The Optical Gravitational Lensing Experiment (OGLE) project is a long term project with the main goal of searching for the dark matter with microlensing phenomena. The idea of employing microlensing for that purpose was originally proposed by Paczynski (ApJ, 304, 1; ApJ Letters, 371, L63). The Magellanic Clouds and the Galactic Bulge are the most natural locations to conduct such search due to a large number of background stars that are potential targets for microlensing. The LMC and SMC stars may be lensed mostly by the Galactic halo objects, in case of the Galactic Bulge stars one expects an additional component - microlensing by low-mass disk stars. In both cases the optical depth for microlensing is very small - about 10-6. Therefore a massive, long term survey must be conducted to a) detect and b) collect statistatically significant sample of microlensing events to draw any conclusion about the nature of dark matter.

The first phase of the project (OGLE-I) started in 1992 and observations were continued for four consecutive observing seasons through 1995. The 1-m Swope telescope at the Las Campanas Observatory, Chile, operated by Carnegie Institution of Washington, with 2048×2048 Ford/Loral CCD camera were used during the entire program.

The project was very successful. The main scientific results are gathered here.

Unfortunately the project suffered from many limitations with the most severe - limited availability of the telescope time. Therefore the observations were limited to the Galactic bulge, and the covered area on the sky was relatively small. It was clear from the very beginning that OGLE-1 is only the pilot phase of the project and a substantial upgrade - a dedicated telescope and new generation instruments - are necessary to accomplish the main goals of the OGLE project.

In the late 1991 the funds for the OGLE microlensing project were granted by the State Committee for Scientific Research to the team of astronomers from the Warsaw University Observatory. One of the goals of the grant was to design and build a 1-m class telescope dedicated for massive photometric surveys of dense stellar fields.

After careful inspection of the market the DFM Engineering Inc. (Longmont, Colorado, USA) was selected as a manufacturer of the telescope. The company offered 1-m class telescope at reasonable price and fulfilling requested parameters. The contract was signed on November 24, 1992.

The good location of the telescope was the next crucial point. Long collaboration of Polish astronomers with astronomers from Carnegie Institution of Washington, excellent climate condition and visibility of Magellanic Clouds and Galactic center made the Las Campanas Observatory, Chile, operated by Carnegie Institution of Washington the best choice. Thanks to a very friendly and cooperative attitude of Carnegie's astronomers the "Memorandum of Understanding" was signed in 1995 between Carnegie Institution of Washington, Princeton University and Warsaw University Observatory defining the terms of presence of the 1.3-m Warsaw Telescope on the mountain.

First civil engineering works started in August 1995. In October 1995 the telescope enclosure manufactured in Poland and shipped to Chile was assembled. At the same time construction of the observer's house started. In January 1996 the dome was assembled and mounted atop of the telescope enclosure. A week later assembling of the telescope started.

The "first light" of the telescope was collected on February 9, 1996. The telescope was ready for operation in the mid-February 1996 and the next step was assembling and testing the instruments. The first "electronic light", taken with the main CCD camera was registered on July 18, 1996. To the end of 1996 remaining instruments: autoguiding system and filter wheel were commissioned. Also the software for hardware control and data reduction was finished. Regular observations of the second phase of the OGLE project started on January 6, 1997.

An article by Udalski, Kubiak and Szymański (1997, Acta Astron., 47, 319) describes the system in great detail.

Starting June 11, 2001, the Optical Gravitational Lensing Experiment entered its third phase, OGLE III, and resumed regular observations at the Las Campanas Observatory, Chile. Observations are collected with the new "second generation" CCD mosaic camera, commisioned in May 2001. The 8k-mosaic camera consists of eight thin SITe 2048×4096 CCD chips (8192×8192 pixels of 0.26 arcsec/pixel) giving the total field of view equal to 35' x 35'. The readout time of the camera is 98 seconds with readout noise of 6-9 e (depending on chip) and the gain of 1.3 e/ADU. See article by Udalski (2003, Acta Astron., 53, 291) for more details.