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Wizard: New observation system of zodiacal light in Kobe University
WIZARD: New observation system of zodiacal light in Kobe University M.Ishiguro ~ T.Mukai b, R.Nakamura c, F.Usui, and M.Ueno d ~The Institute of Space and Astronautical Science(ISAS), Yoshinodai 3-1-1, Sagamihara, Kanagawa 229-8510, Japan bGraduate School of Science and Technology, Kobe University, Rokko-dai-cho 1-1,Nada, Kobe 657-8501, Japan CNational Space Development Agency of Japan (NASDA), Harumi 1-8-10, Chuou-ku, Tokyo 104-6023, Japan dGraduate School of Arts and Sciences, University of Tokyo, Komaba 3-8-1, Meguro, Tokyo 153-8902, Japan We describe a new system(WIZARD 1) for zodiacal light observations, developed by a group in Kobe University, Japan. Since the zodiacal light is faint and widespread all over the sky, the system consists of a very sensitive CCD camera with a quantum efficiency of 90% at 500 nm and a wide angle lens with a FOV of 92~ ~ WIZARD will be able to measure the absolute brightness of diffuse sky in visible wavelengths. The zodiacal component will be separated from the integrated star light, airglow continuum and the scattered light in the atmosphere, in the data reduction procedure. We report the design of WIZARD and the expected performance 2. 1. I N T R O D U C T I O N The zodiacal light is the diffuse sunlight scattered by interplanetary dust particles. It is a difficult task to do high quality photometry of zodiacal light from a ground-based observatory because of its faintness as well as the contamination of diffuse light sources. Active ground-based studies were done in the 1960's and 1970's by using a photo-multiplier attached to a telescope on a high altitude mountain (see e.g. [1]). Their efforts have provided us with an overview of the zodiacal light, but they have also revealed limits of the ground-based observation with photomultiplier mounted on a telescope i.e. low spatial resolution and enormous observation time, compared with the spaceborne observations (see e.g. [21). About 20 years later, the high sensitivity and imaging capability of the Charge Coupled Device (CCD) has enabled us to obtain a 'snapshot' of diffuse faint objects with a portable 1Wide-field Imager of Z___odiacallight with ARray D_etector 2We obtained the first image from WIZARD in 2001 at Mauna Kea(4200 m, Hawaii) in collaboration with the SUBARU Telescope. In this paper, we describe the design and expected performance of WIZARD in this developing phase. - 98-
WIZARD: New observation system of zodiacal light in Kobe University
Figure 1. Plan for the lens unit. Rays at the center and edge of frame are drawn. Table 1 Comparison of the observation systems New Instruments f (mm) (F) 32.5(2.8) QE(%) at 500 nm 90 FOV(o) 92 x 46 Spatial resolution (arcmin pixe1-1) 1.35 Peak wavelength (nm) 480 Uncertainty of the zero point (e-) < 5 *cited from [4]
Previous Instruments* 24.0(2.8) 20 32 x 21 2.50 440 10-100
and inexpensive system [3]. It appeared however, that there were many problems in a CCD's properties i.e. dark current, read-out noise, flat fielding, and so on. By taking flat field frames inside a integrating sphere, we have succeeded to find the dust bands from the ground [4]. It is known that there still remains a problem of how to reduce the noise originating from the CCD itself. In this paper, we will report the development of new system with much smaller instrumental noise. 2. D E S C R I P T I O N
OF THE
SYSTEM
CCD camera head As described above, the stability of the CCD camera is a critical factor in measuring the absolute brightness. A fluctuation in the temperature of the CCD chip will cause uncertainty in the zero point of the photometric system, and its sensitivity. We have employed a liquid-N2 cooled CCD chip (EEV CCD 42-80) with a compact dewar (,,~ 1 kg) for WIZARD. The chip has a huge imaging area (27.6x55.3 mm), 2048x4096 pixels, as well as high sensitivity (90% at 500 nm). Since our previous CCD chip was cooled electronically, that system was consequently subject to the unclear temperature 2.1.
