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Second-order Light

While the aluminum filter in aperture 3 blocks long-wavelength radiation for a clean view of the second-order spectrum, the only filter available to HUT for blocking short-wavelength light is the natural absorption below 912 Å due to the Lyman edge of neutral hydrogen in the local ISM. Since the red end of the HUT spectrum extends to 1860 Å diffracted light in second order appears over the 1824-1860 Å interval. This is removed by using the observed first order spectrum over the 912-930 Å interval, and scaling it by the measured ratio of the second order to first order efficiency of the grating. This ratio was measured to be 0.226 for Astro-1.