As part of an international collaboration with the Geophysical Institute of the University of Alaska, we have developed two all-sky imagers (CRL-ASIs). A sensitivity calibration of the CRL-ASIs was performed using an integrating sphere belonging to the National Institute of Polar Research (NIPR). The two-dimensional sensitivities of the CRL-ASIs produced symmetrical distributions. Using this sensitivity data, we converted airglow/aurora images into two-dimensional distributions of absolute intensity. The sensitivity of the CRL-ASIs was measured for 13 wavelengths between 427.8 nm and 866.5 nm, and the relationship between the sensitivity and the wavelength was investigated for both imagers. The peak sensitivity occurred at about 550 nm

Mamoru Ishii

Masa-yuki  Yamamoto

Masaki Ejiri

Minoru Kubota

Shu Takeshita

Yasuhiro Murayama

National Institute of Polar Research Repository

Adv. Polar Upper Atmos. Res .. 16, l 73-180, 2002 © 2002 National Institute of Polar Research Calibration of CRL all-sky imagers using an integrating sphere Masa-yuki Yamamoto\ Minoru Kubota', Shu Takeshita', Mamoru Ishii', Yasuhiro Murayama' and Masaki Ejiri' 'Communications Research Laboratory, 4-2-1 Nukui-kitamachi, Koganei-shi, Tokyo 184-8795 'National Institute of Polar Research, Kaga 1-chome, ltabashi-ku, Tokyo 173-8515 Abstract: As part of an international collaboration with the Geophysical Institute of the University of Alaska, we have developed two all-sky imagers (CRL-AS1s). A sensitivity calibration of the CRL-ASls was performed using an integrating sphere belonging to the National Institute of Polar Research (NIPR). The two-dimensional sensitivities of the CRL-ASls produced symmetrical distri­butions. Using this sensitivity data, we converted airglow /aurora images into two-dimensional distributions of absolute intensity. The sensitivity of the CRL­AS!s was measured for l 3 wavelengths between 427.8 nm and 866.5 nm, and the relationship between the sensitivity and the wavelength was investigated for both imagers. The peak sensitivity occurred at about 550 nm. 1. Introduction Multiple wavelength observations of aurora/airglow emissions obtained with an all-sky imager, consisting of a bare cooled CCD camera and interference filters with a narrow band passage, have been frequently used as a practical method of producing sensitive images during the past decade (e.g., Taylor et al., 1995; Ejiri et al., 1997; Kubota et al., 2000). The Joint Program of the Middle and Upper Atmosphere Observation (Alaska Project) was begun in 1995 by the Communications Research Laboratory (CRL) and the Geophysical Institute, University of Alaska Fairbanks, USA. As part of this project, we developed two all-sky imagers (CRL-ASls). Continuous observation of aurora/airglow emissions by the CRL-ASls began in October 2000, at the Poker Flat Research Range, Alaska (Latitude: 65. ITN, Longitude: 147.43°E, and Geomagnetic latitude: 65.60"N). Since the two CRL-AS!s are located at the same observation site, we can simultaneously observe auroral images of two independent wavelengths with no time separation. Using this advantage, we plan to estimate the two-dimensional energy deposition rate of precipitating auroral particles. The energy flux and the characteristic energy of the precipitating particles can be estimated using the absolute intensity of 427.8 nm emissions and the ratio of auroral emission intensities of two different wavelengths, respectively (Rees and Luckey, 1974; Rees et al., 1988; Ono, 1993). For this estimation, the absolute intensity distribution of the auroral emissions must be obtained. Therefore, we first 173 174 M.-Y. Yamamoto et al calibrated the sensitivitie of the CRL-ASls using an integrating sphere. Absolute two-dimensional caJibration of an all-sky imaging system can be obtained u ing an integrating sphere, whicb has a uniform light source that fills a 2n-steradian field of view. The uniformjty of the radiance in the 1.9-m integrating sphere, which belongs to the National Institute of Polar Research (NI PR), is better than ± 5% for a 2n field of view (Okano et al., 1998). Sbiokawa et al. (2000) reported calibration results for three imagers developed by the Solar-Terrestrial Environment Laboratory of Nagoya University. In their repot , they mentioned that filters with highly flattened image-quality surfaces may cause serious Newton's Ring patterns on the final images. In our study, we measured the two-dimensional sensitivities of the CRL-ASJs at J3 wavelengths and investigated the relationship between sensitivity and wavelength. 