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Figure 2. Transmission of the filter. The peak wavelength and FWHM are 480 nm and ~45 nm respectively.
variation. Dark noise of the new chip will be negligible. Also, the readout noise will be significantly reduced by adopting an excellent device control system; COGITO3 [5]. At 190 K, measured readout noise and dark current is 20 e- and <<1 e- respectively. 2.2. O P T I C S We used a small F number in a design of lens unit (where F is the ratio of focal length to aperture) because an observed flux of diffuse light source is proportional to F -2. Moreover, it is preferable to cover the sky from the zenith to the horizon in the same frame in order to estimate the brightness of atmospheric diffuse light at the same time. The lens is developed by the Genesia Corporation (Figure 1). Its F value and a focal length f are 2.8 and 32.5 mm respectively. With the EEV CCD 42-80, it gives a FOV of 92~ ~ The optical filter is designed to avoid the prominent airglow emissions (see Figure 2). Both of the optical filter and shutter are set at pupil position. Due to an oblique path from off-optical axis, the wavelength of light transmitted through the interference filter is shifted toward a shorter wavelength (e.g. roughly 10 nm for the 50 degree field position). To avoid this effect, the filter is made of colored glass. The whole lens system is mounted on the thick honeycomb plate and supported by the shutter and filter box. The dewar is also mounted on the plate (see Figure 3). This honeycomb plate is fixed on an equatorial mount, which is useful to track the stars in the region between 48 ~ N and 48 ~ S terrestrial latitude. 3. A N T I C I P A T E D P E R F O R M A N C E
AND FUTURE PLANS
In this section, we estimate an expected count of zodiacal light observed by WIZARD based on the table provided in [6]. The table represents an annually averaged brightness
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WIZARD: New observation system of zodiacal light in Kobe University
Figure 3. Side view of the CCD system.
with low spatial resolution. Kelsall et al. [2] reported a detail model of the zodiacal cloud based on COBE infrared observations. We assume the dust distribution of the visible zodiacal light is the same as that of the infrared emission, and employ this model as a visible zodiacal cloud model. We use a volume scattering phase function given by Hong [7], and adjust the absolute brightness referring to Levasseur-Regourd and Dumont [6]. As a result, we can simulate not only the absolute brightness in the visible band, but also the minute spatial structures, which are absent in the original table. Taking into account the photon and read-out noise, we estimate an observed intensity, which includes the zodiacal light, by the new CCD system at Mauna Kea, Hawaii (4200 m). An optical depth for the diffuse light is assumed to be 0.10, and the brightness of the airglow and scattered light by the Earth's atmosphere are assumed to be 30 and 12 Sx0| respectively, at the zenith. These are the typical values deduced from our previous observations. The brightness of airglow and scattered light are extrapolated by the van Rhijn function and Dumont's formula (see e.g. [4]). Integrated star brightness is assumed to be 20Sx0| which is independent of the galactic coordinate. The observed time is set to the end of astronomical twilight (19:08 HST on December 22). The dots in Figure 4 show the anticipated brightness of the night sky observed by the new CCD system with an exposure time of 5 rain. It should be noted that these are the results of a simulation with 1.35'xl.35' spatial resolution. In the case where the pixels are combined to give same resolution as IRAS ZOHF (20'x20'), the noise was reduced to ,-~ 1 / ~ (lines in Figure 4). It is apparent that even a five minute exposure will allow us to detect asteroidal dust bands near the ecliptic plane (bump structures around/3 ,,~ - 1 ~ and the north-south asymmetry of zodiacal light brightness. This new observation system will also yield information on the brightness of airglow -101-
M. Ishiguro et aL
Figure 4. Scattered dots denote the expected intensity observed by the new CCD system. Each line corresponds to the combined data with 20'x20' resolution.
