The CCD Array Camera for the Macho Project

Abstract

We have developed an astronomical imaging system tailored to our search for gravitational microlensing by compact objects in the halo and disk of the Galaxy. The challenge of detecting rare microlensing events is to monitor ∼10 million stars per night and distinguish genuine events from other sources of variability. The Large Magellanic Cloud and the Galactic bulge provide the high surface density of resolvable stars necessary for this task. A dedicated 50 inch telescope at Mt. Stromlo Observatory has been producing science data since the fall of 1992. Our system incorporates eight 2048 x 2048 CCDs into two focal planes for simultaneous imaging in two passbands (4500-6300 and 6300-8100 Å). Each focal plane consists of four ‘edge-buttable’ CCDs in a custom mounted 2 x 2 array. The 0.62 arcsecond pixel scale (15 μ) yields a 40 x 40 arcminute square field of view in each frame. A sophisticated point spread fitting photometry package extracts up to 600,000 useful magnitudes per color per frame. The data collection rate we need is obtained by simultaneously reading out all sixteen CCD outputs (two per chip) at 34 KHz with 16 bit digitization. With exposure times of 150-300 seconds and a 70 second readout time we can collect up to 100 fields per night. These rates are designed to allow us to detect or rule out massive compact halo objects (MACHOs) in the 10-⁶ - 10 ¹ M⊙range.