Selenology Today # 7 July 2007 - Home

GEOLOGIC LUNAR RESEARCH SELENOLOGY TODAY # 7
Because the moon is illuminated by reflected sunlight, lunar spectra contain the same
Fraunhofer lines and these absorption lines can be used to accurately calibrate
wavelengths along the x axis in reflectance vs wavelength plots. An example of a lunar
spectrum showing labelled Fraunhofer lines is provided in Figure 1-13 below.
Figure 1-13
Once the wavelength scale is calibrated, it will remain valid for the same spectograph
setup as long as the same instrumentation, slit, transmission grating, and imager are used.
Increasing the effective focal length of the telescope will not alter the spectral dispersion
or the wavelength calibration, but a smaller area of the lunar surface will be imaged.
Table 1-1 below shows the true wavelength of Fraunhofer lines used to calibrate the
spectrograph used in this paper in reference and compares them to calibrated values
obtained by noting the pixel position of the Fraunhofer lines on the spectral image.
Table 1-1
Fraunhofer Band Actual Wavelength
(nm)
Wavelength Calculated
from Spectral Image
(nm)
K 393.4 397.2 3.8
G 430.8 429.7 -1.1
F 486.1 486.1 0
b 518.4 518.6 0.2
D 589.6 583.7 -5.9
C 656.3 653.1 -3.2
B 686.7 685.7 -1.0
unlabelled band 719.5 716 -3.5
A 759.4 759.4 0
page 37
Difference (nm)