continuum resulted from chemiluminescence of NO2 [8], and atmospheric diffuse light produced mainly by the scattering from byproduct aerosol particles [9]. Acknowledgements. First light observations by WIZARD were supported by a colleague from the SUBARU telescope. We thank Dr. M. Nakagiri, K. Sekiguchi and A. Miyashita (NAOJ) for help with our observations at Mauna Kea. We also thank Dr. N. Takeyama (Genesia Corporation) and S.M. Kwon for useful comments and discussion. REFERENCES
R. Dumont, Astron. Astrophys. 38 (1975) 405. 1. 2. T. Kelsall, J.L. Weiland, B.A. Franz, W.T. Reach, R.G. Arendt, E. Dwek, H.T. Freudenreich, M.G. Hauser, S.H. Moseley, N.P. Odegard, R.F. Silverberg and E.L. Wright, Astrophys. J. 508 (1998) 44. 3. J.F. James, T. Mukai, T. Watanabe, M. Ishiguro and R. Nakamura, Mon. Not. R. astr. Soc. 288 (1997) 1022. 4. M. Ishiguro, R. Nakamura, Y. Fujii, K. Morishige, H. Yano, H. Yasuda, S. Yokogawa and T. Mukai, Astrophys. J. 511 (1999) 432. 5. M. Ueno and T. Wada, COGITO-3; A flexible imaging device control system, in prep. 6. A.C. Levasseur-Regourd and R. Dumont, Astron. Astrophys. 84 (1980) 277. 7. S.S. Hong, Astron. Astrophys. 146 (1985) 67. 8. S.B. Mende, G.R. Swenson, S.P. Geller, R.A. Viereck, E. Murad and C.P. Pike, J. Geophys. Res. 98 (1993) 19117. 9. S.S. Hong, S.M. Kwon, Y.-S. Park and C. Park, Earth Planets Space 50 (1998) 487.
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WIZARD: New observation system of zodiacal light in Kobe University
Figure 1. Plan for the lens unit. Rays at the center and edge of frame are drawn. Table 1 Comparison of the observation systems New Instruments f (mm) (F) 32.5(2.8) QE(%) at 500 nm 90 FOV(o) 92 x 46 Spatial resolution (arcmin pixe1-1) 1.35 Peak wavelength (nm) 480 Uncertainty of the zero point (e-) < 5 *cited from [4]
Previous Instruments* 24.0(2.8) 20 32 x 21 2.50 440 10-100
and inexpensive system [3]. It appeared however, that there were many problems in a CCD's properties i.e. dark current, read-out noise, flat fielding, and so on. By taking flat field frames inside a integrating sphere, we have succeeded to find the dust bands from the ground [4]. It is known that there still remains a problem of how to reduce the noise originating from the CCD itself. In this paper, we will report the development of new system with much smaller instrumental noise. 2. D E S C R I P T I O N
OF THE
SYSTEM
CCD camera head As described above, the stability of the CCD camera is a critical factor in measuring the absolute brightness. A fluctuation in the temperature of the CCD chip will cause uncertainty in the zero point of the photometric system, and its sensitivity. We have employed a liquid-N2 cooled CCD chip (EEV CCD 42-80) with a compact dewar (,,~ 1 kg) for WIZARD. The chip has a huge imaging area (27.6x55.3 mm), 2048x4096 pixels, as well as high sensitivity (90% at 500 nm). Since our previous CCD chip was cooled electronically, that system was consequently subject to the unclear temperature 2.1.
- 99-
M. Ishiguro et aL
100
80 v r
0 if} if;
E r
60
40
t~ t_
20
i
300
400
500
600
700
i
i
800
900
1000
Wavelength (nm)
Figure 2. Transmission of the filter. The peak wavelength and FWHM are 480 nm and ~45 nm respectively.
variation. Dark noise of the new chip will be negligible. Also, the readout noise will be significantly reduced by adopting an excellent device control system; COGITO3 [5]. At 190 K, measured readout noise and dark current is 20 e- and <<1 e- respectively. 2.2. O P T I C S We used a small F number in a design of lens unit (where F is the ratio of focal length to aperture) because an observed flux of diffuse light source is proportional to F -2. Moreover, it is preferable to cover the sky from the zenith to the horizon in the same frame in order to estimate the brightness of atmospheric diffuse light at the same time. The lens is developed by the Genesia Corporation (Figure 1). Its F value and a focal length f are 2.8 and 32.5 mm respectively. With the EEV CCD 42-80, it gives a FOV of 92~ ~ The optical filter is designed to avoid the prominent airglow emissions (see Figure 2). Both of the optical filter and shutter are set at pupil position. Due to an oblique path from off-optical axis, the wavelength of light transmitted through the interference filter is shifted toward a shorter wavelength (e.g. roughly 10 nm for the 50 degree field position). To avoid this effect, the filter is made of colored glass. The whole lens system is mounted on the thick honeycomb plate and supported by the shutter and filter box. The dewar is also mounted on the plate (see Figure 3). This honeycomb plate is fixed on an equatorial mount, which is useful to track the stars in the region between 48 ~ N and 48 ~ S terrestrial latitude. 3. A N T I C I P A T E D P E R F O R M A N C E
AND FUTURE PLANS
In this section, we estimate an expected count of zodiacal light observed by WIZARD based on the table provided in [6]. The table represents an annually averaged brightness
- 100-
WIZARD: New observation system of zodiacal light in Kobe University
Figure 3. Side view of the CCD system.
