<p> Microcalorimeter x-ray instruments are non-dispersive, high spectral resolution, broad-band, high cadence imaging spectrometers. We have been developing these instruments for x-ray astrophysics for over 25 years and have successfully flown them on both suborbital and orbital observatories. Microcalorimeter spectrometers are true spatial-spectral event-driven instruments. The core instrument for the Astro-H observatory to be launched in 2014 and for the International X-ray Observatory planned for around 2022 [1] are both microcalorimeter spectrometers. For the past two years, supported by the Solar and Heliospheric ROSES program, we have been adapting this highly successful technology to the very different requirements of solar physics. This leverages the large NASA investment in this technology to produce instruments optimized for solar physics with only a moderate development program.</p> <p> During the past two years, we have developed a high spatial resolution, high cadence microcalorimeter optimized for solar observations with a ground-breaking spectral resolving power of nearly 3000 at 6 keV. This exceeds the performance goals of our program. In fact, the single-pixel performance achieved during this program is already sufficient for a solar optimized instrument as described in section 2, albeit using small arrays of detectors. We propose here to continue this successful program by developing large focal-plane arrays, optimized for solar physics and their read-out systems. This complements our existing development programs in astrophysics, where we have already produced and tested kilo-pixel arrays for IXO. The end-result of the proposed work will be a solar-optimized detector system proven and ready for integration into a suborbital payload and then onto a space-borne observatory. Both the suborbital program and an orbital instrument would allow high cadence spatial-spectral observations across the x-ray band from 0.1 to above 10 keV, enabling new science as described in section 1.4. Ultimately this will produce instrumentation suitable for deployment on an Explorer-class mission and, possibly, a remote sensing contribution to the Solar Energetic Particle Acceleration and Transport (SEPAT) Solar-Terrestrial Probe [2]. &nbsp;</p>