A system for calibrating a broadband detector includes a first narrowband telescope for viewing a celestial body, and an earth viewing telescope. The broadband detector is selectively coupled to the first narrowband telescope or the earth viewing telescope. A first narrowband filter is selectively inserted in an optical path of the first narrowband telescope, or the earth viewing telescope. A processor is configured to calibrate the broadband detector based on viewing the celestial body with the first narrowband telescope or the earth viewing telescope, and selectively inserting the first narrowband filter in the optical path. The first narrowband telescope includes a first narrowband filter inside its optical train. The first narrowband filter, which is selectively inserted in the optical path, is spectrally similar to the first narrowband filter inside the optical train of the first narrowband telescope. In addition, a second narrowband telescope, which receives a different wavelength from the first narrowband telescope includes a second narrowband filter inside its optical train. In addition, a second narrowband filter, which is selectively inserted in the optical path of the second narrowband telescope, is spectrally similar to the second narrowband filter inside the optical train of the second narrowband telescope. By using the first and second narrowband telescopes and the first and second narrowband filters, the broadband detector may be calibrated at two different wavelengths.