TID: FMail-lnx64 2.1.0.18-B20170815   
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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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