with low spatial resolution. Kelsall et al. [2] reported a detail model of the zodiacal cloud based on COBE infrared observations. We assume the dust distribution of the visible zodiacal light is the same as that of the infrared emission, and employ this model as a visible zodiacal cloud model. We use a volume scattering phase function given by Hong [7], and adjust the absolute brightness referring to Levasseur-Regourd and Dumont [6]. As a result, we can simulate not only the absolute brightness in the visible band, but also the minute spatial structures, which are absent in the original table. Taking into account the photon and read-out noise, we estimate an observed intensity, which includes the zodiacal light, by the new CCD system at Mauna Kea, Hawaii (4200 m). An optical depth for the diffuse light is assumed to be 0.10, and the brightness of the airglow and scattered light by the Earth's atmosphere are assumed to be 30 and 12 Sx0| respectively, at the zenith. These are the typical values deduced from our previous observations. The brightness of airglow and scattered light are extrapolated by the van Rhijn function and Dumont's formula (see e.g. [4]). Integrated star brightness is assumed to be 20Sx0| which is independent of the galactic coordinate. The observed time is set to the end of astronomical twilight (19:08 HST on December 22). The dots in Figure 4 show the anticipated brightness of the night sky observed by the new CCD system with an exposure time of 5 rain. It should be noted that these are the results of a simulation with 1.35'xl.35' spatial resolution. In the case where the pixels are combined to give same resolution as IRAS ZOHF (20'x20'), the noise was reduced to ,-~ 1 / ~ (lines in Figure 4). It is apparent that even a five minute exposure will allow us to detect asteroidal dust bands near the ecliptic plane (bump structures around/3 ,,~ - 1 ~ and the north-south asymmetry of zodiacal light brightness. This new observation system will also yield information on the brightness of airglow -101-
M. Ishiguro et aL
Figure 4. Scattered dots denote the expected intensity observed by the new CCD system. Each line corresponds to the combined data with 20'x20' resolution.
continuum resulted from chemiluminescence of NO2 [8], and atmospheric diffuse light produced mainly by the scattering from byproduct aerosol particles [9]. Acknowledgements. First light observations by WIZARD were supported by a colleague from the SUBARU telescope. We thank Dr. M. Nakagiri, K. Sekiguchi and A. Miyashita (NAOJ) for help with our observations at Mauna Kea. We also thank Dr. N. Takeyama (Genesia Corporation) and S.M. Kwon for useful comments and discussion. REFERENCES
R. Dumont, Astron. Astrophys. 38 (1975) 405. 1. 2. T. Kelsall, J.L. Weiland, B.A. Franz, W.T. Reach, R.G. Arendt, E. Dwek, H.T. Freudenreich, M.G. Hauser, S.H. Moseley, N.P. Odegard, R.F. Silverberg and E.L. Wright, Astrophys. J. 508 (1998) 44. 3. J.F. James, T. Mukai, T. Watanabe, M. Ishiguro and R. Nakamura, Mon. Not. R. astr. Soc. 288 (1997) 1022. 4. M. Ishiguro, R. Nakamura, Y. Fujii, K. Morishige, H. Yano, H. Yasuda, S. Yokogawa and T. Mukai, Astrophys. J. 511 (1999) 432. 5. M. Ueno and T. Wada, COGITO-3; A flexible imaging device control system, in prep. 6. A.C. Levasseur-Regourd and R. Dumont, Astron. Astrophys. 84 (1980) 277. 7. S.S. Hong, Astron. Astrophys. 146 (1985) 67. 8. S.B. Mende, G.R. Swenson, S.P. Geller, R.A. Viereck, E. Murad and C.P. Pike, J. Geophys. Res. 98 (1993) 19117. 9. S.S. Hong, S.M. Kwon, Y.-S. Park and C. Park, Earth Planets Space 50 (1998) 487.
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