2. Instrumentation and calibration method Each CRL-ASl consists of two sections: an optical section, with a wide-field lens (Nikon, 6 mm, Fl .4, FOY= 180°), interference filters, and tele-centric lens system; and a detector section, with a bare cooled CCD camera (Pixel Vision, 512 x 512 pixels, back­illuminated). Both CRL-ASTs have the same configuration. Five filters can be installed in the filter turret of each CRL-ASJ. Figure I shows one of the CRL-ASls being calibrated with the integrating sphere (Optronic Laboratories Inc., OL462-80A). The integrating sphere has an internal diameter of 1.9 m. Jts specially coated inner surface was carefully manufactured to achieve uniform reflection. The lamp luminosity of the inte­grating sphere was constantly monitored during the experiment. The transmi sion profiles Fig. 1. CRL-ASI being calibrated with the 1.9-m integrating sphere from NIPR. Calibration of CRL all-sky imagers 175 of the interference filters were measured using the auto spectra-photometer at NIPR (Hitachi. Ltd., U-3300). During the aurora/airglow emission measurements using the CRL-ASI, the observed value C AG [ Count] was given by the following equation (e.g., Shiokawa et al., 2000; Kubota et al., 2001 ): ( CAc;) =a· E Ali 1 ·� Top< (,l )) • (Scco(,l )) · {(/A<; (,l )) + (/ sd-l))} • TAG (,l)d,l, (I) where a[ Count/ Rayleigh/s] is a constant, EAG[s] is the exposure time, To,,[%] is the transmission of the optical section without the interference filter, Scrn[%] is the sensitivity of the CCD camera (determined by multiplying the quantum efficiency of the CCD [electrons/ photon J and the conversion efficiency [Count/ electron J ), / AG [ Rayleigh J is the absolute intensity of the aurora/airglow emission, / HG [Rayleigh/nm J is the absolute intensity of the background continuum, and TAG[%] is the transmission of the interference filter. The values enclosed by < > indicate the two-dimensional variables at the focal plane. In general, TAG [ ,l J becomes a two-dimensional variable because the filter trans­mission peak is shifted towards shorter wavelength at the edge of an image as a result of oblique incoming light. In eq. (I), however, TAG[A] is described as a fixed value at the center of the image. This point will be discussed in grater detail in the Discussion section. From eq. (I). the following equation can be derived: (CG)= a• EA<,l�< Tort (,l)) • (Scrn(,l)) · {(/AG (,l))o'(,1-11Au)+(/sG(,l))} • TAc;(,l )d,l =a• ( Top< Cl Ac,))· (Scrn {,l Ac;)) · EAG {(/AG Cl Au))· TAG (,l,G) +<Is,; ( A Ac,))· 100 T,G ( ,l )d,l }, (2) where the variation of T0p, and S crn with different wavelengths within the filter pass band is assumed to be negligible. To obtain the absolute intensity /Au, simultaneous measurement of the background intensity I BG is needed to enable the background to be subtracted. T,c;(AAu), 1= TA<,(,l)d,l, and a•< T0,1(,lAG)) • (ScrnClAG)) are required for the calibration: TAG(,l,c;) and (00T,0(,l)d,l can be obtained by measuring the transmission profiles of Jo the interference filters, while a • < T"'' (AAG)) • (Scrn (,lAc;)) [Count/ Rayleigh/ s J can be derived using the integrating sphere in the manner described below. The integrating-sphere image < C,m,,) [ Count] obtained by the CRL-ASis can be described as where E,m,,[ s J is the exposure time and (f,,,.,,(,l)) [Rayleigh/nm J is the luminosity of the integrating sphere. Since <I,m,,Cl )) has been carefully determined beforehand, a •  (To,,Cl AG)) • (Scrn(AAG)) can be calculated using eq. (3). 176 M.-Y. Yamamoto et al. 3. Results Figure 2a shows the two-dimensional distribution of CRL-ASl sensitivity at a wavelength of 557.7 nm. The sensitivity drops symmetrically from the center toward the edge of the image due to vignetting. Figure 2b shows two cross sections of the sensitivity along the horizontal and venical lines that cross at the center of Fig. 2a. Both cross sections are nearly identical, suggesting that the alignment of the CRL-AST optical system was well adjusted and that the CRL-ASI can be used for all-sky imaging with excellent results. The sensitivity values along the vettical cross line are slightly higher than the (a) CRL-ASI system-1 Wavelength: 557.7 nm 200 100 0 -100 -200 -200 -100 0 100 200 (b) Cross sections of sensitivity � 0.20 .