Hervey S. Stockman

The James Webb Space Telescope

The James Webb Space Telescope (JWST) is a large (6.6 m), cold (<50 K), infrared (IR)-optimized space observatory that will be launched early in the next decade into orbit around the second Earth–Sun Lagrange point. The observatory will have four instruments: a near-IR camera, a near-IR multiobject spectrograph, and a tunable filter imager will cover the wavelength range, 0.6 < ; < 5.0 μ m, while the mid-IR instrument will do both imaging and spectroscopy from 5.0 < ; < 29 μ m.The JWST science goals are divided into four themes. The key objective of The End of the Dark Ages: First Light and Reionization theme is to identify the first luminous sources to form and to determine the ionization history of the early universe. The key objective of The Assembly of Galaxies theme is to determine how galaxies and the dark matter, gas, stars, metals, morphological structures, and active nuclei within them evolved from the epoch of reionization to the present day. The key objective of The Birth of Stars and Protoplanetary Systems theme is to unravel the birth and early evolution of stars, from infall on to dust-enshrouded protostars to the genesis of planetary systems. The key objective of the Planetary Systems and the Origins of Life theme is to determine the physical and chemical properties of planetary systems including our own, and investigate the potential for the origins of life in those systems. Within these themes and objectives, we have derived representative astronomical observations.To enable these observations, JWST consists of a telescope, an instrument package, a spacecraft, and a sunshield. The telescope consists of 18 beryllium segments, some of which are deployed. The segments will be brought into optical alignment on-orbit through a process of periodic wavefront sensing and control. The instrument package contains the four science instruments and a fine guidance sensor. The spacecraft provides pointing, orbit maintenance, and communications. The sunshield provides passive thermal control. The JWST operations plan is based on that used for previous space observatories, and the majority of JWST observing time will be allocated to the international astronomical community through annual peer-reviewed proposal opportunities.

Cosmic-Ray Rejection and Readout Efficiency for Large-Area Arrays

We present an algorithm to optimally process uniformly sampled array image data obtained with a nondestructive readout. The algorithm discards full wells, removes cosmic-ray (particle) hits and other glitches, and makes a nearly optimum estimate of the signal on each pixel. The algorithm also compresses the data. The computer requirements are modest, and the results are robust. The results are shown and compared to results of Fowler-sampled and -processed data. Nonideal detector performance may require some additional code, but this is not expected to cost much processing time. Known types of detector faults are addressed.

Independent Testing of JWST Detector Prototypes

The Independent Detector Testing Laboratory (IDTL) is jointly operated by the Space Telescope Science Institute (STScI) and the Johns Hopkins University (JHU), and is assisting the James Webb Space Telescope (JWST) mission in choosing and operating the best near-infrared detectors. The JWST is the centerpiece of the NASA Office of Space Science theme, the Astronomical Search for Origins, and the highest priority astronomy project for the next decade, according to the National Academy of Science. JWST will need to have the sensitivity to see the first light in the Universe to determine how galaxies formed in the web of dark matter that existed when the Universe was in its infancy (z~10-20). To achieve this goal, the JWST Project must pursue an aggressive technology program and advance infrared detectors to performance levels beyond what is now possible. As part of this program, NASA has selected the IDTL to verify comparative performance between prototype JWST detectors developed by Rockwell Scientific (HgCdTe) and Raytheon (InSb). The IDTL is charged with obtaining an independent assessment of the ability of these two competing technologies to achieve the demanding specifications of the JWST program within the 0.6-5 μm bandpass and in an ultra-low background (<0.01 e-/s/pixel) environment. We describe results from the JWST Detector Characterization Project that is being performed in the IDTL. In this project, we are measuring first-order detector parameters, i.e. dark current, read noise, QE, intra-pixel sensitivity, linearity, as functions of temperature, well size, and operational mode.

Validation of Up-the-Ramp Sampling with Cosmic-Ray Rejection on Infrared Detectors

We examine cosmic-ray rejection methodology on data collected from InSb and Si:As detectors. The application of an up-the-ramp sampling technique with cosmic-ray identification and mitigation is the focus of this study. This technique is valuable for space-based observatories which are exposed to high-radiation environments. We validate the up-the-ramp approach on radiation-test data sets with InSb and Si:As detectors which were generated for SIRTF. The up-the-ramp sampling method studied in this paper is over 99.9% effective at removing cosmic rays and preserves the structure and photometric quality of the image to well within the measurement error.

