Introduction

The Arecibo 305-m radio telescope is the largest single-dish radio telescope on our planet, and is available to the global scientific community for astronomical observations at wavelengths between $\lambda$6 m and $\lambda$3 cm (frequencies of 47 MHz to 10 GHz). Over the past few years it has contributed significantly to the investigationly of solar system bodies via radar imaging, the discovery and subsequent study of new pulsars, the use of binary pulsars as laboratories for general relativity, the measurement of celestial magnetic fields via the Zeeman effect, the compiling of huge redshift surveys of galaxies, the detection of OH megamasers and other molecules in ultra-luminous infra-red galaxies, and much more besides. The study of a large number of molecular species (e.g. OH, CH, H$_{2}$CO, HCN, HC$_{3}$N, CH$_{2}$NH and CH$_{3}$OH) is now possible, the high end of the Arecibo frequency range having been ``opened up'' by the telescope upgrade of the mid-1990s. Additionally, new instrumentation has allowed the telescope's participation in wide-band Very Long Baseline Interferometry (VLBI) studies, adding enormously to the sensitivity of this endeavor for the imaging of the smallest scale structure in both line and continuum radio emitters. The Arecibo Telescope is a regular contributor to the HSA, EVN and Global VLBI Arrays, and participates in EVN eVLBI sessions.

With the arrival at the Observatory of the Arecibo L-band Feed Array (ALFA) in mid-2004, an exciting new facility with wide user-community appeal was added to the telescope's receiver ensemble. ALFA is a seven-feed receiver system that allows large-scale surveys of the sky to be conducted with unprecedented sensitivity. In the past, use of the telescope as a survey instrument has been limited by the relatively small field of view of its single-pixel receivers. ALFA, operating over the band 1225 - 1525 MHz, enables deep surveys for a variety of Galactic and extragalactic investigations.

The present document is intended to provide an introduction to Arecibo Observatory and the 305-m telescope both for radio astronomers wishing to have an overview of telescope capabilities, etc., and for other interested parties wanting to know whether the instrument could be an appropriate tool with which to further their research. It also aims at informing potential new users concerning the procedures for obtaining observing time.

We note here that for U.S.-based scientists without financial support from other sources, and with a scheduled research program on the telescope, NAIC will reimburse transportation costs associated with the conduct of the research program, NAIC budget permitting. The details and conditions of this are to be found at
http://www.naic.edu/~astro/proposals/proposal.shtml#11. (Travel support cannot be provided for investigators not based in the U.S.A.) Similarly, NAIC will contribute towards the publication costs of results from observations carried out at Arecibo. Details of this are at http://www.naic.edu/~astro/proposals/proposal.shtml#12.

In Section 2, we provide an introduction to the Observatory, the telescope and its instrumentation. Section 3 lays out how to compute expected sensitivities as required for justifying the time requirements in a proposal. Section 4 deals with VLBI use of the 305-m telescope, while Section 5 informs the potential user as to when and how to submit a telescope proposal, and elaborates on the proposal procedure. Section 6 provides general information and a list of contact addresses.