Habitable Worlds Observatory (HWO) imposes the highest level of temporal and spatial stability requirements on the mirror segments that will comprise its entrance pupil. Some questions of ultrastability realization are addressed by simple models of lightweighting cells and response timescales to thermal stimulus. Closed Back (CB) lightweighted segments are attractive from the eigenfrequency and areal density perspectives but have a long thermal time constant associated with applied thermal stimulus changes, radiated from a heater plane behind the mirror. Open Back (OB) lightweighted segments enjoy a rapid thermal time constant but are not presently meeting the eigenfrequency and areal density of the CB. Thus, neither CB nor OM appears to be optimal for HWO stability. We explore a third option, Partially Closed Back PCB lightweighted segments. PCB segments appear to exhibit eigenfrequencies approaching that of the CB, with thermal time constant characteristics close to those of the OB, thus the best of both forms. We explore preliminary questions of ultrastability for HWO inclusive of these three segment models.
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