SIRTF, the Shuttle Infrared Telescope Facility, will be a versatile astronomical telescope that can accommodate photometric, spectroscopic and polarimetric instruments. It is expected to be 100 to 1000 times more sensitive than any existing infrared telescope over much of its 2 to 1000 micrometer spectral range. A study that demonstrated the feasibility and provided a tentative design of such a telescope was completed by Hughes Aircraft Corp. in 1976. Since then, more detailed designs of cooled IR telescopes have been carried out for the Infrared Astronomical Satellite (IRAS) and the Small Helium Cooled Infrared Telescope for Spacelab 2. Analyses of off-axis rejection of these new systems have suggested improvements to the SIRTF optical and baffle designs. Rocket tests have verified the capability of using superfluid helium as a cryogen in zero gravity. Constraints on funds for Shuttle payloads favor an evolutionary approach to the development of the full potential of SIRTF. All these factors require a consideration of design alternatives involving the optical configuration, the cryogen, the mechanical structure, and size of SIRTF. Studies are currently underway at Perkin-Elmer Corporation and at Ames Research Center to examine the alternatives in terms of performance, cost and reliability. At this time it appears that the baseline optical configuration will be changed to an aplanatic Cassegrainian (Ritchey-Chretien) system.