Fig. 6

a The brightness ratio of the 2nd-order to the 1st-order images is plotted against wavelength. The ratios were measured in laboratory calibrations and are used in the correction for the 2nd-order image. b The brightness fraction of the zero-order image in the zero-, 1st-, and 2nd-order images. The contribution of the other order images is negligible. The fraction is equivalent to the relative transmittance of the grating for the zero-order if the other factors that reduce lights (e.g., absorptions in the glass) are independent of wavelengths. For the calculations in Eq. (2), the fraction was assumed to be constant at 800 nm and more