Science with the James Webb space telescope

The scientific capabilities of the James Webb Space Telescope (JWST) fall into four themes. The End of the Dark Ages: First Light and Reionization theme seeks to identify the first luminous sources to form and to determine the ionization history of the universe. The Assembly of Galaxies theme seeks to determine how galaxies and the dark matter, gas, stars, metals, morphological structures, and active nuclei within them evolved from the epoch of reionization to the present. The Birth of Stars and Protoplanetary Systems theme seeks to unravel the birth and early evolution of stars, from infall onto dust-enshrouded protostars, to the genesis of planetary systems. Planetary Systems and the Origins of Life theme seeks to determine the physical and chemical properties of planetary systems around nearby stars and of our own, and investigate the potential for life in those systems. To enable these for science themes, JWST will be a large (6.5m) cold (50K) telescope with four instruments, capable of imaging and spectroscopy from 0.6 to 29 microns wavelength.

The next generation space telescope design reference mission

We review the Next Generation Space Telescope (NGST) design reference mission (DRM). The NGST DRM contains the scientific goals of the observatory and consists of primary and secondary targets and their characteristics. The purpose of the DRM is to establish a metric against which cost/capability trades, instrumental configurations, and operational scenarios can be tested.

Next Generation Space Telescope

The Next Generation Space Telescope, planned for launch in 2009, will be an 8-m class radiatively cooled infrared telescope at the Lagrange point L2. It will cover the wavelength range from 0.6 to 28 micrometers with cameras and spectrometers, to observe the first luminous objects after the Big Bang, and the formation, growth, clustering, and evolution of galaxies, stars, and protoplanetary clouds, leading to better understanding of our own Origins. It will seek evidence of the cosmic dark matter through its gravitational effects. With an aperture three times greater than the Hubble Space Telescope, it will provide extraordinary advances in capabilities and enable the discovery of many new phenomena. It is a joint project of the NASA, ESA, and CSA, and scientific operations will be provided by the Space Telescope Science Institute.

Independent detector testing laboratory and the NGST detector characterization project

The Independent Detector Testing Laboratory (IDTL) has been established by the Space Telescope Science Institute (STScI) and the Johns Hopkins University (JHU), and it will assist the Next Generation Space Telescope (NGST) mission in choosing and operating the best near-infrared detectors. The NGST is the centerpiece of the NASA Office of Space Science theme, the Astronomical Search for Origins, and the highest priority astronomy project for the next decade, according to the National Academy of Science. NGST will need to have the sensitivity to see the first light in the Universe to determine how galaxies formed in the web of dark matter that existed when the Universe was in its infancy (z ~10-20). To achieve this goal, the NGST Project must pursue an aggressive technology program and advance infrared detectors to performance levels beyond what is now possible. As part of this program, NASA has selected the IDTL to verify comparative performance between prototype NGST detectors developed by Rockwell Scientific (HgCdTe) and Raytheon (InSb). The IDTL is charged with obtaining an independent assessment of the ability of these two competing technologies to achieve the demanding specifications of the NGST program within the 0.6-5 μm bandpass and in an ultra-low background (<0.01 e-/s/pixel) environment. We describe the NGST Detector Characterization Project that is being performed in the IDTL. In this project, we will measure first-order detector parameters, i.e. dark current, read noise, QE, intra-pixel sensitivity, linearity, as functions of temperature, well size, and operational mode.

James Webb Space Telescope

The James Webb Space Telescope ( JWST ) is the scientific successor to the Hubble and Spitzer missions. Its wavelength range (1 - 28μm) and sensitivity (1 nJy - 1 μJy) complement the submillimeter facilities of the coming decade, Herschel and ALMA. The JWST development is on schedule for a June 2013 launch to L2 on an Ariane 5.

Intra-pixel sensitivity in NIR detectors for NGST

Intra-Pixel Sensitivity (IPS) is defined as the spatially varying response of the pixel to incoming flux. IPS plays a crucial role when the Point-Spread Function (PSF) is critically, or under-, sampled. Variations in IPS lead to photometric and astrometric errors. The Next Generation Space Telescope (NGST) requires high quality photometry and astrometry, so an accurate estimation of the IPS function is necessary for a successful NGST mission. Photo-electrons generated in a pixel may be detected in the depletion region (detection of the flux) of the same pixel, or might diffuse and end up in the microstructure of the detector, the electric field distribution therein, wavelength of the incident radiation, and diffusion processes of the excess charge carriers generated determines the IPS function of a pixel that can vary from pixel to pixel. The total detected flux is proportional to the convolution of the PSF and the IPS function. If we approximate the profile of the PSF, then the problem of determining the IPS function reduces to deconvolving using the experimentally obtained Sensitivity variation profile and the calculated PSF. We aim to obtain a highly undersampled PSF, scan it over a single pixel on a grid of 10 x 10 points, and retrieving IPS function using deconvolution. We present our results, experiment design, and the scope of further work, using an NGST detector, to estimate the IPS function at various wavelengths.