Mirror, Mirror…On Its Way!
James Webb Space Telescope
NASA Mission Update
Jan. 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
14 responses to “Mirror, Mirror…On Its Way!”
Leave a Reply
You must be logged in to post a comment.
Complicated.
And all these motors have to keep working, without hope of repair, for over a decade. Dang. Well, here’s hoping! There wasn’t much talk about this part, so I guess the engineers are quite confident in the motors
This was one of the things that was given a ‘wink and a nudge’ about in the early days of the program. The assumption was that the military had perfected this technique and so the funding went forward. I doubt that the military has actually done it at all because the amateur satellite observers community would have imaged such a large satellite by now. So yes, you’ve touched on the high end system that has to work in remote and ultra harsh conditions over very long periods of time. Give it 10 years and we should start hearing of ultra large diameter imaging satellites. Because now that NASA has all this working, the military is going to start using the technology pioneered here.
quite sure they have done this in geo.
GEO is the easiest orbit to spot something like this. I want to believe the spooks have been doing this. I’m even ready to believe that they have and would not share the technology with NASA and made NASA figure it all by their lonesome again. But I think these large satellites would have been detected and reported over the past 20 years.
So far as I know, all the Earth-pointing imaging spooksats operate in LEO and use monolithic mirrors. One doesn’t see Earth-imaging spooksats with huge, segmented mirrors in GEO because an imaging spooksat wants to be as close as possible to what it’s trying to image. If there are any imaging spooksats in or near GEO, they are probably also built with monolithic mirrors because they are probably “situational awareness” birds sent to look at other things in or near GEO and not at things on Earth.
Bottom line? You’re almost certainly right. The military/NRO never developed JWST-style segmented mirror technology because there is no need for such in terms of their taskings.
there are some awefuly bright vehicles up in Geo. some of them are large 100 meter antennas and some not. Its hard for amateurs at that distance to have the resolution to see actual parts of the vehicle. particularly when the MISTY experiements were pretty good at working through the disguis part of the effort.
what they look at in GEO are large real time events…the more detailed stuff they save for the Low orbit where they dont need them and in fact cannot use them dues to smear rate.
they are quite different environments however . Webb needs to be very cold…as you know
So a 6m mirror at GEO would give a diffraction resolution of about 400mm or about 1.5 feet. You’d want a huge image sensor to take in a large FOV boy the data rate would be through the roof, and as such the power requirements. Mirror would be big, solar panels would be big antenna would be big if it used radio. Laser might be the way to go.
they have high speed laser links on those birds. all I know is rumors. I think that they are looking both at EArth and away ie at satellites in super synch and other orbits.
I watched a launch-related webcast where one of the scientists (or it could have been the one engineer interviewed, I forget) went on a bit about how innovative these actuators are. Apparently the temperatures involved are one of the big challenges.
The basic idea though seems somewhat similar to adaptive optics, was my impression. I don’t mean similar in terms of changing the mirror shape to compensate for atmospheric seeing of course (what with there being no atmosphere out at L2), but just in terms of the basic idea of being able to change, as needed and in real time, a mirror’s shape during operations.
I guess another way to think about it is extremely short distance (from adjacent segment to adjacent segment) optical (IR) interferometry — making minor adjustments to the various mirrors to get them to act as one.
But there’s two systems at work here: the actuator system for changing the curvature of the primary, and then the actuators for changing the positions [tilts?] of the primary and secondary mirror segments. Oh man, a bit involved.
Any illuminating thoughts/info/other on all that?
I think you have a good understanding of what’s going on. Terrestrial large diameter mirrors are actively supported by push-pull actuators integrated into the mirror cell.
Maybe. But doing so would run decidedly counter to the plans for milsats of all kinds to be smaller, far cheaper and deployed in far larger quantities than traditional milsats and spooksats of whatever type. I think the military is less interested in the ability to read the agate type on an insurance contract from one or a very few places in orbit than it is in having, say, 1-meter imagery available of the entire planet in real-time.
Yeah, I would agree on that. There’s much more to be gained by watching vehicles move real time. If you can account for every tank, truck, APC, SAM battery, and road mobile ICBM etc, that’s far more important than measuring diameters of missile tubes.
they have to keep moving?
Yeah.
From NASA’s website:
“Once in space, getting these mirrors to focus correctly on faraway galaxies is another challenge entirely. Actuators, or tiny mechanical motors, provide the answer to achieving a single perfect focus.
The primary and secondary mirror segments are both moved by six actuators that are attached to the back of the mirrors. The primary segment has an additional actuator at the center of the mirror that adjusts its curvature. The third mirror segment remains stationary.“
So it seems that the actuators collectively constitute a kind of fine focusing system, but being used tediously here for gross positioning/collimation. I say “tediously” because it’s going to take [virtually] cranking on the ol’ fine focus knob for about 2 weeks 😀 to get the mirrors into the basic position from which fine adjustments will be made thereafter