Daniel MacDonald

Advances in edge sensors for the Thirty Meter Telescope primary mirror

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 installation errors. 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.

Current progress on TPFI nulling architectures at Jet Propulsion Laboratory

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.

Cornell Caltech Atacama Telescope primary mirror surface sensing and controllability

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.

Measurements of fluctuations in the hard X-ray background with RXTE

During normal observations, the High Energy X-ray Timing Experiment instrument (HEXTE) on the Rossi X-ray Timing Explorer (RXTE) Spacecraft makes nearly simultaneous observations of two pairs of background fields separated by 3.0 degrees. Differences in fluxes from these pairs of background fields can be used to detect a source population much deeper than can be studied from individual detections. We have measured the fluctuations in the X-ray background (XRB) from 15–40 keV using differences between adjacent HEXTE background fields. The fluctuations as a fraction of the XRB flux are: 0.092±0.014% from 15 to 20 keV, 0.11±0.02% from 20 to 25 keV and 0.23±0.08% from 34 to 41 keV for a 1.1 square degree field of view. The measured fractional fluctuations increase with energy, which implies that the effective number of contributing sources is decreasing with energy. The fluctuation level and energy dependence are consistent with HEAO-1 results.