Christopher G. Edwards

Precision spectro-polarimeter for high-resolution observations of solar magnetic fields

As a Japanese National space mission with international collaboration, Solar-B (2005 launch) will carry a spectro- polarimeter (SP) to be operated in visible light to obtain the first high angular resolution, precision measurements of solar vector magnetic fields from space. The SP is part of the Focal Plane Package (FPP) fed by a diffraction-limited 50-cm optical telescope. The SP will be operated exclusively at the photospheric 630 nm Fe I lines. It features a rotating, low-order crystalline quartz retarder for polarization modulation and a reflecting Littrow spectrograph design that is shortened by using diffraction from the 12micrometers wide slit to fill the grating. Polarization analysis is accomplished by a modified Savart plate beam splitter. A custom CCD detector with two active areas, one for each beam from the beam splitter, allows continuous high duty-cycle sampling of polarization. The spectrograph slit will sample a 0.16 x 164 arcsec2 rectangle of the solar image, which may be scanned across the slit by up to +/- 160 arcsec in order to build up vector magnetic field maps of the solar photosphere. Along with simultaneous, co-spatial imaging and polarimetry with the filter imagers of the FPP, the SP will provide a precise view of active and quiet solar magnetic fields that control the structure, dynamics, and energetics of the upper solar atmosphere.

Precision pointing and image stabilization for the transition region and coronal explorer solar observatory

This paper presents a detailed description of the precision pointing system and the image stabilization system (ISS) for the Transition Region and Coronal Explorer (TRACE) satellite mission. The TRACE spacecraft is the fourth in NASAs small explorer series of missions and is scheduled for launch in September 1997. The primary TRACE science objective is to explore the relationship between the fine scale magnetic fields in the solar surface and features in the photosphere, chromosphere, transition region and corona. Quantitative images of these regions will be collected and used to study the structure and evolution of the suns magnetic field with a spatial and temporal resolution of one arc-second and one second, respectively. TO meet the science objectives, the instrument payload and the spacecraft attitude control system are coupled using a guide telescope. The guide telescope provides both the targeting mechanism and pointing error signals for the spacecraft feedback control system. In addition, the guide telescope generates signals used to control the active mirror of the ISS. Simulation results show that precision target pointing is maintained to less than 5 arc-seconds, while analysis indicates that the ISS reduces image motion jitter below the 0.1 arc- second level.

A system to reduce jitter for the GOES-N/O/P solar X-ray imager

A jitter compensation system is incorporated in the Solar X-ray Imager (SXI) that will be mounted to the solar array wing of the GOES N spacecraft, the next in the series of NOAA weather satellites. The SXI obtains images in a back-thinned CCD with 5 arcsec pixels. The SXI incorporates a pointing aspect sensor manufactured by the Adcole Corporation that is used in a semi-closed loop system with the SXI flight computer to shift the detected image during an exposure along the readout columns of the CCD in order to compensate for jitter in one dimension. Simulations of the predicted motions caused by the GOES spacecraft and self-induced by the SXI filter wheels indicate that the jitter as experienced by the SXI instrument will be primarily along one axis, parallel to the east-west direction, with amplitudes in the tens of arcseconds and with dominant frequencies less than approximately 10 Hz. The SXI CCD columns are aligned along this direction in order to make possible on-chip compensation. The SXI motion compensation system has been evaluated with realistic models for the expected spacecraft jitter and assuming a performance algorithm for the SXI instrument. Our analysis indicates that the X-ray spatial imaging performance will be improved when the jitter compensation system is used. We discuss the design and analysis predictions.