  The VIRMOS Consortium has the task to design and manufacture two
spectrographs for ESO VLT, VIMOS (Visible Multi-Object Spectrograph) and NIRMOS
(Near Infrared Multi-Object Spectrograph). This paper describes 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 components and the software of
the two main parts of the MMU, the Mask Manufacturing Machine and the Mask
Handling System, are illustrated together with the mask material and with the
slit properties. Slit positioning is accurate within 15 micron, equivalent to
0.03 arcsec on the sky, while the slit edge roughness has an rms on the order
of 0.03 pixels on scales of a slit 5 arcsec long and of 0.01 pixels on the
pixel scale (0.205 arcsec). The MMU has been successfully installed during
July/August 2000 at the Paranal Observatory and is now operational for
spectroscopic mask cutting, compliant with the requested specifications.
