James Webb Telescope Deployment Timeline

[peter scott]

https://www.jwst.nasa.gov/content/webbLa...sWebb.html

[Doodlebug]

That's a fine link. Thanks.
I wonder what back up systems are in place
(yet more weight) ?

[Mike Watterson]

There is a link to switch to Metric.
Curiously labelled
English <> Metric

[Doodlebug]

There is a video on that web site that says that the telescope has a means
of shaking itself if some deployment get stuck. To free gears/motors etc etc.
That's a wonderful idea that we all do on the bench at times in exasperation. :-)

[Doodlebug]

Latest details....    

[Craig]

Miles? Miles per hour, Fahrenheit?

At least the NASA site works in sensible units https://webb.nasa.gov/content/about/orbit.html

But even they quote the temperature as -233C. It is better to convert this to Kelvin, the temperature above absolute zero. The is 40K, which is pretty cold - liquid nitrogen boils at 77K, so the detector modules are 37K colder than liquid nitrogen.

So how do they maintain that sort of temperature? Well the business end points at deep space, which is at 3K - so provided you can shield the heat coming from the sun (and earth and moon) you get passive cooling for free.

Craig

[Nick]

There is a button on the home page that switches to metric, i.e. km & ℃ (not K ).

Interesting about mirrors A3 and A6 - their position sensors were broken before launch but couldn't be replaced in time, so they are handled in a different way.

If you look at the status page at https://www.jwst.nasa.gov/content/webbLa...sWebb.html, you will notice that A3 & A6 are showing as still effectively stowed. An explanation:

See 2018 SPIE paper by Wolf et al. "JWST mirror and actuator performance at cryo-vacuum" (find it by searching on "JWST A3 and A6 position sensors").

See sec 1.2 and 2.4.2 of the paper.

It turns out that the mirror segments at A3 and A6 have "faulty" linear variable differential transformers (LVDT) (position sensors). Each LVDT is suppose to have two coils to cancel out differential thermal effects. In each of A3 and A6, one of the two coils is faulty. The engineers had to work out a way of getting accurate readouts using only one coil in each LVDT. So they developed a different readout procedure that could use only the one good coil in each LVDT. I do not know why it was impossible to replace the faulty LVDTs before launch.

One thing that caught my eye is that all 132 actuators are capable of sub-10 nano-meter (nm) accuracy over a 20mm range BOTH AT AMBIENT AND CRYOGENIC CONDITIONS (they were all tested at 40K).

That is an insane level of precision over an equally insane temperature range (40K - ≈300K).

[Doodlebug]

BTW the speed of telescope is shown in Miles per second, not MPH.

The mirror actuators are tiny motors.

As there is a shaking mechanism if something mechanical 'sticks', I wonder if
they have a reverse/over voltage to engage the motors if they jam?