Back
ASTRONOMY
Detecting planets with binoculars
ON top of Mount Graham in Arizona, United States, there sits a unique instrument that is set to initiate the next stage in astrophysics, boasting the potential to see the light of gas-giant planets around other stars and peer deep into the hearts of distant quasars to obtain 10 times more detail than the Hubble Space Telescope. This instrument is called the Large Binocular Telescope (LBT).
As the name suggests, the LBT is not just a telescope - it borrows its design from the binoculars. The main difference between the LBT and an ordinary pair of binoculars, apart from size and cost, is the LBT's interferometric capability. Interferometry is a common technique that links telescopes electronically to create a combined, bigger telescope. The LBT has two 8.4-metre mirrors that combine into an instrument that has the resolution equivalent to that of a single mirror 22.8 metres across. The first of the LBT's 8.4-metre mirrors saw first light in October 2005 and the second mirror should be completed within the next few months.
Optics and mechanics of binoculars
By far, the best type of optical aid for the first-time stargazer is a pair of binoculars. They are less expensive than a telescope, have a wider field of view, are easy to use, are portable and have uses other than those relating to astronomy. Even for the more accomplished observer, there is a great deal to be said for binoculars. Many features on the moon can be picked out, as can a large number of deep-sky objects, and this type of instrument has been fundamental in discovering comets and novae.
Essentially, a pair of binoculars consists of two small refracting telescopes. This eliminates the sometimes tiring need to close one eye. The key to their compactness is that the light paths are folded inside the binocular barrels by a series of prisms. The most important consideration when buying a pair of binoculars (or a telescope) is aperture (the diameter of the objective, or main, lens). This should be as large as possible, permitting it to gather more light and allowing the observation of even faint objects. However, binoculars with very large lenses will be heavy and difficult to hold steady. Binoculars are graded as, for example, 8x 40, (eight denotes magnification and 40 the aperture in millimetres). The greater the magnification, the more difficult it is to hold the binoculars steady. As a rule, the highest magnification for hand-held binoculars should be 7x, although some observers are comfortable with 10x or even 12x. Probably, the best size of binocular for general use is 7x 50.
Another advantage of 7x 50 binoculars is that their "exit pupil" matches the diameter of the pupil of the dark-adapted eye. The exit pupil of any optical system is the breadth of the beam of light leaving that system. It is calculated by dividing the aperture by magnification: in the case of a 7x 50 this is 50 mm divided by 7, that is 7 mm approximately. The aperture of the fully dark-adapted eye pupil is about 7 mm. Therefore, the full diameter of the light beam leaving the binoculars can be used.
Holding the binoculars in the viewing position, but away from your eyes, will allow you to see the exit pupils as discs of light in the eyepieces. Ideally, these discs should be complete: if they appear squared off, the prisms are not allowing all the light through to the eyepieces. Binoculars with high magnifications can be held steady with a camera tripod. A special clamp, known as a binocular adapator, locates around the central pivot of the binocular and is fixed to the tripod by a screw on the tripod head.
One should test the binoculars before buying them. The general standard of workmanship should be good, the lenses should not be damaged and the barrels should not have dents. Now, extend the eyepieces to their full extent. Ensure that the eyepiece barrels are firm and rigid and do not tilt in any way. The focussing mechanism should run smoothly and with steady resistance. The same applies to the central pivot, which alters the spacing of the eyepieces. This should stay firmly in place once it is set.
Anti-reflection coating
Most binoculars have anti-reflection coating on their object glasses. These increase contrast and improve light transmission. Ideally, all the other optical surfaces should also be coated. Look at the reflection of light in the objective lens. There will be two reflections, one from the outer surface of the objective and one from the inner surface. If both surfaces are coated, then both reflections should be coloured. By carefully adjusting the position of the binoculars one will also get reflections from the prisms inside the barrels, which indicate whether or not they are coated. Applying the same tests at the eyepiece end ensure that they are coated.
Focus the binoculars on a point source of light and then move it towards the edge of the field of view. The point should remain in sharp focus until somewhere around two-thirds of the way out towards the edge. Binoculars with a so-called flat field should give sharp focus regardless of the position of the point within the field. Now focus the binoculars while centred on a straight line, such as the edge of a building. Inevitably, the line will appear to be bent at the edge of the field of view. Reject the binoculars if the line becomes bent more than a third of the way in from the edge. If the barrels are not set parallel to each other, the binoculars will produce a double image. Look through the binoculars while opening and closing your eyes. Look carefully for a momentary double image - it will become single as your eyes adjust to compensate - and reject binoculars that display this fault.
