Charter and activities of the SIRTF focus in-orbit checkout (IOC) integrated products team (IPT)and Optical Performance of the CTA

Abstract

The NASA Space Infrared Telescope Facility (SIRTF) contains an 85 cm cryogenically cooled beryllium Ritchey-Chretien telescope. This Cryogenic Telescope Assembly (CTA) will operate at about 5 K. Once in orbit, the telescope may be focused by moving the secondary mirror using a cryogenic focus mechanism to vary the separation between the primary and secondary mirrors. The risk of failure of the motor is unknown but is believed to be non-negligible. It is therefore desirable to evaluate and achieve best focus with a minimum number of motor activations. The SIRTF Project has charged an Integrated Products Team (IPT) with conducting this activity. We describe a strategy to determine the initial mirror spacing by quantitatively evaluating the shapes of the images formed by the telescope using the Infrared Array Camera (IRAC) and other science instruments (SI's). We show that this information can be used to predict the direction and magnitude of the secondary mirror move that will result in the telescope best focus. The tools used to evaluate focus position and ptical quality of the in orbit CTA have been qualified during the ground-based BRUTUS test are here described. Future activities of the IPT to meet IOC objectives are summarized.