The Cornell Caltech Atacama Telescope (CCAT) is a 25 m diameter telescope that will operate at wavelengths as short
as 200 microns. CCAT will have active surface control to correct for gravitational and thermal distortions in the
reflector support structure. The accuracy and stability of the reflector panels are critical to meeting the 10 micron
HWFE (half wave front error) for the whole system. A system analysis based upon a versatile generic panel design has
been developed and applied to numerous possible panel configurations. The error analysis includes the manufacturing
errors plus the distortions from gravity, wind and thermal environment. The system performance as a function of panel
size and construction material is presented. A compound panel approach is also described in which the reflecting surface
is provided by tiles mounted on thermally stable and stiff sub-frames. This approach separates the function of providing
an accurate reflecting surface from the requirement for a stable structure that is attached to the reflector support structure
on three computer controlled actuators. The analysis indicates that there are several compound panel configurations that
will easily meet the stringent CCAT requirements.
To meet the 10 µm RMS half wavefront error requirement for the 25 m diameter Cornell Caltech Atacama Telescope
(CCAT), active control of the approximately 200 primary mirror panels is required. The CCAT baseline design includes
carbon fiber aluminum honeycomb sandwich mirror panels. Distortions of the panels due to thermal gradients, gravity
and the mounting scheme need to be taken into consideration in the control system design. We have modeled the
primary mirror surface as both flat and curved surfaces and have investigated mirror controllability with a variety of
sensor types and positions.
To study different mirror segmentation schemes and find acceptable sensor configurations, we have created a software
package that supports multiple segment shapes and reconfigurable panel sizing and orientation. It includes extensible
sensor types and flexible positioning. Inclusion of panel and truss deformations allows modeling the effects of thermal
and gravity distortions on mirror controllability.
Flat mirrors and curved mirrors with the correct prescription give similar results for controlled modes, but show
significant differences in the unsensed flat mirror modes. Both flat and curved mirror models show that sensing
schemes that work well with rigid, thermally stable panels will not control a mirror with deformable panels. Sensors
external to the mirror surface such as absolute distance measurement systems or Shack-Hartmann type sensors are
required to deal with panel deformations. Using a combination of segment based sensors and external sensors we have
created a promising prototype control system for the CCAT telescope.
The out-of-plane degrees of freedom (piston, tip, and tilt) of each of the 492 segments in the Thirty Meter Telescope
primary mirror will be actively controlled using three actuators per segment and two edge sensors along each intersegment
gap. We address two important topics for this system: edge sensor design, and the correction of fabrication and
The primary mirror segments are passively constrained in the three lateral degrees of freedom. We evaluate the segment
lateral motions due to the changing gravity vector and temperature, using site temperature and wind data, thermal
modeling, and finite-element analysis.
Sensor fabrication and installation errors combined with these lateral motions will induce errors in the sensor readings.
We evaluate these errors for a capacitive sensor design as a function of dihedral angle sensitivity. We also describe
operational scenarios for using the Alignment and Phasing System to correct the sensor readings for errors associated
with fabrication and installation.
Infrared interferometric nulling is a promising technology for exoplanet detection. Nulling research for the Terrestrial Planet Finder Interferometer has been exploring a variety of interferometer architectures at the Jet Propulsion Laboratory (JPL). Three architectures have been identified as having promise for achieving deeper broadband IR null depths. Previous nulling research concentrated on layouts using dispersive elements to achieve a quasi-achromatic phaseshift across the passband. However, use of a single glass for the dispersive phase shift method inherently limits the nulling bandwidth. JPL is researching use of multiple glass types to increase null depth and bandwidth. In order to pursue nulls over much broader wavelength regions, nondispersive interferometer architectures can be employed. Toward this end, JPL has been researching two reflective architectures as nulling interferometers. The key enabling technology for this and other nondispersive field flip architectures is single mode spatial filtering devices. We have obtained results with both pinhole spatial filtering and single mode fibers.
By the middle of 2006, the Interferometry Technology development program for NASA's Terrestrial Planet Finder (TPF) Mission has the goal of demonstrating deep and stable interferometric nulling of broadband Mid-IR thermal radiation under conditions that are traceable to the expected on-orbit conditions. Specifically, the task is to demonstrate null levels of 10-6, with a 50% bandwidth centered at 10 μm, with null stabilities of 10-7 all at cryogenic temperatures for observational periods of a couple of hours. The Achromatic Nulling activity at JPL addresses this concern in two testbeds: the warm nulling testbed and the cryonulling testbed. The warm nulling testbed will demonstrate the physics of nulling broadband thermal sources in an environment that is conducive to efficient research. We'll explore nulling techniques, optical-mechanical alignment methods, motion control, and path-length metrology for a single beam interferometer, as well as preliminary planet detection techniques. Ultimate nulling capabilities under conditions that are more flight-like will be demonstrated in the cryogenic nulling testbed. Knowledge gained from operation at room temperature will be applied to the cryogenic experiment where we face the additional challenges of extreme temperatures, cryogenic actuators, component survivability and fluxes that are within an order of magnitude of expected flux levels on orbit. Concurrently, we will develop a low flux mid-IR camera that will allow us to measure the nulls at these faint photon fluxes. This talk will review this development activity and will include recent nulling experimental results and plans for future work.
The scientific objectives and future requirements of high energy x-ray astronomy are discussed and concepts for imaging instruments based on CdZnTe detectors and coded masks are reviewed. An instrument concept based on CdZnTe strip detectors, HEXIS, is described in detail. Technical requirements for large area CdZnTe strip detectors are discussed and recent work at UCSD and WU on the capabilities of CdZnTe strip detectors is described in detail. Studies with small, approximately 50 micron beams demonstrate that crossed strip detectors have good properties for both spatial and spectral measurements.
The High Energy X-Ray Timing Experiment (HEXTE), currently under development for the X-Ray Timing Explorer (XTE) mission, employs a closed loop gain control system to attain 0.5 percent stabilization of each of eight-phoswich detector gains. This Automatic Gain Control (AGC) system utilizes a split window discriminator scheme to control the response of each detector pulse height analyzer to gated Am-241 X-ray events at 60 keV. A prototype AGC system has been implemented and tested within the gain perturbation environment expected to be experienced by the HEXTE instrument in flight. The AGC system and test configuration are described. Response, stability and noise characteristics are measured and compared with theoretical predictions. The system is found to be generally suitable for the HEXTE application.
The HEXTE, part of the X-Ray Timing Explorer (XTE), is designed to make high sensitivity temporal and spectral measurements of X-rays with energies between 15 and 250 keV using NaI/CsI phoswich scintillation counters. To achieve the required sensitivity it is necessary to provide anticoincidence of charged cosmic ray particles incident upon the instrument, some of which interact to produce background X-rays. The proposed cosmic ray particle anticoincidence shield detector for HEXTE uses a novel design based on plastic scintillators and wavelength-shifter bars. It consists of five segments, each with a 7 mm thick plastic scintillator, roughly 50 cm x 50 cm in size, coupled to two wavelength-shifter bars viewed by 1/2 inch photomultiplier tubes. These segments are configured into a five-sided, box-like structure around the main detector system. Results of laboratory testing of a model segment, and calculations of the expected performance of the flight segments and particle anticoincidence detector system are presented to demonstrate that the above anticoincidence detector system satisfies its scientific requirements.