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The Micro‐X Imaging X‐ray Spectrometer is a sounding rocket payload slated for launch in 2011. An array of Transition Edge Sensors, which is operated at a bath temperature of 50 mK, will be used to obtain a high resolution spectrum of the Puppis‐A supernova remnant. An Adiabatic Demagnetization Refrigerator (ADR) with a 75 gram Ferric Ammonium Alum (FAA) salt pill in the bore of a 4 T superconducting magnet provides a stable heat sink for the detector array only a few seconds after burnout of the rocket motors. This requires a cold stage design with very short thermal time constants. A suspension made from Kevlar strings holds the 255 gram cold stage in place. It is capable of withstanding loads in excess of 200 g. Stable operation of the TES array in proximity to the ADR magnet is ensured by a three‐stage magnetic shielding system which consists of a superconducting can, a high‐permeability shield and a bucking coil. The development and testing of the Micro‐X payload is well underway.

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.

We have measured the relation between sensor noise and the steepness (α = TR⋅ and β = IR⋅dRdI) of the superconducting transition for various superconducting transition edge sensors. The measurements are performed on square Mo/Cu bilayer thermistors of three sizes (125 μm, 250 μm and 400 μm), which are fabricated for X‐ray detection. We have identified the devices in two regimes: a) α>100 and b) α

Issues regarding the energy calibration of high dynamic range microcalorimeter detector arrays are presented with respect to new results from a minor actinide‐mixed oxide radioactive source. The need to move to larger arrays of such detectors necessitates the implementation of automated analysis procedures, which turn out to be nontrivial due to complex calibration shapes and pixel‐to‐pixel variability. Some possible avenues for improvement, including a more physics‐based calibration procedure, are suggested.