Physics of auroral phenomena : proceedings of the 36th Annual seminar, Apatity, 26 February – 01 March, 2013 / [ed. board: A. G. Yahnin, A. A. Mochalov]. - Апатиты : Издательство Кольского научного центра РАН, 2013. - 215 с. : ил., табл.

*Physics o f Auroral Phenomena", Proc. XXXVI Annual Seminar, Apatity, pp. 151 -154, 2013 © Kola Science Centre, Russian Academy of Science, 2013 Polar Geophysical Institute PRACTICE OF CCD CAMERAS’ CALIBRATION BY LED LOW-LIGHT SOURCE B.V. Kozelov1, B.U.E.Brandstrom2, F. Sigemes3, A.V. Roldugin1, S.A. Chemouss1 'Polar Geophysical Institute, Apatity Murmansk region, 184209 Russia Swedish Institute o f Space Physics, Kiruna, Sweden The Kjell Henriksen Observatory, UNIS, Longyearbyen, Norway Abstract.Two identical CCD cameras used for auroral observations (Kozelov et al., 2 0 1 2 ) have been calibrated by LED low-light source PGI-Chemouss-38AM. The calibration factor as a function o f the wavelength and the camera gain was deduced. Previously the light source PGI-Chemouss-38AM (SID 105) was absolutely calibrated during the intercalibrational workshop of optical low light sources (Brandstromet al., 2 0 1 2 ), and it was found some issues motivated additional studies of the light source. The current-voltage and emission intensity characteristics o f the light source have been measured. It have been found recommendation for the light source users: i) the current value should be measured; ii) the light source should be equipped by external stabilized electric power source; iii) the setting “3” of the lamp and region o f wavelengths < 500 nm should be avoided due to peaked spectrum. 1. Introduction The Multiscale Aurora Imaging Network (MAIN) system contains 4 cameras located near Apatity, Kola Peninsula, Russia (Kozelov et al., 2012). Two of these cameras are identical, they are based onAVT Guppy F- 044B NIR (1/2”CCD) digital cameras with Fujinon HF25HA-1B (l:1.4/25mm) lens and additionally equipped by glass filter on blue-green region.These cameras are using as a stereoscopic pair for measurements of altitude o f auroral luminosity. Comparison o f the altitude with results of numerical models can give us an important geophysical characteristic - the energy o f electrons precipitated to the ionosphere and excited the aurora. Knowledge of absolute intensity of auroral emission can be used to deduce the second characteristics - the energy flux of precipitated electron. More details about geophysical application o f the camera system were presented in (Kozelov et al., 2012). Each auroral camera contains of not only the CCD camera, but lens, optical filter(s) and income window o f camera dome, therefore this rather complicated optical system as a whole is a subject of special absolutely calibration. The “black” (dark current) and “white” fields needed to compensate the CCD sensor inhomogeneity are also usually measured during calibration. The absolute calibration o f light intensity observed by CCD camera can be formally described by the expression: J (x ,y ) = Here: ( x,y ) are the pixel coordinates (column and line numbers) in CCD matrix, Cobs(x,y) [CTS s'1] is the raw count rates in the CCD pixels, CBF(x,y) and CWf(x,y) are “black” and “white” fields, CBF(x0,yo) and Clrf{x0,y0) are average values of count rates in calibration region of “black” and “white” fields,^[R s CTS'1] is a calibration factor. Here we use traditional units for aurora light intensity 1 Rayleighs = 10 6 photons cm '2 s'1. The quantity inthe curly braces of (l)wewill refer as Q(xj>). This matrix characterizes the relative inhomogeneity of the CCD response due to given optical system and individual features of the CCD pixels. In common case for CCD cameras all quantities should be dependent on gain control, and in our case Kx= FAcxp(-kjg), (2) whereg is the camera gain level, Fk and £яаге camera- type-dependent constants, which should be obtained during calibration by test source. So, we need to measure the following quantities: Fh kh Q(x,y, g) and CBF(x,y, g). First two constants have alsobeen dependent on the wavelength Я under consideration, but the rest two are not. The only first quantity gives information about absolute intensity.As a result of ( 1 ) we will have intensity distribution J(x,y) in Rayleighs for given wavelength Л. To obtain the calibrated intensity for another wavelength A we need to have correspondent values F x and кл. To obtain the^values gives absolute value of the light intensity and it we use the LED-based low-light source named as ‘PGI-Chemouss-38AM’, referred as SID 105 in paper (Brandstrom et al., 2012). The light source has been additionally tested and equipped by stabilized electric power source. Here we report: i) the results of calibration of two identical cameras from MAIN system, ii) the results of additional testing of the light source SID 105 and its power source. 2. The cameras’ calibration Two identical cameras (named hereinafter as “G -l” and “G-2”) used in the MAIN system are based on AVT Guppy F-044B NIR (1/2”CCD) digital cameras which contained type 1 / 2 (diag. 8 mm) interlaced sensors 151