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* Originally in ASTRONOMY
* Crossposted in ASTRONET

Hi All,

Why does it take half a year to allign the mirrors on the James Webb Space
Telescope... Read below


Alise Fisher
Posted on January 13, 2022

With major deployments complete, Webb continues its journey to its final halo
orbit around L2. In the meantime, there are several smaller deployments in the
next couple of weeks, which constitute the beginning of a several-month phase
of aligning the telescope's optics. This week, we have started the process of
moving the mirror segments (all primary plus secondary) out of their stowed
launch positions. For more details, here is Marshall Perrin from the Space
Telescope Science Institute, home of the Webb Mission Operations Center:

"To support the movable mirrors during the ride to space, each of them has on
its back three rigid metal pegs which can nestle into matching holder sockets
in the telescope structure. Before launch, the mirrors were all positioned
with the pegs held snug in the sockets, providing extra support. (Imagine Webb
holding its mirrors tucked up close to its telescope structure, keeping them
extra safe during the vibrations and accelerations of launch.) Each mirror now
needs to be deployed out by 12.5 millimeters (about half an inch) to get the
pegs clear from the sockets. This will give the mirrors -room to roam- and let
them be readied in their starting positions for alignment.

"Getting there is going to take some patience: The computer-controlled mirror
actuators are designed for extremely small motions measured in nanometers.
Each of the mirrors can be moved with incredibly fine precision, with
adjustments as small as 10 nanometers (or about 1/10,000th of the width of a
human hair). Now we're using those same actuators instead to move over a
centimeter. So these initial deployments are by far the largest moves Webb?s
mirror actuators will ever make in space.

"And we don't do them all at once. The mirror control system is designed to
operate only one actuator at a time. That way is both simpler (in terms of the
complexity of the control electronics) and safer (since computers and sensors
can closely monitor each individual actuator as it works). Furthermore, to
limit the amount of heat put into Webb?s very cold mirrors from the actuator
motors, each actuator can only be operated for a short period at a time. Thus,
those big 12.5-millimeter moves for each segment are split up into many, many
short moves that happen one actuator at a time. Scripts sent from the Mission
Operations Center will direct this process under human supervision, slowly and
steadily moving one actuator at a time, taking turns between segments. At full
speed, it takes about a day to move all the segments by just 1 millimeter.
It's about the same speed at which grass grows!

"This may not be the most exciting period of Webb's commissioning, but that's
OK. We can take the time. During the days that we're slowly deploying the
mirrors, those mirrors are also continuing to slowly cool off as they radiate
heat away into the cold of space. The instruments are cooling, too, in a
gradual and carefully controlled manner, and Webb is also continuing to gently
coast outwards toward L2. Slow and steady does it, for all these gradual
processes that get us every day a little bit closer to our ultimate goal of
mirror alignment."

-- Marshall Perrin, deputy telescope scientist, Space Telescope Science
Institute

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