Too much current
MIRI has four different observing modes, and Navarro’s team was responsible, among other things, for designing the medium-resolution spectroscopy mode for this instrument. His team also has the optical design and roast made. They are rods that unravel light in all colors. These grates sit on a mechanism, actually a motorized wheel that can turn. There are two of these rotation mechanisms in MIRI, which come from a foreign consortium partner. According to Navarro, there was a ‘hiccup’. They saw something strange; one of the two rotation mechanisms used more power than the other. It could be a warning that you wanted to break up. Everything then instantly stopped and this MRS mode could no longer be used.
“On July 12, James Webb was transferred to all scientists, and on August 24, our piece that we contributed to was stopped. They saw something strange there, and then the ‘mode suspended’ was immediately set. So everything continued to work, but not this MRS mode anymore. They then conjured up all the data that was there for two months about the materials, the suppliers and all the test data and investigated the reason. Normally when you test you look for , prove why it would be good . In this case, however, they went through the raw data to prove if they had seen anything so strange before. This was done very rigorously. We know which screw was tightened 15 years ago, with how much force, as a person there and who signed the signature for verification. It comes in handy at times like this. In the end it was decided to do something different for the electronic control. Instead of the wheel a rig screws it up, it now flips back and forth. Everything has been operational ig one since November 2.”
A far less worrisome hiccup was seen when the temperature of the various instruments was lowered. Once the sun shield was exposed, the telescope cooled to about 40 Kelvin, or -233 degrees Celsius. For about a month, the instruments were kept warm to get rid of all the water vapor. After a month, these heaters were turned off and the temperature of most instruments dropped to around 40 Kelvin, as can be seen in the image below.
Only MIRI’s temperature drop lagged behind. It was according to plan, because this instrument is extra thermally insulated. MIRI is the only instrument that also has active cooling on board, actually an additional cryocooler because this instrument must be colder than 7 Kelvin before it can be turned on. This active cooling works with five stages and has only been switched on after 90 days, which can be seen by the sharp drop in temperature from the red dot. However, a hiccup can be seen just after 100 days. “I didn’t expect that,” Navarro says. “I was disappointed here: it can’t be true, can it? We’ve been working on this project for years, and now it wouldn’t get cold enough to turn on, but after turning on the fifth cooling stage, it turned into order.”
The cryocooler is visible here after completing a test phase. On the right is the Cryocooler Compressor Assembly, the primary component of the cryocooler that compresses and precools the helium gas before it is pumped around.
By far the most important issue that has actually caused irreversible effects is a micrometeorite impact that occurred in May. One of the eighteen mirror segments, which is completely unprotected, was hit. We’re talking about particles the size of a grain of sand, but because of the speeds of tens of kilometers per second, these tiny particles have an energy that Navarro describes as: ‘like being hit by a city bus’. Effects of these kinds of particles, which pass right through the large solar shield, were already taken into account beforehand. It is inevitable. It creates small holes which gradually reduce the effect of the shield and the whole telescope will also work worse.
This scenario is taken into account with an increasingly ‘perforated’ sun shield, but it was not entirely expected that there would be a big impact so quickly at the mirrors. This time it was a relatively large particle. Impacts by these kinds of larger particles should really only happen once every five or ten years, but they happened after only a few months.
“It wasn’t nice,” says Navarro. “The impact tilted the mirror segment and created a small crater with a deformation in the area around it. The crater is so small that we cannot see it directly. The amount of light we receive has not changed, but the shape has actually changed .After correction with the motors at the rear of the segment this is reduced to 150nm On the whole mirror this has a contribution of +9nm out of a total deviation of 80nm The impact has an effect because it is less perfect than before the impact but the other mirror segments manage say better than hoped so it is not that bad Even with the performance of this affected segment the telescope is within the specifications also because it remains very stable and does not change over time. of impact would take the telescope beyond the 150nm specification, and the images would be really blurry. James Webb flies through our solar system at a speed of about 30 km per second. To reduce future effects, the telescope now rarely looks in the direction of flight.”
The deformation of this mirror segment will mainly affect the high contrast measurements. It’s about how big the contrast can be between the stars and the planets, and how big a planet has to be before you can observe it. When you take a normal picture you hardly notice the distortion, but according to Navarro it is different with high contrast measurements. “Then you sort of pull pictures apart. For example, a picture is taken of a comparable star without a planet. You pull the two apart, and if there’s a difference between them, it’s blown up. At the moment, observing we planets that are ten thousand times fainter than their parent star.”