Calibration
This explains the calibration method used for the lightmeter network. For a step by step example see Calibration example Linz
We calibrate the lightmeters in-situ, ie at the site of measurements with globally available natural sources – the Sun, the twilight and the Moon. The goal is to obtain a maximum of network and long term measurement- system- stability and control by repeated calibration over time.
The strategy is to use the exceptional linearity and high signal to noise (~ 0.7 dm² of surface similar to a 10 cm aperture telescope) of the instruments to determine a single calibration factor for the “sub-lunar” night-time part. Then straighten out the non-linear daytime part and connect to sunshine (the Sun and an atmospheric model). [1] show detailed multi-frequency modelling. For simplicity less complex models are available: total radiation according to [2] with air-mass [3], twilight tables [4], ephemerids [5], with a visual lunar phase function [6]. A constant “background” offset is allowed to account for light pollution and air-glow. Only the earlier twilight is used in light-polluted places.
Solar heights between 0 and 30° are excluded from the fits and the Moon is used as a check. That results in clear-sky residuals (rms) of <10% under typical and <5% under “observatory” (usually dry and high) conditions. This overall accuracy is usually determined by the daytime part, limited by the accuracy of the atmospheric model and the available meteorological data. Thus for high accuracy applications the full approach, [1] is recommended. Overall accuracy is checked by multiple calibrations at the same site (every clear twilight to noon data set may be used) and by comparison to daylight total radiations instruments, where available, [7].
The IYA-lightmeter, Mark 2.3 had main calibration constant differences of up to a factor of ten. The present post-year of astronomy series (Mark 2.4pro, 2.3l) generally have instrumental constants within 20%, say. Station differences caused be the local met-conditions, perturbing light (non-constant in cities) and partial sky obstruction are likely to dominate variations in the station constants.
--GW (talk) 17:18, 30 July 2014 (CEST)
[1] Müller, A.; Wuchterl, G.; Sarazin, M. (2011), Measuring the Night Sky Brightness with the Lightmeter, in Astronomical Site Testing Data in Chile (Eds. M. Curé, A. Otárola, J. Marín, & M. Sarazin) Revista Mexicana de Astronomía y Astrofísica (Serie de Conferencias) Vol. 41, pp. 46-49,
[2] Duchon, C. E. and O'Malley, M.S., (1999). Estimating Cloud Type from Pyranometer Observations, Journ. Appl. Metr. 38, 134,
[3] Rozenberg, G. V. (1966). Twilight: A Study in Atmospheric Optics (New York: Plenum Press), translated from the Russian by R. B. Rodman, p. 160.
[4] Siedentopf, H. and Scheffler, H., (1965) Dämmerungs- und Nachthimmelshelligkeit --- Brightness of twilight and of the night sky, sect.1.5.3., p.60, in Landoldt-Börnstein, Zahlenwerte und Funktionen aus Naturwissenschaft und Technik, Neue Serie, Gruppe IV, Bd. I, H. H. Voigt, ed., Springer, Berlin.
[5] The pyephem / Xephem packages that use VSOP87 from Bretagnon P., Francou G.,, Astron. Astrophys. 202, 309 (1988) for the epochs of interest.
[6] Graff, K. (1936), Die physische Beschaffenbeit des Planetensystems, Handbuch der Astrophysik, 7, pp. 410, http://esoads.eso.org/abs/1936HDA.....7..410G,
[8] Nobuaki Ochi and Günther Wuchterl (2014), Long-term measurement of the night sky brightness in Japan using Lightmeters: 2009-2012 data , Journal of Toyo University, Natural Science, No.58 : in press (2014)
[9] Wuchterl, G. in Clive Ruggles and Michel Cotte (2011), Heritage Sites of Astronomy and Archaeoastronomy in the context of the UNESCO World Heritage Convention, with contributions by 41 other authors, Case Study 16.2 pp 149,. Ocarina Books. Online version at http://www.astronomicalheritage.org/
[10] Wuchterl, G. in Clive Ruggles et al., “Extended Case Studies for Astronomy and World Heritage”, IAU in press, on line at http://www.astronomicalheritage.net
[11] Reithofer, Wuchterl, Chwatal, Posch, Linhardt, Kopper (2012), Licht über Wien – Energieaufwand und Quellen. Erstellung eines exemplarischen Lichtkatasters. Wiener Umweltanwaltschaft (Hrsg), http://wua-wien.at/home/naturschutz-und-stadt-kologie/lichtverschmutzung/lichtkataster-2012-2
[12] Linhardt, F., Kopper, M. Reithofer, M., Wuchterl, G. (2013) Licht über Wien II. Kontinuierliche Messungen der nächtlichen Globalstrahlung und Energieaufwand für die Wiener Lichtglocke im Jahr 2012, Wiener Umweltanwaltschaft (Hrsg.), http://wua-wien.at/home/naturschutz-und-stadt-kologie/lichtverschmutzung/lichtkataster-2012
[DB] Lightweather database of the GAVO (German Astrophysical Virtual Observatory) http://dc.zah.uni-heidelberg.de/lightweather
[MF] Manufacturer of the lightmeter: K2W Lights KG, Mozartstraße 3, 07607 Eisenberg, Germany, http://k2wlights.de
Useful Tools
- Calculations of dew point and relative humidity;
- Conversion of reduced (sea-level) and measures pressure;
- OGIMET Weather Information, METARS.
Additional Models / Sources
This section provides links and sources of models and other useful data for calibration purposes of the Lightmeter.
- Reference Solar Irradiance Spectra (ASTM E490): http://rredc.nrel.gov/solar/spectra/
- Air mass: Fritz Kasten and Andrew T. Young, Applied Optics, Vol. 28, Issue 22, pp. 4735-4738 (1989), Revised optical air mass tables and approximation formula
- Extinction model: Hayes, D. S. and Latham, D. W., Astrophysical Journal, vol. 197, p. 593-601 (1975), A rediscussion of the atmospheric extinction and the absolute spectral-energy distribution of VEGA
- Wavelength dependent extinction coefficients for La Silla observatory: http://www.eso.org/sci/observing/tools/Extinction.html
- Photopic and Scotopic functions: http://cvision.ucsd.edu/
- Radiometry and photometry in astronomy with examples: http://www.stjarnhimlen.se/comp/radfaq.html
- Lunar Irradiance model: http://www.moon-cal.org/index.php, see also: Kieffer et al., The Astronomical Journal, Volume 129, Issue 6, pp. 2887-2901, The Spectral Irradiance of the Moon
