GASP: High Speed Optical Detectors for Polarimetric Observations

We present an overview of the work currently ongoing into the detector system used on the Galway Astronomical Stokes Polarimeter (GASP). GASP is based upon a Division of Amplitude Polarimeter and has no moving parts or modulated components. The complete Stokes vector is measured from just one exposure. This means that the timing resolution depends only on the frame rate of the detector system. GASP has been used on the 2.2 m Telescope at Calar Altro (2009) and on 4.2 m WHT (2010). The system consists of the instrument box to which either an L3Camera system or an APD detector system is attached. A GPS Time and Frequency system controls all timing aspects for both detector systems. The choice of detector depends on the science target and objectives. For frame rates up to 650 Hz the L3Camera system can be used, but for faster varying targets, the system must be used with APD detectors. The latter can operate at resolutions of < 1 ?sec with a timing accuracy of 25 nsec. Data loads for the L3Camera system are typically 86 GB per Hr per camera (for full frame). The APD system only stores data if a photon is detected and each photon event is individually time tagged to a 40 MHz clock (25 nsec). For the APDs, 1TB of disc space would provide a minimum of 8.5 hours recording time.