Leaving aside the LBT, there are other interferometers currently in use. The Very Large Telescope at the European Southern Observatory in Chile joins four eight-metre telescopes into an interferometer. The Keck Observatory on Mauna Kea, Hawaii, has an interferometer that combines two 10-metre telescopes. By having the LBT mirrors relatively close together (14 metres) compared to the Very Large Telescope, it has been possible to combine the light over a larger field of view. The instruments that are now being built for the LBT will have a field of view up to one arcminute. An arcminute is the area of one-sixtieth of a degree, projected onto the night sky. In astronomical terms, this is a large field of view when peering deep into space.
Design for the LBT
The design for the LBT originated way back in the 1980s, and a lot of thought went into how far apart the mirrors should be placed because of the trade-off between resolution and field of view. It was decided that the two mirrors would be placed less than one mirror diameter apart, as this ensured that there would be a continuous coverage of wavelengths of light extending into the ultraviolet.
Professional telescopes, including the LBT, are not the sort of telescopes that astronomers `look through'. To do serious scientific work, the light collected has to be fed into various instruments, such as spectrometers, and analysed by computer. When completed, the LBT should be able to take a picture 10 times more detailed than those of the Hubble Space Telescope.
The problem is that the earth's own atmosphere can disrupt this level of details, like the disrupted reflection in a pool during a rainstorm. This problem is overcome by using a remarkable, modern computer-correction technique known as adaptive optics. An adaptive optics system utilises a computer to analyse image distortions. A series of actuators then distort parts of the mirror in such a way as to correct the distortion. All this happens in real time and provides `still' images devoid of atmospheric blurring. Even so, the structure of the telescope has been conceived to be as rigid as possible. These modern computer techniques have allowed another breakthrough, one that is key to realising the full potential of the instrument, and it all lies in the interferometry.
Set for full operation sometime in 2008, the LBT interferometer will have the capability to detect directly Jupiter-sized planets around nearby stars. The technique it will use, called nulling interferometry, is deviously clever. The main problem with direct imaging is that stars are millions of times brighter than any planet orbiting them and everything gets lost in the star's glare. What the LBT interferometer will hope to do is to combine the light from stars, using the two mirrors, in such a way as to make the stars disappear. The LBT will take advantage of the wave-like nature of light (in this case infrared light). Two beams from the same source would be made to match up so that the peak of one beam would coincide with the trough of another, cancelling them out. This is called destructive interference, and would literally take the star out of the picture. Anything left would, hopefully, be a planet.
On the scale of millionths of a metre, which infrared light waves are measured in, the atmosphere is very turbulent and this technique calls for sophisticated computer control. To a question whether the LBT could itself detect terrestrial planets, the answer is that the contrast ratio would probably be too high. But there are often surprises when investigating new planetary systems, so we can never say for sure. So, though it may not be likely to image earth-like planets directly, one of the other science goals of the LBT is to survey nearby stars for planet-forming dust discs that may surround them; the study of which may shed more light on the formation of our own solar system.
But why stop at mere solar systems when one could see how the universe formed? Indeed, that is one of the other major science goals of the LBT. The universe is composed of clusters of galaxies that form giant chains. In between are large voids. The LBT will be able to help determine how the early universe came to look the way it did and why galaxies form the way they do by using a powerful spectrograph to measure the movement and rotation of galaxies. It would also be able to peer into active galactic nuclei - the violent centres of some distant galaxies that contain supermassive black holes and quasars. The LBT does not just stop there - any detected gravitational effects may help explain the mystery surrounding the missing mass in the universe, the so-called `dark matter'. The LBT's gaze seems to have no limits.
Professor Amalendu Bandyopadhyay is a senior scientist at the M.P. Birla Institute of Fundamental Research, M.P. Birla Planetarium, Kolkata.
The Large Binocular Telescope is a unique instrument that is set to initiate the next stage in astrophysics.
A tripod is a useful accessory to have when viewing with binoculars. It eliminates shake and gives your arms a rest.
LIGHT STRIKES THE LBT's two primary 8.4-metre mirrors and is reflected back to smaller, secondary mirrors that can be moved to adjust the path length of the light.
The twin eyes of the LBT, perched on a hilltop in Arizona, will peer into the depths of the universe.
THE FIRST OF the LBT's mirrors began operations on October 12, 2005, when it saw first light, taking this picture of the galaxy NGC 891.
ENGINEERS WORK AT computer monitors as one of the giant primary mirrors looms above.