----�---�---�--�----=--�----. °E 0.15 ::J 0 � 0.10 -� -I; 'vi 0.05 C Q) (/) 0.00��---�--�---�--�� -200 -100 0 100 Radial position [Pixel) 200 Fig. 2. (a) Two-dimensional distribution of the CRL-ASI sensitivity (at 557.7 nm). The circle seen in the image represents the joint where the two hemispheres that form the integrating sphere meet. (b) Cross sections of the sensitivity data shown in (a). The solid and broken lines represent the cross sections of the sensitivily data along the horizontal and vertical fines that cross at the center of !he image in (a), respectively. The joint where the hemispheres meel appears as a dip at about ± 170 pixels. The small "bump" seen at - 250 pixels in the vertical cross section represents a localized nonuniformity of the inner swface of the integrating sphere. (a) 0.25 en ix 0.20 § 0.15 0 � :=, :� 'iii 0.10 C Q) (/) 0.05� O.OJ 300 (b) 0.25 L · t 0.20 � § 0.15 0 � :=, ·;;;: :� 0.10 C Q) (/) 0.05 Calibration of CRL all-sky imagers .... ······ 400 500 I'"" 'I" .... ······ ... 600 700 800 Wavelength [nm] II 'I " " I ii ' I I ., ......... ,... ,.J 900 1000 " � 1 , 1 4 j 0.00����������������������� 300 400 500 600 700 Wavelength [nm] 800 900 1000 Fig. J. (a) Relationship between sensitivity and wavelength for CRL-ASI system-I. The dolled line shows the reference sensitivity derived from the transmission rate of the optical system and the quantum efficiency of the CCD device, based on data provided by the ma11ufac1urer. (b) Relationship between sensitivity and wavelength for CRL-ASI system-2. 177 values along the horizontal one between -240 and -20 pixels, as seen in Fig. 2b, and a "bump" occurs on the edge of the image at -250 pixels. The higher value een at the bottom edge of the image seems to be caused by a localized nonuniformity of the inner surface radiance of the integrating sphere. However, the difference between the two cross sections is less than 7% if the "bump" is excluded. Since the integrating sphere consi t of two hemispheres, the mating joint of the two hemispheres is seen as small dips at about 178 M.-Y. Yamamoto et al Table 1. Listing of filter specifications. All filters were made by Andover Co. with options of 'type 3'. 'block to Fl R ', and 'non image quality: Center FWHM wavelength (nm) T"; U";)(%J Objective emission (nm) 427.9 2.8 41.0 N,+ ING(O.l) 481.25 1.7 45.1 Background for H,6 485. 75 1.7 46.8 H,6 557.6 2.0 52.8 OJ 572.2 2.0 61.6 background 589.05 2.3 59.7 Na 630.0 2.2 68.6 OJ 670.65 2.9 53.9 N, 732.05 2.1 54.8 o+ 770.0 3.0 59.6 K 777.6 2.8 62.5 01 844.7 2.6 54.5 OJ 866.5 9.4 82.8 0,(0.1) ± 170 pixels in the cross sections, which is also seen as a circle on the image. However, this does not seriously affect the results of the observation because the dips can be removed using a smoothing process over a region of 10 X 10 pixels. Figure 3a shows the variations in sensitivity between 427.8 nm and 866.5 nm for CRL-ASI system-I, and Fig. 3b shows the same data for CRL-ASI system-2. Each sensitivity value indicates the averaged value for !Ox 10 pixels at the center of the image (zenith direction). The maximum sensitivity value was observed at about 557.7 nm. A list of the 13 interference filters used in the present investigation is shown in Table I. The sensitivity of system-2 was somewhat higher than that of system-I, though the two CRL-AS!s were manufactured using the same design. However, the sensitivity distribu­tions of the two CRL-ASTs indicate that their features are similar. 4. Discussions In eq. (]), TAc; (,l) can be expressed as a fixed value at the image center if AA<, is situated on the shorter wavelength side of the flat portion of the filter transmission. Most of the Andover Type 3 filters listed in Table I satisfy this condition. For the OJ (557.7 nm) and Hp (485.9 nm) emissions, however, the corresponding filters fail to meet this condition. Nevertheless, the absolute intensity of these emissions can be correctly calibrated at almost any portion of the image except for at the edge. As is well known, filter transmission shifts with temperature. This may cause the absolute calibration factor to vary when the imagers are used at a different temperature from that in the integrating sphere. In the present case, however, the shift was negligible because the imagers were placed in an air-conditioned room at room temperature in the Calibration of CRL all-sky imagers 1 79 observatory building at Poker Flat. By comparing Figs. 3a and 3b, the sensitivity of CRL-ASI system-2 was found to be about 1 .4 times higher than that of CRL-ASI system-I at each wavelength. This difference seems to be caused by a difference in the gain setting of the conversion efficiency from the number of electrons to the output count rate. In general, the quality of an image can be evaluated by the S/N ratio. The difference in sensitivity between the two CRL-AS!s is not considered to affect the S/N ratio of the image for the following reason. Since the noise of the imager system consists of thermal noise, readout