Extended focal-plane array development for the International X-ray Observatory

We are developing arrays of transition-edge sensors (TES's) for the International X-ray observatory (IXO). The IXO microcalorimeter array will consist of a central 40 x 40 core of 300 km pitch pixels with a resolution of 2.5 eV from 0.3-10 keV. To maximize the science return from the mission, an outer extended array is also required. This 52 x 52 array (2304 elements surrounding the core) of 600 km pitch pixels increases the field-of-view from 2'to 5.4' with a resolution of 10 eV. However, significantly increasing the number of readout channels is unfavorable due to the increase in mass and power of the readout chain as well as adding complexity at the focal plane. Consequently, we are developing position-sensitive devices which maintain the same plate scale but at a reduced number of readout channels. One option is to use multiple absorber elements with different thermal conductances to a single TES. Position discrimination is achieved from differences in the pulse rise-time. Another new option is to inductively couple several TES's to a single SQUID. Position discrimination can be achieved by using different combinations of coupling polarity, inductive couplings and heat sink conductances. We present first results demonstrating < 9 eV across four 500 km pixels coupled to a single SQUID. A further possibility is to increase the number of channels to be time-division multiplexed in a single column at some expense in resolution. In this paper we discuss experimental results and trade-offs for these extended array options.