High-Speed Time-Series CCD Photometry with Agile

We have assembled a high-speed time-series CCD photometer named Agile for the 3.5 m telescope at Apache Point Observatory, based on the design of a photometer called Argos at McDonald Observatory. Instead of a mechanical shutter, we use the frame-transfer operation of the CCD to end an exposure and initiate the subsequent new exposure. The frame-transfer operation is triggered by the negative edge of a GPS pulse; the instrument timing is controlled directly by hardware, without any software intervention or delays. This is the central pillar in the design of Argos that we have also used in Agile; this feature makes the accuracy of instrument timing better than a millisecond. Agile is based on a Princeton Instruments Acton VersArray camera with a frame-transfer CCD, which has 1K × 1K active pixels, each of size . Using a focal reducer at the Nasmyth focus of the 3.5 m telescope at Apache Point Observatory, we yield a field of view of 2.2 × 2.2 arcmin2 with an unbinned plate scale of 0.13'' pixel-1. The CCD is back-illuminated and thinned for improved blue sensitivity and provides a quantum efficiency ≥80% in the wavelength range of 4500–7500 Å. The unbinned full-frame readout time can be as fast as 1.1 s; this is achieved using a low-noise amplifier operating at 1 MHz with an average read noise of the order of rms. At the slow read rate of 100 kHz to be used for exposure times longer than a few seconds, we determine an average read noise of the order of rms. Agile is optimized to observe variability at short timescales from one-third of a second to several hundred seconds. The variable astronomical sources routinely observed with Agile include pulsating white dwarfs, cataclysmic variables, flare stars, planetary transits, and planetary satellite occultations.