Mapper of the IGM Spin Temperature: Instrument Overview

The observation of the global 21 cm signal produced by neutral hydrogen gasin the intergalactic medium (IGM) during the Dark Ages, Cosmic Dawn, and Epochof Reionization requires measurements with extremely well-calibrated widebandradiometers. We describe the design and characterization of the Mapper of theIGM Spin Temperature (MIST), which is a new ground-based, single-antenna,global 21 cm experiment. The design of MIST was guided by the objectives ofavoiding systematics from an antenna ground plane and cables around theantenna, as well as maximizing the instrument's on-sky efficiency andportability for operations at remote sites. We have built two MIST instruments,which observe in the range 25-105 MHz. For the 21 cm signal, this frequencyrange approximately corresponds to redshifts 55.5 > z > 12.5, encompassing theDark Ages and Cosmic Dawn. The MIST antenna is a horizontal blade dipole of2.42 m in length, 60 cm in width, and 52 cm in height above the ground. Thisantenna operates without a metal ground plane. The instruments run on 12 Vbatteries and have a maximum power consumption of 17 W. The batteries andelectronics are contained in a single receiver box located under the antenna.We present the characterization of the instruments using electromagneticsimulations and lab measurements. We also show sample sky measurements fromrecent observations at remote sites in California, Nevada, and the CanadianHigh Arctic. These measurements indicate that the instruments perform asexpected. Detailed analyses of the sky measurements are left for future work.