The Nasmyth Corrector is a system of lenses that allows the exploitation of the full 25 arcmin diameter field of view of the Nasmyth focus of the VLT. The diameter of the lenses is approximately 90 cm. The selected lens material is equivalent to BK7 with a single layer MgF2 coating guaranteeing a very good transmission over the whole 370-1400 nm range.

The corrector has three main purposes:

• to correct the field aberrations,
• to reduce the field curvature and
• to provide a concentric exit pupil to the field curvature.

The chosen design fulfills very well these requirements and provides an image quality better than 0.15 arcsec (diameter of a circle encompassing 80% energy) over the whole field of view. The lateral shift of the pupil is less than 3% of the pupil diameter except for the bluest wavelengths (370 nm) where it is reaches up to 6.5% at the edge of the field.

The corrector is mounted on a mechanical structure which serves two purposes: 1) to attach the corrector to the Nasmyth adapter and 2) to support the positioner plate during observations. Provision has also been made in the mechanical structure to allow easy access to the interior of the Nasmyth adapter. The VLT sensor arm is located before (following the direction of the light) the corrector.

The corrector has been successfully mounted and commissioned at Paranal in the first nights of October 2001.