Speaking of Hubble...

Archive: March 2010

A Cold Triumph

March 26, 2010 by Massimo Stiavelli
Engineers dressed from head to toe to avoid contaminating the Webb mirror pieces with unwanted particles are reflected in the mirror segments.

Engineers dressed from head to toe to avoid contaminating the Webb mirror pieces with unwanted particles are reflected in the mirror segments.

The James Webb Space Telescope, Hubble’s successor, is an infrared telescope. Warm materials glow in the infrared, and for this reason Webb optics have to be kept cold — all the way down to 40 Kelvin (or -233C or -388 F). Unfortunately, our technology doesn’t allow us to polish mirrors while working at 40K. Thus, the conundrum for Webb was that the mirrors had to be polished at ordinary temperatures but still had to be the right shape at 40K. When temperatures change so dramatically, mirrors warp and deform.

About 10 years ago, a study took place to select the best material for the Webb mirrors. Instead of glass, we decided to use a substance called Beryllium. We found that by using computer models, we could accurately predict the ways Beryllium would deform. The plan was to polish the 18 mirror segments that make up our primary mirror at a warm temperature, but give them exactly the wrong shape that would deform into the right shape once they were brought down to 40K.

It sounds like a bold plan and, while confident in our testing, we were all a bit concerned about whether this would work well and on the first try. One can always try again — but this entails extra cost and delays, so we were hoping to avoid it.

Finally, in January 2010, the first demonstration mirror segment for Webb went through the full polishing process and was frozen to 40K at the X-ray and Cryogenic Facility at the Marshal Space Flight Center in Huntsville, Ala. We measured the segment at the end, and found it had deformed to the right shape. This is a major success for the Webb project and lets us move on to developing the rest of the primary mirror segments.

A Glimpse of the Past, a Taste of the Future

March 5, 2010 by Massimo Stiavelli
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Wide Field Camera 3's infrared view of the Hubble Ultra Deep Field

I am Massimo Stiavelli, an astrophysicist at the Space Telescope Science Institute (STScI). I am the STScI Project Scientist for Hubble’s successor, the James Webb Space Telescope. I came to the institute in 1995, and before working on Webb, I worked on all of Hubble’s cameras.

My main scientific interest is understanding the formation and evolution of galaxies and the processes occurring in the early universe. I led the team responsible for the Hubble Ultra Deep Field observations, the deepest view of the distant universe ever done in visible light. The telescope had to stare at the same spot for 27 days to make these observations, and I have continued to study this field, trying to understand as much as possible from this large investment of Hubble’s time.

Last summer, I was part of a team that used one of Hubble’s new cameras, Wide Field Camera 3, to look for infrared light in the Ultra Deep Field region. The ultraviolet light from galaxies in the early universe has been stretched by the expansion of space, transforming it into infrared light. Therefore, to see the early universe, we must look for infrared light.

The results were interesting not just because they involve the Ultra Deep Field — they also hint at what Webb’s sensitive infrared vision might detect once it is launched.

Early on, some were nervous that Webb had been optimized to study a population of objects we didn’t know for sure existed, even if they were predicted by models.

Now, thanks to Wide Field Camera 3, we can all breathe a sigh of relief. These galaxies are there and they are faint: the best possible outcome for Webb.

Wide Field Camera 3’s observations of the Ultra Deep Field beat even the most optimistic expectations. In particular, they revealed a population of galaxies existing in the first billion years of the universe. A preliminary analysis suggests that these objects are rare and dim, possibly indicating that we are beginning to see the era when galaxies first form.

This is very good news for the James Webb Space Telescope. With its large mirror  — about seven times the area of Hubble’s — and incredible infrared sensitivity, Webb will be able to study in greater detail this population of galaxies and measure their properties.