On-orbit performance testing of the pointing calibration & reference sensor for the Spitzer Space Telescope

We present the on-orbit performance results of the Pointing Calibration and Reference Sensor (PCRS) for the Spitzer Space Telescope. A cryogenic optical (center wavelength 0.55 μm) imager, the PCRS serves as the Observatoryis fine guidance sensor by providing an alignment reference between the telescope boresight and the external spacecraft attitude determination system. The PCRS makes precision measurements of the positions of known guide stars; these are used to calibrate measurements from Spitzerís star tracker and gyroscopes to obtain the actual pointing of the Spitzer telescope. The PCRS calibrates out thermomechanical drifts between the 300 K spacecraft bus and the 5.5 K telescope. By using only 16 pixels, the PCRS provides high precision centroiding with extremely low (∼64 μW) power dissipation, resulting in minimal impact to Spitzerís helium lifetime. We have demonstrated that the PCRS meets its centroiding accuracy requirement of 0.14 arcsec 1-σ radial, which represents ∼1/100 pixel centroiding. The Spitzer Space Telescope was launched on 25 August, 2003 and completed its In-Orbit Checkout phase two months later; the PCRS has been operating failure-free ever since.