noise, and statistical random noise, the S/N ratio of the image data is determined before the conversion process. Namely, the signal and noise are amplified together at the same rate in the conversion process, so the S/N ratio remains constant and is independent of the conversion process' gain setting. Using the sensitivity data, the optimum exposure time for the aurora/airglow emis­sions can be determined for each wavelength. The exposure time is determined by considering the time resolution and S/N ratio. For the present CRL-ASI set-up, the dark noise is about 0.5 [ Counts/pixel/s] and the readout noise is about 12 [ Counts, rms] . The statistical random noise associated with the quantum effect in the CCD is equal to the square root of the signal count rate. In the case of a typical green aurora with an intensity of 3 [ k Rayleigh J when observed at a wavelength of 557.7 nm, a high-quality image with an S/N ratio of 35 can be obtained using a 3-second exposure. For an aurora with an intensity of 500 [ Rayleigh J at a wavelength of 844.6 nm, an image with an S/N ratio of 8 can be obtained using a 45-second exposure. 5. Summary The sensitivity of two CRL-AS!s was calibrated using an integrating sphere. The two-dimensional sensitivities of the CRL-ASls exhibited symmetrical distributions .  Calibration data were obtained for 13 wavelengths between 427.8 nm and 866.5 nm, and the relationship between sensitivity and wavelength was analyzed for each CRL-ASI. The absolute intensity of the aurora/airglow emission can be calculated from the CRL-ASI observations using the results of the accurate sensitivity calibration. The two-dimensional absolute intensity of the aurora emission is useful for estimating the energy flux and characteristic energy of the precipitating auroral particles. Using the filters listed in Table I, observations of aurora/airglow emissions at altitudes ranging from the mesopause region to the F-region can be used to quantitatively investigate atmospheric and ionospheric dynamics in this region. Acknowledgments The authors would like to express their sincere thanks to an anonymous referee for his/her kind comments and suggestions. The editor thanks Dr. S. Okano for his help in evaluating this paper. 1 80 M.-Y. Yamamoto et al. References Ejiri, M., Okano. S., Okada. M., Taguchi, M. and Takeshita. S. ( 1997): All sky imager observation ot aurora and airglow at South Pole: System design and the initial test results (extended abstract). Proc. NIPR Symp. Upper Atmos. Phys., 11, 1 59-162. Kubota. M., Shiokawa, K .• Ejiri. M.K., Otsuka. Y .. Ogawa, T.. Sakanoi, T., Fukunishi, H., Yamamoto, M .. Fukao. S. and Saito, A. (2000): Traveling ionospheric disturbances observed in the OI 630-nm nightglow images over Japan by using a multipoint imager netv ..'ork during the FRONT campaign. Geophys. Res. Lett.. 27. 4037-4040. Kubota, M., Fukunishi. H. and Okano. S. (2001 ): Characteristics of medium- and large-scale TIDs over Japan derived from 01 630-nm nightglow observation. Earth Planets Space, 53, 741- 751. Okano, S., Takeshita. S. and Taguchi, M. ( 1998): Absolute calibration system at NIPR for aurora/ airglow measurements using a 1 .9-m integration sphere. Proc. 24th Annual European meeting on Atmospheric Studies by Optical method, 30. Ono, T. ( 1993 ) :  Derivation of energy parameters of precipitating auroral electrons by using the intensity ratios of auroral emissions. J. Geomagn. Geoelectr., 45, 455-472. Rees. M.H. and Luckey. D. ( 1974): Auroral electron energy derived from ratio of spectroscopic emissions I .  Model computations. J. Gcophys. Res., 79, 5 181 - 5186. Rees. M.H .. Lummcrzheim. D., Roble, R.G., Winningham, J.D., Craven, J.D. and Frank L.A. (1988): Auroral energy deposition rate, characteristic electron energy, and ionospheric parameters derived from Dynamics Explorer I images. J. Geophys. Res .. 93. 12841-12860. Shiokawa. K., Katoh, Y .. Satoh, M .. Ejiri, M.K. and Ogawa, T. (2000): Integrating-sphere calibration of all-sky cameras for nightglow measurements. Adv. Space Res .. 26. 1025-1028. Taylor. M.J .. Bishop, M.B. and Taylor, V. (1995): All-sky measurements of short period waves imaged in the 01(557.7 nm). Na(589.2 nm) and near infrared OH and 00(0,1) nightglow em1ss10ns during the ALOHA-93 campaign. Geophys. Res. Lett., 22, 2833 2836. (Received December 20, 2001; Revised manuscript accepted April 4, 2002) 

Calibration of CRL all-sky imagers using an integrating sphere

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Calibration of CRL all-sky imagers using an integrating sphere

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