The total sky imager (TSI) provides time series of hemispheric sky images during daylight hours and retrievals of fractional sky cover for periods when the solar elevation is greater than 10 degrees. The instrument works by taking an image of the sky with a convex mirror every 30 seconds. The TSI begins capturing images when the sun is 0 degrees above the horizon and begins to process these images into cloud fraction values when the sun is 5 degrees above the horizon. To reduce the saturation of direct sunlight, a black band is placed on the mirror to reduce the sun's direct reflectance. The mirror rotates at the same rate as the sun across the sky. A common problem occurs when the sun position does not match the black band and needs adjusting. The local solar noon image is the best time of day to check the alignment. The noon image is shown with the plots if available. Also, a link to the MPEG movie from the tsimovie datastream is provided on the Diagnostic Plots page if the movie is available. Only the last 3 months of data is available.
The TSI has four datastreams.
tsiskycover – NetCDF data file containing fractional sky cover and sun obscuration by cloud
tsiskyimage – hemispheric sky image every 30 seconds (JPEG)
tsimovie – daily movie of hemispheric sky images (MPEG)
tsicldmask – processed fractional sky cover image every 30 seconds(PNG).
The TSI has four masks in addition to the cloud masks. First is the mask around the sun (region_sun), which has a 25 degree radius. Next is the horizon mask (region_horizon), which is 40 degrees high by 50 degrees wide. Then there is the 50 degree radius zenith mask (region_zenith). Finally, there is the shadowband mask which covers the shadowband. It is defined as a function of the camera to mirror distance.
This portion of the TSI has two simultaneous analysis regions. The first is the standard 80 degree radius (160 degree field of view) used to determine the "count_opaque" and "count_thin" values. The second is the user defined 50 degree radius (100 degree field of view) used to determine the "region_zenith_count_opaque" and "region_zenith_count_thin" values.
List of past problems for this instrument that DO need to be mentioned in your DQA's:
Camera out of focus
Shadow Band Alignment
Water infiltration into lens
Mirror stopped rotating
The mirror stopped rotating and then allowed the full solar intensity into the camera. This overloaded the camera and damaged the CCD. See DQPR 1157.
Rust forming on the mirror
Equatorial TSI Shadowband
The NIM deployment was near the equator, which resulted in the sun moving from the south side of nadir to the north side during the deployment (and visa versa). This requires the shadowband to change from operating on the bottom of the image to the top. In the transition time the shadowband is capable of shading the sun even if it is not in the normal local solar noon vertical position. Although, this transition period should not last longer than a few weeks to a month. During this period the alignment should be monitored carefully to distinguish a shading problem from normal migration between hemispheres. See DQPR 1608.
Missing bird spike
Occasionally, the processes that make the videos will be interrupted. If this occurs, the tasks will be completed when the system is restarted. Subsequently, the video created will likely contain images from the previously interrupted run as well as the ones queued up for that day. This means that part of a previous day's video may be added on to the following day's. This doesn't affect data quality but should be mentioned in the DQA.