Description of System Configuration and Measurement Methods
The WSI consists of the sensor assembly and the controller. The sensor assembly includes the optical system with a solid state CCD camera and a filter changer, the solar/lunar occultor, and environmental protection.
The radiance images are acquired through a Nikkor 8mm, f/2.8 fisheye lens. The lens has a full 180 degree field of view for viewing the complete sky dome simultaneously. The lens has equi-distant projection, i.e., the zenith angle in objectspace is nearly linear with respect to the position of the corresponding pixel in image space. This lens creates a 24 mm circular image covering the sky. A fiber optic minifier reduces the size of the image to under 10 mm for compatability with the CCD chip.
For the WSI to acquire images under all natural lighting conditions (sunlight, moonlight, and starlight), it must be able to handle an approximately a 9-decade range of lighting. To help accomplish this, a neutral density (ND) filter wheel containing 2- and 3-log ND filters is positioned between the lens and CCD imaging chip. The spectral filter wheel containing the red (650-nm) and blue (450-nm) filters is also positioned between the lens and CCD chip. We have actually achieved a dynamic range of over 10 decades. The optical filter assembly is a custom built optical-mechanical subassembly which provides an optical link between the Nikor fisheye lens and the photometrics camera. This subassembly is electrically driven by manual or computer control. It contains the independently controllable filter wheels, each of which contains four spectrla or neutral density optical filters. The two wheels are stacked along the systems optical axis, so that a broad variety of filter combinations may be selected to best match the technical task at hand.
The imaging camera is a Photometrics Series 300 Slow Scan CCD camera. This camera generates very low noise, has outstanding sensitivity characteristics, and produces a high quality image. Its 16-bit digitization (approximatley 4-log range) allows for fine radiometric resolution. It is very linear in comparison with other cameras we have characterized and has very low dark current due to a 3-stage TE cooler on the chip. Exposures from less than 100 msec to over 60 seconds yield high quality imagery.
To prevent direct sunlight or moonlight from hitting the protective dome covering the lens and producing artifacts in the image from scattering, a solar/lunar occultor is actively positioned between the sun/moon and dome. The occultor has drive controls for 2 degrees of freedom and is automatically positioned in the correct place by the controlling computer.
This full automatic sub-system is designed to maintain the fisheye lens and its protective dome in fall shadow under all solar or lunar orientations. As with all sub-systems and assemblies within the Day/Night WSI package, continuous on-line monitoring of sub-system operational status and fault-alert conditions are recorded.
A modified solar/lunar occultor has been designed for deployment to remote locations. The modified design has 1 degree of freedom; that is the trolley is replaced with a fixed arc shade. The arc moves from East to West, and the arc shade is built of sufficient length to cover the North-South motion of the sun and moon at a given site. The shade is larger for sites near the Arctic than is required for sites near the Equator.
The environmental housing is the primary system enclosure which contains or supports all of the sub-systems which must operate outdoors and without supplemental protection from the elements. The enclosure is water-tight and insulated. It is temperature regulated inside and mechanically strong enough to survive strong winds. The interior temperature of the enclosure is maintained near 16 deg c. Sub-assemblies contained within the enclosure include a closed-loop recirculating liquid system to cool the hot side of the imaging chip's Peltier device. Failure of the coolant-flow results in immediate cut-off of power to the camera. Temperatures and related system status readings are monitored by the computer and result in power cut-off when conditions are out of bounds.
The computer-driven control system includes two accessory control panels (ACP). The sensor ACP provides control of the filter changes and monitoring of the system states (temperature, flow rate, etc.). The occulator ACP provides control of the occultor trolley and arc. A variety of field-hardened PC systems have been used, depending on the age and program requirements for the individual instrument. All systems include a GPS card for determining time and location.
by the Marine Physical Laboratory, SIO.