The VLT-VIRMOS mask manufacturing unit

The VIRMOS Consortium had the task to design and manufacture two spectrogra phs for the ESO Very Large Telescope, VIMOS (Visible Multi-Object Spectrogr aph) and NIRMOS (Near Infrared Multi-Object Spectrograph). This paper descr ibes how the mask manufacturing unit (MMU), which cuts the slit masks to be used with both instruments, meets the scientific requirements and manages the storage and the insertion of the masks into the instrument. The compone nts and the software of the two main parts of the MMU, the mask manufacturi ng machine and the mask handling system, are illustrated together with the mask material and with the slit properties. Slit positioning is accurate wi thin 15 mum, equivalent to 0."03 on the sky, while the slit edge roughness has an rms on the order of 0.03 pixels on scales of a slit 5" long and of 0 .01 pixels on the pixel scale (0."205). The MMU was successfully installed during 2000 July/August at the Paranal Observatory and is now operational f or spectroscopic mask cutting, in compliance with the requested specificati ons.