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We present a new method, fan-beam modulation, for observing weak extendedx-ray sources with the Reuven Ramaty High-Energy Solar Spectroscopic Imager(RHESSI). This space-based solar x-ray and gamma-ray telescope has much greatersensitivity than previous experiments in the 3-25 keV range, but is normallynot well suited to detecting extended sources since their signal is notmodulated by RHESSI's rotating grids. When the spacecraft is offpointed fromthe target source, however, the fan-beam modulation time-modulates thetransmission by shadowing resulting from exploiting the finite thickness of thegrids. In this paper we detail how the technique is implemented and verify itsconsistency with sources with clear known signals that have occurred duringRHESSI offpointing: microflares and the Crab Nebula. In both cases the resultsare consistent with previous and complementary measurements. Preliminary workindicates that this new technique allows RHESSI to observe the integrated hardx-ray spectrum of weak extended sources on the quiet Sun.Comment: Publishe

A new method of observing weak extended x-ray sources with the Reuven Ramaty High-Energy Solar Spectroscopic Imager