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The SIRTF Phase A baseline provided a 10 to 20 K cold telescope facility to allow for infrared astronomy to be conducted onboard the shuttle. SCHe cryogen tanks provided the necessary cooling for the duration of the 15-day sortie-type mission. Recently, LPARL completed a study to assess the feasibility for extending the lifetime of the baseline SIRTF concept to between 6 and 24 months. From this study, LPARL concluded that extending the lifetime by simply increasing the size of the SCHe tank size was not practical. An alternative approach was found, one in which solid hydrogen was used to cool the facility components. With solid hydrogen, 6 months of cooling could be provided by an 1870-liter supply having a combined tankage plus cryogen mass of less than 400 kg, both comparable figures to those required by the 15-day SCHe baseline. Detailed analyses have shown that facility temperatures can be maintained below 10 K and that the temporal temperature gradients of both the primary and secondary mirrors are less than one-tenth the maximum allowed excursion of 3.6 K/h. Study constraints imposed were that the telescope must allow for pointing to within 60 deg of the earth and sun, optics temperature must be less than 10 K, and that the total instrument power will be at 1.0 W.
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