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Rescue robots are put through their paces
The GMT will have seven very large mirrors (see “Future telescopes”), each made of a Pyrex-like material with a honeycomb structure, which reduces weight while providing strength. Air at a controlled temperature will be pumped into the honeycomb, bringing
mirror. This mirror comes after the tertiary and is thin, flexible and usually a few tens of centimetres across. It changes shape between 50 and 100 times a second to compensate for the effects of the atmosphere. However, such add-on systems mean there is an extra mirror in
“We are going to depths of the universe and resolutions we have never achieved before”
beam of light it receives towards a smaller secondary mirror. This then focuses the light further and sends it on to a tertiary mirror, which directs the beam towards one of the telescope’s detectors. The biggest primary mirror that can be cast from a single block of glass today is 8.4 metres across, partly because a mirror any wider would be too heavy and difficult to manoeuvre. Also the greater depth of a larger mirror makes it almost impossible to ensure that it is the same temperature throughout. That is a problem because if different parts of the mirror are at different temperatures the image quality will degrade. The only way, then, to build a bigger telescope is to have a primary consisting of a mosaic of smaller mirrors. www.newscientist.com
the mix. That’s not ideal, because every mirror absorbs some light, reducing the amount that gets to the instruments, and also adds thermal noise of its own, affecting observations in the infrared. The GMT’s solution to this is to turn the secondary into the deformable mirror. “This means you haven’t lost any [light], because you would have to bounce light off the [secondary] mirror anyway,” says Pat TEXAS ENGINEERING EXTENSION SERVICE
the entire mirror into thermal equilibrium in 20 minutes. That’s not bad, considering that the 100-inch telescope on Mount Wilson in California, which saw first light in 1917, took a whole night for its 33-centimetre-thick primary mirror to reach a uniform temperature. The TMT’s and E-ELT’s primaries will have much smaller segments than the GMT, inspired by the success of the twin 10-metre Keck telescopes on Mauna Kea, Hawaii. Going with smaller segments has its advantages, not least that each piece is thinner and easier to manufacture. The downside is that it’s much harder to keep all the segments in perfect alignment as the telescope moves. So-called edge sensors are needed to keep track of any displacement between the segments, while large numbers of pistons, or actuators, will push or pull each segment to keep the primary mirror’s curvature precise to within a few nanometres. The other significant technology that these telescopes will be exploiting is adaptive optics (AO). The various layers of the atmosphere, which are at different temperatures or move at different speeds, can distort the light reaching the telescope. Today’s telescopes have add-on AO systems that monitor either a guide star or an artificial star created by firing a laser into the upper atmosphere. Software compares the image of the star with the expected image to work out the atmospheric aberrations, which are then corrected for in real time using a deformable
McCarthy of the Observatories of the Carnegie Institution of Washington in Pasadena, California, a member of the GMT consortium. However, secondary mirrors are large, so making them deformable presents a considerable challenge. Mindful of this, the E-ELT’s designers are sticking with the smaller quaternary mirror approach. The TMT will have a fourth mirror too, but to reduce the thermal noise the light will leave the telescope and enter a separate AO system chilled to -30 °C. Despite the challenges, each telescope team is aiming for first light by 2017 and the chance to open a new era for astronomy and cosmology. “We are going to depths of the universe and to a spatial resolution that we have never achieved before,” says Kissler-Patig. “You are likely to discover things that you absolutely hadn’t expected.” ●
Rescue robots are put to the test Search and rescue robots hunting for survivors after an earthquake will need to develop an accurate picture of their surroundings. So last month would-be rescue robots from around the world were put through their paces creating an instant map of an unknown area. In the tests, run by the US National Institute of Standards and Technology, the robots had to travel through a
simulated forest, with uneven terrain and randomly placed PVC “trees”, and map the area using their on-board sensors and software. The event, at the Disaster City training site in College Station, Texas, included tests of the robots’ ability to clamber over obstacles – in this case ramps and steps, including some with unequal gaps between them.
