In 2001, the GONG+ instruments began acquiring solar magnetic field images (magnetograms) every minute.
These observations offer a useful resource for the solar physics community. However, the quality of the magnetograms
was reduced by a significant zero point error in the observations that varied across the solar image and with time. This
precluded using the magnetograms for meaningful extrapolations of weak photospheric fields into the corona. The error
was caused by the slow, asymmetric, locally varying switching of the LCD modulator (LCM) from one retardation state
to the other. This generated a false magnetic field pattern as a result of different responses to weak instrumental linear
polarization ahead of the LCM. The original modulator driver used a very simple design to excite the LCM. Liquid
crystals like those in the LCM take different times to switch from one polarization state to the other than to return to the
first polarization state. To eliminate the difference in switching times, a driver capable of varying its output during the
change in LCM state was needed. A microcontroller-based design was chosen. The final driver design resulted in a
factor of 100 improvement in the zero point error.
Understanding the Sun's magnetic field related activity is far from complete as reflected in the limited ability to make
accurate predictions of solar variability. To advance our understanding of solar magnetism, the National Solar
Observatory (NSO) constructed the Synoptic Optical Long-term Investigations of the Sun (SOLIS) suite of instruments
to conduct high precision optical measurements of processes on the Sun whose study requires sustained observations
over long time periods. The Vector Spectromagnetograph (VSM), the principal SOLIS instrument, has been in operation
since 2003 and obtains photospheric vector data, as well as photospheric and chromospheric longitudinal magnetic field
measurements. Instrument performance is being enhanced by employing new, high-speed cameras that virtually freeze
seeing, thus improving sensitivity to measure the solar magnetic field configuration. A major operational goal is to
provide real-time and near-real-time data for forecasting space weather and increase scientific yield from shorter
duration solar space missions and ground-based research projects. The National Solar Observatory proposes to build two
near-duplicates of the VSM instrument and place them at international sites to form a three-site global VSM network.
Current electronic industry practice of short lifetime cycles leads to improved performance and reduced acquisition costs
but also to redesign costs and engineering impacts that must be minimized. The current VSM instrument status and
experience gained from working on the original instrument is presented herein and used to demonstrate that one can
dramatically reduce the estimated cost and fabrication time required to duplicate and commission two additional
The National Solar Observatory operates two facilities with demanding needs for rapid image collection (i.e. > television frame rates). The first is GONG, a global network of six identical small telescopes devoted to nearly continuous observations of the sun's surface vibrations in order to study its internal properties by helioseismology. The second, SOLIS, is a suite of three instruments that collects images and spectra of the sun needed to study the behavior of solar activity on time scales of minutes to decades. Five different types of cameras are installed in these instruments. High speed, high sensitivity, large dynamic range, and good photometric performance are key factors for cameras used to make measurements of subtle solar signals that pass through the noisy terrestrial atmosphere. A camera that combines all these characteristics is elusive. The combination of high speed and good photometric performance, when observing small intensity changes, is particularly
hard to get in practice. High speed in large format CCD and hybrid FPA cameras is achieved by dividing the array into multiple channels that are read simultaneously. An unwanted result of this technique is cross talk between signal channels. It is of order 1 percent in the case of Silicon Mountain Design 1M60_20 cameras (1k x 1k, 60 fps) and Rockwell Scientific Company HyViSI-1024 cameras (1k x 256, 92 fps). Cross talk (and also successive-frame image retention) are particularly hard to deal with since they may exhibit non-linear characteristics that depend on illumination light level. We describe these and other phenomena, attempts to mitigate the effects, and results from solar observations.
SOLIS (Synoptic Optical Long-term Investigations of the Sun) is a suite of three innovative instruments under construction that will greatly improve ground-based synoptic solar observations. The Vector Spectromagnetograph (VSM) is a compact, high-throughput vector-polarimeter with an active secondary mirror, an actively controlled grating spectrograph, and two high-speed cameras with silicon-on-CMOS-multiplexer hybrid focal plane arrays. It will measure the magnetic field strength and direction over the full solar disk within 15 minutes. The Full-Disk Patrol (FDP) takes full-disk solar intensity and Doppler images in various spectral lines and in the continuum at a high cadence through liquid-crystal tuned birefringent filters. The Integrated Sunlight Spectrometer (ISS) uses a fiber-fed spectrograph to measure minute changes of the Sun-as-a-star in many spectral lines. A high degree of automation and remote control provides fast user access to data and flexible interaction with the data-collection process. SOLIS is currently in the final assembly phase and will become operational early in 2003.
We present a novel concept for a solar magnetograph that uses a photo-refractive crystal to reflect and focus the light from the wings of many spectral lines onto a camera. The crystal acts simultaneously as multiple, narrow-band filters and as an off-axis telescope. Polarization measurements are performed close to the final focus. Since this approach uses the light from many spectral lines simultaneously, the required telescope aperture is substantially reduced and exposure times can be so short that accurate tracking is not necessary. Such a concept is particularly attractive for NASA's Minimum Solar Mission where very compact, light-weight instruments are required.