Calibration of the EPIC O2 absorbing bands using Lunar observations
To calibrate the EPIC O2 absorbing bands, we used EPIC lunar observations at the time of the Full Moon as seen from the earth. Lunar reflectance Rλ does not increase much with a small wavelength change Δλ; a 10 nm difference in λ leads to a difference in Rλ in the range of 0.0006-0.0013 or 0.8-1.2% (e.g., Ohtake et al., 2010, 2013). It follows from this that the difference in moon reflectance between the O2 B-band (688 nm) and the 'red' (680 nm) channels as well as between the O2 A-band (764 nm) and the near-IR (780 nm) channels will be within 1.5%.
Since the calibration factors, Kλ, for λ=680 and 780 nm are assumed to be known from in-flight comparisons between EPIC Earth observations and well-calibrated measured Earth reflectances obtained from the Terra and Aqua MODIS LEO satellite observations, we can obtain the calibration factors for the O2 absorbing channels at 688 and 764 nm. Indeed, the ratio F(λ1,λ2) of the lunar reflectance values measured in counts/sec at two neighboring channels λ1 and λ2 is very stable (Fig. 1). Thus, the calibration factor Kλ for λ=688 nm, can be approximated as
K688 = R688/R688counts = R688/[R680counts F(680,688)] = = [R688/R680] K680/F(680,688) ≈ K680/F(680,688)
Similarly to 688 nm, the calibration factor for 764 nm can be estimated as
K764 ≈ K780/F(780,764)
Here, Rλ and Rλcounts are the values of calibrated reflectance and measured C/S at wavelength λ, respectively; Kλ is the multiplicative calibration coefficient expressed as a conversion from counts/second to reflectance at wavelength λ and the ratio F(λ1,λ2) = Rλ2counts/Rλ1counts. The calibration factors Kλ for these channels are given in Table 1.
The calibration factors for all 10 EPIC channels are also publicly available at https://eosweb.larc.nasa.gov/project/dscovr/DSCOVR_EPIC_Calibration_Factors_V02.pdf.
|λ (nm) Center||FWHM (nm)||Kλ|
Table 1. Calibration factors for two O2-band channels.
Ohtake, M., T. Matsunaga, Y. Yokota, S. Yamamoto, Y. Ogawa, T. Morota, C. Honda, J. Haruyama, K. Kitazato, H. Takeda, A. Iwasaki, R. Nakamura, T. Hiroi, S. Kodama and H. Otake, 2010. Deriving the absolute reflectance of lunar surface using SELENE (Kaguya) Multiband Imager data. Space Sci. Rev. 154, 57-77.
Ohtake M., C.M. Pieters, P. Isaacson, S. Besse, Y. Yokota, T. Matsunaga, J. Boardman, S. Yamomoto, J. Haruyama, M. Staid, U. Mall and R.O. Green, 2013, One Moon, many measurements 3: Spectral reflectance. Icarus, 226, 364-374.
Figure 1. Moon observations. Ratios of Moon reflectance F in counts/sec at 688 over 680 nm and at 764 over 780 nm channels averaged over Moon pixels.