The ESO 1m telescope was the first telescope to be installed at the La Silla observatory, in 1966. It has been used until 1994 as a photometric telescope, both in visible with a single channel photometer, and in infrared with an InSb photometer and a bolometer.

Since 1994, it is fully dedicated to the DENIS project.

The telescope is a classical Cassegrain, used at its Cassegrain focus. The fork mounting and the driving system have been built by Rademakers N.V., Netherlands. The optics were figured by Jenoptik, Jena, Germany. The f/4.5 paraboloid primary mirror has been made from a low expansion glass (Therman of Schott, Jena, # 25526), while the hyperboloid secondary is from fused quartz (Heraeus). The main mirror weight of 350 kg is supported by 12 radial astatic levers, and 9 axial levers (3 internal, close to the Cassegrain hole, and 6 external, 3 of which are fixed).

The telescope main characteristics follow:

• primary mirror diameter: 1.08 m (drawings & measured)
• useful primary mirror diameter: 1.00 m (measured)
• secondary mirror diameter: 0.295 m (drawings & measured)
• useful secondary mirror diameter: 0.290 m (measured)
• secondary baffle diameter: 0.365 m (measured)
• obscuration ratio: 0.365
• collecting area: 0.68 m^2
• focal distance of primary mirror: 4.585 m (f/4.5) (drawings)
• focal distance of secondary mirror: 1.712 m (assumed)
• focal distance of telescope: 15.279 m (f/15) (assumed)
• plate scale at nominal focus: 13.5 arcsec/mm (drawings)
• back focal length: 0.600 m (drawings)
• DENIS focal plane (from primary): 0.720 m (measured)
• best focal plane (from primary): 1.009m (measured)
• distance between primary and secondary mirrors: 3.387 m (drawings), 3.356 m (measured at best focal plane)
• conic constant of secondary mirror: -3.5031 (-3.4503?)
• focus encoder equation: F = -1298 + 220 d, where d is the secondary displacement in mm. Max. range for d is 50 mm, but display is limited to +1700 to -1500 (15 mm). At -1298, secondary is at 3.353 m from primary.
• length and width of spider arms: 0.43 m and 5.7 mm (measured)
• internal diameter of top ring: 1.17 m (measured)