Monday, 12 September 2016

Calibration of the All-Sky Cameras

As nights are getting dark enough to observe the northern light, it is time to perform the yearly absolute calibration of the cameras and imagers which monitor the aurora. This week, scientists from the Finnish Meteorological Institute, EISCAT, the University of Southampton and the University Centre in Svalbard (UNIS) are calibrating their instruments in Tähtelä.

Four all-sky imagers are being calibrated: two ICCDs (Sodankylä and Longyearbyen) and two EMCCDs (Kilpisjärvi and Abisko). The calibration of each instrument takes a couple of hours. It consists in measuring the response curve of the sensor to a known source of light obtained from three lamps illuminating an integrating sphere. The purpose is to be able to convert the number of counts received in each pixel of the sensor into photon flux in rayleigh (1 R = 795,774,716 photons.m-2.s-1.sr-1).

Integrating sphere used to produce the calibrating light source.
Photo: M. Grandin

Since the instruments measure the auroral emission in three wave lenghts (427.8 nm "blue", 557.7 nm "green" and 630.0 nm "red"), the calibration must be done for each of these three channels. In practice, the green and red channels are calibrated during a same procedure, and the blue channel calibration takes place separately. The whole calibration procedure is of course done in a dark room and is controlled remotely.

The Sodankylä ICCD imager set up for calibration.
Photo: M. Grandin
May this auroral season be rich in substorms with clear-sky conditions!

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