6 December 2008 | NewScientist | 27
mirror. This mirror comes after the tertiary and is thin, flexible and usually a few tens of centimetres across. It changes shape between 50 and 100 times a second to compensate for the effects of the atmosphere. However, such add-on systems mean there is an extra mirror in
“We are going to depths of the universe and resolutions we have never achieved before”
beam of light it receives towards a smaller secondary mirror. This then focuses the light further and sends it on to a tertiary mirror, which directs the beam towards one of the telescope’s detectors. The biggest primary mirror that can be cast from a single block of glass today is 8.4 metres across, partly because a mirror any wider would be too heavy and difficult to manoeuvre. Also the greater depth of a larger mirror makes it almost impossible to ensure that it is the same temperature throughout. That is a problem because if different parts of the mirror are at different temperatures the image quality will degrade. The only way, then, to build a bigger telescope is to have a primary consisting of a mosaic of smaller mirrors. www.newscientist.com
the mix. That’s not ideal, because every mirror absorbs some light, reducing the amount that gets to the instruments, and also adds thermal noise of its own, affecting observations in the infrared. The GMT’s solution to this is to turn the secondary into the deformable mirror. “This means you haven’t lost any [light], because you would have to bounce light off the [secondary] mirror anyway,” says Pat TEXAS ENGINEERING EXTENSION SERVICE
the entire mirror into thermal equilibrium in 20 minutes. That’s not bad, considering that the 100-inch telescope on Mount Wilson in California, which saw first light in 1917, took a whole night for its 33-centimetre-thick primary mirror to reach a uniform temperature. The TMT’s and E-ELT’s primaries will have much smaller segments than the GMT, inspired by the success of the twin 10-metre Keck telescopes on Mauna Kea, Hawaii. Going with smaller segments has its advantages, not least that each piece is thinner and easier to manufacture. The downside is that it’s much harder to keep all the segments in perfect alignment as the telescope moves. So-called edge sensors are needed to keep track of any displacement between the segments, while large numbers of pistons, or actuators, will push or pull each segment to keep the primary mirror’s curvature precise to within a few nanometres. The other significant technology that these telescopes will be exploiting is adaptive optics (AO). The various layers of the atmosphere, which are at different temperatures or move at different speeds, can distort the light reaching the telescope. Today’s telescopes have add-on AO systems that monitor either a guide star or an artificial star created by firing a laser into the upper atmosphere. Software compares the image of the star with the expected image to work out the atmospheric aberrations, which are then corrected for in real time using a deformable
McCarthy of the Observatories of the Carnegie Institution of Washington in Pasadena, California, a member of the GMT consortium. However, secondary mirrors are large, so making them deformable presents a considerable challenge. Mindful of this, the E-ELT’s designers are sticking with the smaller quaternary mirror approach. The TMT will have a fourth mirror too, but to reduce the thermal noise the light will leave the telescope and enter a separate AO system chilled to -30 °C. Despite the challenges, each telescope team is aiming for first light by 2017 and the chance to open a new era for astronomy and cosmology. “We are going to depths of the universe and to a spatial resolution that we have never achieved before,” says Kissler-Patig. “You are likely to discover things that you absolutely hadn’t expected.” ●
Rescue robots are put to the test Search and rescue robots hunting for survivors after an earthquake will need to develop an accurate picture of their surroundings. So last month would-be rescue robots from around the world were put through their paces creating an instant map of an unknown area. In the tests, run by the US National Institute of Standards and Technology, the robots had to travel through a
simulated forest, with uneven terrain and randomly placed PVC “trees”, and map the area using their on-board sensors and software. The event, at the Disaster City training site in College Station, Texas, included tests of the robots’ ability to clamber over obstacles – in this case ramps and steps, including some with unequal gaps between them.
6 December 2008 | NewScientist | 27