A cursory glance of the James Webb space-based telescope in my post titled "Twinkle twinkle little star..." does not do it justice. It is one of the most anticipated ever of NASA's space programs and complex preparation and testing of the various components of the instrument has been ongoing for years and will continue until launch in 2013. Plans are to place it 1.5 million km from earth outside the orbit of the moon at the second LaGrange point (L2).
It is possible to see stars on a clear sunny day when the sky is viewed from the bottom of a deep well. This is the principle theory behind the telescope's sun shield. In addition, the sun shield will help to maintain the optimum cryogenic temperatures under which the telescope is designed to operate. It is large, measuring 2600 sq ft. It is comprised of five layers of thin membranes made of a polymer-based film. All testing is being done on a full-scale model to minimize dependence on computer modeling data in the effort to reduce program risk.
One of the instruments aboard the Webb space telescope is the Near Infrared Spectrograph (NIRSpec), which is capable of obtaining simultaneous spectra of 100 objects in a 9-square-arcminute field of view. An object's spectra contains unique characteristics specific to the types and amounts of the elements making up the actual composition of the object under view. In order to make these determinations on very faint and far away objects it is necessary to further reduce the amount of light interference from nearer objects. In the same way that we squint in order to see a clearer image when light gets in the way, the instrument employs an array of micro-shutters to perform the same task. They are arranged in four postage stamp sized grids of over 62,000 shutters per grid. Each shutter can be independently controlled -- opened or closed -- when a magnetic field is applied. Development of this revolutionary technology has been ongoing for over six years.
The telescope is designed to be positioned precisely using the Fine Guidance Sensor (FGS), an instrument consisting of a sensitive camera and a Tunable Filter Imager (FGS-TFI) packaged with the FGS but a functionally independent science instrument. The FGS/TFI is a contribution of the Canadian Space Agency. The job of the FGS is to accurately position the telescope to acquire images and provide error-correction data for correct alignment of the primary mirror. The TFI will be used solely for science observations.
The telescope will be optimized for imaging in the infrared region of the optical spectrum. The primary imager will be the Near Infrared Camera (NIRCam) . It is designed to fulfill all of the JWST mission's core science goals, which are concerned with the evolution of the objects populating the Cosmos. NIRCam will also be used for providing additional error-correcting data necessary to properly align each of the individual segments that make up the primary mirror. It does this through a process called "Wavefront Sensing and Control" (WFSC). Scientists have recently successfully tested the software needed to enable the eighteen individual mirror segments of the primary mirrior to act as a single mirror and work effectively with the secondary mirror.
The search for life elsewhere in the galaxy is not being ignored by the JWST. One of the observing programs allocated to the Mid Infrared Instrument (MIRI) will be to find the source of life supporting elements in planetary systems. The MIRI will consist of an imager and a medium resolution spectrograph. It is being developed by NASA and the European Space Agency (ESA) and construction is being overseen by a science team at the University of Arizona.
The JWST is not cheap. In May, 2007 the cost of the project was estimated at about $4.5 billion. By the time of the Hubble Space Telescope's launch on 24 April, 1990 that telescope's cost was about $2.5 billion (original estimate: $400 million). Hubble's cummulative costs to this day are estimated to be between $4.5 and $6 billion.
Tuesday, January 22, 2008
James Webb Space Telescope close up
Twinkle twinkle little star, all you like, though near or far
The ability of telescopes to finely resolve images from space is compromised by the presence of the atmosphere, which produces the twinkling effect that we see when we stare up at a star. That is why astronomical observatories tend to be situated on islands and/or at high altitudes. Less atmosphere means less atmospheric distortion. The amount of distortion is quantifiable and the term astronomical seeing has emerged to describe the effect of atmosphere on image. A star is said to twinkle, or scintillate as its brightness fluctuates.
It can be seen then that when imaging a point source of light such as a star the exposure time should be as short as possible in order to produce the most coherent image. As the distortion effects change over time, the resulting image forms a speckle pattern in a short exposure; a blurred image of the star during a long exposure. Space-based telescopes such as Hubble have no seeing problems.
Computerised speckle image processing can offer very high resolution of point-source observations of space and in fact have superior resolution of bright sources when seen from Earth than does Hubble, because of that telescope's smaller diameter mirror. However, Hubble allowed us for the first time to view faint images with great clarity of objects which are impossible to see from Earth.
The highest resolution astronomical images possible are made using astronomical interferometers. An astronomical interferometer is an array of telescopes or mirror segments working together. U.S. Route 60 passes through The Very Large Array located about 80 km west of Socorro, New Mexico. The VLA stands at over 2 km above sea level and is comprised of 27 large radio antennae each with a 25 m diameter. The antennae are situated on rails formed into a Y pattern and can be moved around to preset baseline configurations. A baseline is the effective separation between any two telescopes as seen from the radio source. The VLA's 27 radio telescopes give 351 independent baselines at once. In addition, the rotation of the Earth moves the telescopes to new baselines. Aperture Synthesis is the term given to combining all the telescope data into a single image. Very Long Baseline Interferometry transforms data obtained from telescopes thousands of km apart. Advanced computational algorithms can now be used to transform data from irregularly spaced baselines using whatever data is available in a process called synthesis imaging.
The largest optical telescope arrays consist of 6 telescopes with 15 baselines. These offer poorer image resolution than large array radio telescopes. The challenges of creating an optical array are formidable, since light waves are a million times shorter than radio waves. However, it is possible to achieve better than Hubble resolution at a thousandth the cost. The Cambridge Optical Aperture Synthesis Telescope threatens Hubble's dominance in the field of astronomical imaging.
NASA's planned James Webb Space Telescope is currently undergoing cryogenic testing at the Marshall Space Flight Center. 18 individual mirror segments form a 6.5 m mirror assembly protected by a tennis-court sized sunshield. It is a large infrared-optimized space telescope designed to detect the heat of cosmic bodies billions of light years away. It will also be used to observe stars at visible wavelengths. Although its mass is about half that of Hubble, it will be almost 6 times larger in diameter.
A very large array of James Webb Space telescopes would provide the ultimate in astronomical observatories allowing us to clearly see Earth-like planets orbiting stars hundreds of light years away. I predict that a star that twinkles when viewed from space is a star whose system includes a water-bound planet like Earth, and that the scintillation will be caused by the star's light reflecting from and refracting through that world's oceans.
