with Carl Biagetti
A nearly three-month gap passed between the servicing mission and the first new science from the Hubble Space Telescope. During that period, intensive testing and a myriad of adjustments took place as engineers slowly reactived the telescope. Carl Biagetti at the Space Telescope Science Institute led the Servicing Mission Observatory Verification (SMOV) team, responsible for slowly and carefully restoring the telescope to its scientific capabilities.
What is Servicing Mission Observatory Verification?
It's the program to re-commission Hubble after the astronauts have serviced the telescope. When the astronauts — great and heroic as they are — finish working on the telescope, it's not ready to do astronomy. Re-commissioning means doing whatever we need to and making whatever adjustments we need to make in order to start using the telescope for astronomy.
What is it that happens during the SMOV? The astronauts have gone up there and installed these new things — now what?
There are two basic aspects. One is the observatory as a spacecraft needs to reestablish its own pointing control rather than being attached to the shuttle. We have to demonstrate that the Hubble as a spacecraft is able to control its attitude — the direction it's pointing to the sky.
The other aspect is that the new instruments, which are the most important part of any servicing mission, are, for the first time, suddenly in a vacuum and weightless.
Being finely tuned instruments, weightlessness means they may be slightly out of alignment and focus. So we go through a process, once they're ready, of aligning and focusing. Each instrument has to go through that process separately. It takes quite awhile — on the order of a couple of weeks for each instrument.
They also need to "outgas" — all the extraneous molecules that are inside them from being built on the Earth and brought up by Earthlings need to dissipate before we can use them, before we can cool them down or turn on any high voltages, and that takes awhile.
What kind of molecules are we talking about?
I would characterize them as organic molecules. They could be simple atmospheric molecules, like oxygen and nitrogen. But there are also a lot of organic molecules from humans building the instruments and bringing them up into orbit, and also just from the materials the instruments are made of. Instruments are "baked out" — heated up on the ground so a lot of these molecules get pushed out of the instrument anyway. But it's not 100 percent efficient.
What will those molecules do if you start things up before they're gone?
There are two basic concerns. The molecules can interfere with the optics at certain wavelengths. Molecules can absorb certain wavelengths and they can sometimes even get what we call "polymerized" on the optical surface — it gets attached to the optics and that spot on the optics will block UV wavelengths being transmitted to the telescope.
And they can interfere with the electronics at high voltages. If it interferes with the electronics, it can cause a short circuit, and you can ruin the instrument. So it's pretty serious.
What happens once the molecules are gone and the instruments are adjusted?
Once those milestones are achieved — the outgassing is complete, the harmful molecules are gone and the instruments are aligned in a weightless state, then we do calibrations by looking at actual, familiar astronomical objects, in order for the astronomers to know how to precisely interpret the data the instruments are giving them.
What's the difference between alignment and calibration?
Alignment refers just to the optics, the path light takes from the telescope into the instrument, and onto the detecting surface of the instrument. Calibration is something that allows you to understand what the instrument is telling you about the sky, and not about itself.
If the instrument is saying that this pixel has so many electrons in it, the astronomer can interpret that to mean a certain brightness about the star or astronomical object that we're looking at. You need to be able to convert that data into information about the sky, and remove any information that's not about the sky.
What would an instrument be "telling you about itself" with these data?
It can tell you that some of the electrons inside a pixel are there naturally, even if the shutter were closed and you weren't looking at anything. So you aren't interested in those electrons, because they're not part of astronomical target — they're inherent to the instrument. You want to remove them from your scientific data.
How do you do that? Do you adjust the instrument to remove them, or do you ignore them when the data comes back?
Some involve adjustments — like tweaking a voltage here and there. For the most part, it involves knowing how to remove those "instrument signatures," as we call them, on the ground after we've gotten the data. So what an astronomer gets is data that is as close as possible to representing what was observed in the sky. The astronomer can get the raw data if he or she chooses, but the idea is to filter out as much as possible so that the data just represents the sky.
How common are instrument signatures? Is it a big part of the work?
It's a big part of SMOV to collect this data and understand it. In SMOV, in a lot of cases you're seeing a lot of things for the first time and it's a big effort to understand it and separate out instrument signatures from the scientific signal. All instruments have it, and it always has to be done. So it's just a matter of understanding it, so at a certain point we're comfortable scheduling normal science operations.
What's the progression of the SMOV activities? Do you have to do everything separately, or can you do some of these tasks at once?
We have an elaborate plan that we work on a couple years before the mission to determine what the sequence is and whether there are any sequences that can be done in parallel. So if one sequence goes more slowly than expected, we can continue on the other. Once we're at a certain stage — an early milestone — all the sequences for each of the instruments can run independent of one another, and that's by design. We don't want to have one instrument dependent on the commissioning of another. So if one instrument has problems or is slower than expected, it doesn't slow anything else up. We work hard on designing that type of plan.
How does that type of plan help you?
What that allows us to do is start doing useful science on an instrument by instrument basis, or even a channel by channel basis inside the instrument. That's important because you're making use of the observatory efficiently — because observatory lifetime, from a scientific sense, is a precious commodity. Whenever you're not doing a SMOV or calibration activity, you want to be doing science.
How long will the SMOV and calibration activities take?
We started on May 19th, as soon as the astronauts released the telescope from Atlantis. And we expect to go right into early September — so a little over three months of commissioning. Some of the science will start well before that period ends because of the things we just talked about, but it's going to take a good three months to finish all the activities. But the same way we identify places where science can start, we identify places where you can do the Early Release Observations, so they can get done in a timely fashion. Everybody is interested in them.
Is there anything about the Servicing Mission 4 SMOV that differs from previous missions?
This is the first time we've installed two instruments and repaired two old ones, so it's the quantity of the work in this SMOV commissioning that makes it different from all the rest. There are a lot more real-time activities because of that.
What was it like for you to watch the mission?
You work a long time designing a plan for carrying out a SMOV commissioning but it's all Powerpoint and Excel files and Word files. When you see the astronauts installing an instrument or taking out an old one, it starts to feel real to you for the first time.
Do you watch with an eye for things that might affect SMOV down the road?
You do, but it's really hard to extrapolate what might be happening onboard. (Laughs) You're just aware of astronauts gingerly handling these instruments. When (astronaut John) Grunsfeld bumped into the high-gain antenna that was a tense moment, but that turned out to be nothing.
Do you think there'll be any surprises as this moves along?
There are always surprises. But based on the functional tests that were performed during the servicing mission, we feel pretty good about how the instruments are going to look once we get them fired up.