• RSS-1024. Covers 360-1100 nm. Resolution of 0.3 nm FWHM at 360 nm increasing to 3 nm at 1100 nm. The far out-of-band (OOB) rejection ratio is better than 105. Wavelength stability in operation is ±0.1 pixel (±0.02 nm at 360 nm to ±0.2 nm at 1100 nm).
  
• UVRSS-1024. Covers 290-360 nm. The standard model has resolution of 0.3 nm FWHM at 300 nm increasing only slightly through the spectral range, and far OOB rejection ratio between 2 and 5 x 107. Wavelength stability during field operation is ±0.1 pixel (±0.01 nm at 290 nm to ±0.02 nm at 360 nm). The standard 0.3 nm slit width provides the highest OOB; an available slit width option produces effective bandwidths of 0.6 nm FWHM, but degrades OOB rejection.

• Retrieval of atmospheric column quantities for species such as aerosols, NO2 , H2O, and 03
• Field calibration verification via Langley regression
  
• Determination of optical depths
  
• Spectral reflectance and BDRF

Light enters the RSS through an optimized cosine response-optimized diffuser fore optic. An external shadowband alternately shades and exposes the diffuser, making sub one second speed direct-normal, diffuse-horizontal, and total-horizontal spectral irradiance measurements. Smapling rates can be as fast as once every 30 seconds. The technique provides direct-diffuse and direct-total ratios that are fully independent of calibration.

Once inside the diffuser, light passes through an entrance slit, through an entrance lens into dual prisms which refract the light. Finally, a camera lens focuses the spectrum onto a Peltier-cooled, 1024x256 pixel element astronomy-grade slow scan CCD. Careful attention to anti reflection coatings and a large volume of space around the spectrograph itself helps give the system state-of-the- art out of bandwidth performance.

The CCD analog subsystem employs dual slopeintegration and column binning. CCD signals are acquired by a precision 16-bit analog-to-digital processing subsystem controlled by an embedded CPU. A second CPU manages the shadowband, and a third 32 bit CPU manages data calibration, database storage and the web server.

A precision multi-channel analog proportional-differential-derivative thermal management subsystem precisely maintains the temperature of five different thermal zones within the spectrograph. Its job is to keep the temperature of internal optics held stable and above ambient temperature fluctuations. Any environmentally-induced temperature fluctuations would serve to "tune" the wavelength response. In addition, this subsystem also controls the shutter and maintains the CCD’s thermoelectric coolers at about 15°C on the RSS and near 0° on the UVRSS. For data presentation, the Data Visualization Engine and YESDAQ database permit data access via TCP/IP to any web browser.

• Prisms offer excellent far out-of-band (OOB) rejection. Especially in the UVRSS, OOB rejection is critical since low energy measurements in the UV-B are contaminated with intense sunlight light at longer visible wavelengths.
  
• Prisms can span a much wider spectral range while maintaining uniform throughput. In the visible/NIR RSS, a 92% throughput is possible over the region from 360 to 1100.
  
• Prism throughput is highly stable over time vs. gratings. The RSS prisms are made from amorphous fused silica (synthetic quartz)—an extremely tough and durable material that does not degrade or change with radiation or time. Gratings suffer from oxidation effects that degrade their diffraction efficiency over time.
  
• It is much easier to achieve and maintain wavelength stability with a prism. In gratings (like any mirror), the derivative of the deviation angle is two with respect to an angular shift of the grating: the spectrum is shifted by two times the amount of the angular deviation. In an equilateral prism, the same derivative of the deviation angle is zero at the minimum deviation criterion.
• Unlike grating spectrograph designs, prism spectrographs do not require additional diffraction order-sorting filters or optics. These filters can degrade over time.
  
• Gratings in scanning monochromators using photomultiplier detectors must be moved via complex and often fragile mechanisms and typically take several minutes to complete a scan, during which time the sky condition can change dramatically. The motor and mechanical translation stages increases the cost of the instrument substantially and introduces uncertainties related to the wavelength calibration as well as operational reliability

A precision laser machined slit is supplied, and is externally located to permit exchanges by the user. The supplied slit is optimized for outdoor atmospheric work, and may be exchanged if more or less signal is available. The general engineering tradeoff is spectral resolution vs. light gathering (for SNR). However, because the aberration limit for the UVRSS is approximately 0.2 nm FWHM, attempts to drop below approximately 0.3 nm FWHM by decreasing the slit size only results in a loss of light and subsequent loss of system SNR. As the slit size is made larger to gain throughput the expense is a
corresponding loss in resolution.

Optical components within the system operate in a dry nitrogen purged environment to eliminate potential condensation on the surface of the cooled internal CCD that would blur the refracted light and ultimately contaminate it. Internal humidity and pressure are continuously monitored to ensure the integrity of the seal over its lifetime.