Thermal Optimization of IXO SXT FMA to Meet 20?C and Stringent Temperature Gradient Requirements and Heater Power Budget

*The International X-ray Observatory (IXO) Soft X-ray Telescope (SXT) Flight Mirror Assembly (FMA) has a single mirror configuration that includes a 3.3 m diameter and 0.93 m tall mirror assembly. It consists of three rings. The inner ring has 12 identical modules. The middle ring has 24 identical modules. The outer ring has 24 identical modules. The operating temperature requirement of the FMA is 20oC. The spatial temperature gradient requirement within a FMA module is ±0.5°C and is very stringent. The mirror segment material (glass) has a very poor thermal conductivity. The mirror segment coating (iridium) has a small emittance (<0.1). The forward end of the FMA, including the mirror segments and stray light baffles views space (3K). The aft end is inside a metering structure. One side of the metering structure has solar impingement. It is a challenge to meet the thermal requirements and heater power budget. This paper presents thermal optimization of the outer modules which are the largest and most difficult to meet the thermal requirement. The parameters for optimization are: (1) design and dimensions both the non-conductive and aluminum portions of the pre-collimator, (2) heater locations, (3) number of heater zones, and (4) heater controller set points. The optimized thermal design meets the thermal requirement and minimizes the operating mode heater power.