps along the
is true for AO systems. A traditional LGS system lets
astronomers look at more places in the sky than a NGS system does, but the field of view is still limited to the
“It did take some time to tune up,” recalled Ellerbroek.
region of space immediately adjacent to the artificial
“Altair has many background loops that tune the control
(or natural) laser guide star. MCAO systems increase
algorithms to adjust to changing atmospheric conditions.
that available field of view by using more lasers and
And it took a while to wring out the issues there.” An
more deformable mirrors to estimate and correct the full
unexpected delay also occurred when the diameter of
three-dimensional structure of atmospheric turbulence.
a seemingly simple “pinhole” had to be changed for
In other words, more atmosphere is analyzed so more
optimal operation of the wavefront sensor, requiring the
can be subtracted away.
unit to be partially disassembled and then rebuilt. “The thing I remember most is the work I did in
Finally, calibrating the flexure between Altair and the
transitioning the MCAO project from its original stage as
other instruments on Gemini proved to be one of the
a feasibility study, which was done before I arrived, to a
more time-consuming challenges. Every possible tilt
preliminary design-level concept, which was producible
configuration required its own set of deformable mirror
enough that it could be carried forward, built, and be
commands to cancel the optical misalignments due to
implemented in the near future,” Ellerbroek recalled.
flexure at the azimuth and elevation angles. “As the
“Had I not been involved with Gemini, Altair would