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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

Canada.

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

ground level.

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

way.

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

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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

GeminiFocus

Issue 38 - June 2009  

GeminiFocus