baudrunner's space: James Webb Space Telescope close up
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Tuesday, January 22, 2008

James Webb Space Telescope close up

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.

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