FZ
Focused Sun Observation
As the sun drops lower in the sky, the high Mu values result in an increasing amount of scattered radiation in the field of view of a DS observation. The FZ command allows ozone to be determined at solar angles greater than which can be achieved with a traditional DS. Scattered radiation is measured by offsetting the solar image by 1 degree from the entrance slit, and observing radiation in the vicinity of the solar disk. This scattered radiation is then subtracted from the direct sun result to give a more accurate measure of the direct radiation, before the Ozone determination is made. This method will give reasonably accurate O3 values to ZA of 85° (Air Mass =8.2) as compared to the DS which has cut-off angles of 70 to 80° (Air Mass 3.2 to 5) without corrections. This is a useful commandat high latitudes and is normally not used where DS readings are available. For the FZ measurement the iris is closed, FW#1 is set to position 3, and FW#2 is set to position 5. The minimum zenith angle at which an FZ may be taken is 73°.
See also DS command.