Sub-arcsecond performance of the ST5000 star tracker on a balloon-borne platform

The ST5000 is a star tracker developed at the University of Wisconsin and used in NASA\´s sounding rocket payloads. In order to demonstrate the ST5000\´s suitability for pointed balloon-borne telescopes, we flew an ST5000 on a stratospheric balloon on May 6, 2011. This flight addressed our four basic questions: will the ST5000 work from 120,000 ft (it does), what was the rms performance (about 0.6"), what angular rates of motion would cause the ST5000 to fail (greater than 0.5 degrees/sec) and could the ST5000 serve as a daytime star tracker (not without modifications). We will briefly present the results from the flight and describe the ST5000\´s quantitative performance. We will also describe the problems with background light as a function of wavelength, altitude and angle from the Sun. We will discuss approaches to improve the daytime performance using infrared detectors and longer focal lengths/reduced platescales. Solutions that have finer platescales have the potential to improve the star tracker\´s error signal to the 0.1" level, which is better than the diffraction limit of a one meter telescope at 5000 Å.