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In typical algorithms for optimally filtering transition‐edge‐sensor‐microcalorimeter pulses, the average value of a filtered pulse is set to zero. The achieved energy resolution of the detector then depends strongly on the chosen length of the pulse record. We report experimental confirmation of this effect. We derive expressions for the dependence of energy resolution on record length, and apply them to a pair of detector models for the X‐ray Microcalorimeter Spectrometer instrument on NASA/ESA/JAXA’s proposed International X‐ray Observatory. Although the two models have identical pulse time‐constants, they differ by a factor of two in the record length required to achieve a given energy resolution. Finally, we derive an expression for the maximum output count rate at high energy resolution of a TES pixel.

The Micro‐X sounding rocket experiment will deploy an imaging transition‐edge‐sensor (TES) microcalorimeter spectrometer to observe astrophysical sources in the 0.2–3.0 keV band. The instrument has been designed at a systems level, and the first items of flight hardware are presently being built. In the first flight, planned for January 2011, the spectrometer will observe a recently discovered Silicon knot in the Puppis‐A supernova remnant. Here we describe the design of the Micro‐X science instrument, focusing on the instrument’s detector and detector assembly. The current design of the 2‐dimensional spectrometer array contains 128 close‐packed pixels with a pitch of 600 μm. The conically approximated Wolter‐1 mirror will map each of these pixels to a 0.95 arcmin region on the sky; the field of view will be 11.4 arcmin. Targeted energy resolution of the TESs is about 2 eV over the full observing band. A SQUID time‐division multiplexer (TDM) will read out the array. The detector time constants will be engineered to approximately 2 ms to match the TDM, which samples each pixel at 32.6 kHz, limited only by the telemetry system of the rocket. The detector array and two SQUID stages of the TDM readout system are accommodated in a lightweight Mg enclosure, which is mounted to the 50 mK stage of an adiabatic demagnetization refrigerator. A third SQUID amplification stage is located on the 1.6 K liquid He stage of the cryostat. An on‐board 55‐Fe source will fluoresce a Ca target, providing 3.69 and 4.01 keV calibration lines that will not interfere with the scientifically interesting energy band.

The International X-Ray Observatory (IXO) is under formulation by NASA, ESA and JAXA for deployment in 2022. IXO emerged over the last 18 months as the NASA Constellation-X and ESA/JAXA X-Ray Evolving Universe Spectrometer (XEUS) missions were combined. The driving performance requirements for the X-Ray Microcalorimeter Spectrometer (XMS) are a spectral resolution of 2.5 eV over the central 2'x2' in the 0.3-7.0 keV band, and 10 eV to the edge of the 5'x5' field of view (FOV). The XMS is now based on a microcalorimeter array of Transition-Edge Sensor (TES) thermometers with Au/Bi absorbers and a SQUID MUX readout. One of the concepts studied as part of the mission formulation has a core 40x40 array corresponding to a 2'x2' FOV with 3'' pixels surrounded by an outer, annular 52x52 array of 6'' pixels that extends the field of view to 5.4'x5.4' with better than 10 eV resolution. There are several options for implementing the readout and cooling system of the XMS under study in the US, Europe and Japan. The ADR system will have from two to five stages depending on the performance of the cryocooler. Mechanical coolers with sufficient cooling power at 4K are available now, and {approx}2K coolersmore » are under development. In this paper we give an overview of the XMS instrument, and some of the tradeoffs to be addressed for this observatory instrument.« less