The Next-Generation Space Telescope (NGST), the successor of the Hubble Space Telescope, is scheduled for launch in the year 2008. NGST will make unprecedented discoveries in the realms of galaxy formation, cosmology, stellar populations, star formation, and planetary systems. The telescope is currently in the conceptual design phase of development, and Ames has been involved in defining and studying the scientific instrumentation it will need to conduct its observations.
Along with scientists at the University of Arizona (Kimberly Ennico, Jill Bechtold, George Rieke, Marcia Rieke, Jim Burge, Roland Sharlot, and Rodger
Thompson), and in partnership with Lockheed Martin (Larry Lesyna and the Advanced Technology Center staff), Ames has led and completed a conceptual study of the entire NGST scientific instrument complement. This team was one of several international teams selected to study NGST science instruments. The Ames-led team conducted trade studies of specific instrument technologies and implementations, and developed a comprehensive integrated science instrument module (ISIM) concept.
The team found that the science drivers of NGST justify observations from visible (0.4 microns) to far-infrared (35 microns, about 50 times longer than visible to the human eye) wavelengths of light. Imaging capability is required throughout this wavelength range, while spectroscopic capability is required at all wavelengths greater than or equal to
1 micron. Several technologies are key to achieving these capabilities within the cost, schedule, and environmental requirements of the NGST mission. Visible and far-infrared detectors could be developed in time for NGST, and several detector cooling options - including pulse tubes and solid H2
systems - are viable. The team also found that dispersive slit spectrographs are superior to imaging Fourier Transform spectrometers when complete spatial coverage is not required. Dispersive spectro-scopy may be best accomplished with conventional slits or micro-shutter arrays instead of micro-mirror arrays because of the large background rejection (greater than a factor of 1000) required at near-to-far infrared wavelengths.
A complement of seven instrument modules resulted from these scientific and technical considerations by the study team. Two wide-field cameras (each covering about 0.01 square degrees) located directly at the focus of the NGST telescope image visible to near-infrared wavelengths (0.4 to
2.5 microns) in a few broadband filters. A separate visible-light camera covers a smaller field at the maximum resolution of the NGST telescope. The other four modules cover near-to-far infrared wavelengths of light (1 to 34 microns) and are capable of conducting either imaging or dispersive spectroscopic observations with modest fields and good spatial resolution. The capability of providing both imaging and spectroscopy with relatively simple optical designs is enabled by using transmissive dispersion elements (called grisms) for spectroscopy. The conceptual layout of one of these modules is shown in figure 1.
Several technologies must be developed further in order to implement this design concept and to ensure the success of NGST. Increasing the size (number of picture elements) and reducing the noise of infrared detectors will have the greatest scientific impact at relatively modest cost. Ames is already leading this effort for NGST. Reliable closed-cycle cryogenic coolers must also be developed to cool these detectors to temperatures of 4 to 30 kelvin above absolute zero. This task will be eased by the fact that the NGST telescope will always be behind a sun shade and will cool down to approximately
40 kelvin. However, these and all other NGST systems must be very reliable because NGST will be located approximately a million miles from Earth, far beyond the reach of the Space Shuttle, and will not be serviceable by astronauts.
Point of Contact: T. Greene
(650) 604-5520
tgreene@mail.arc.nasa.gov
Back To Top
Previous Paper
Return to Astrophysics
Next Paper 
