ELMER is an optical instrument for the GTC designed to observe between 3650 and 10000 Armstrong. The observing modes for the instrument at Day One shall be: Imaging, Long Slit Spectroscopy, Mask-multi-object spectroscopy, Slit-less multi-object spectroscopy, Fast Photometry and Fast short-slit spectroscopy. It will be installed at the Nasmyth-B focal station at Day One, but it will also be designed to operate at the Folded Cassegrain focal station. The physical configuration of the instrument consists of a front section where the focal plane components are mounted (Slit Unit) and a rear section with the rest of the components (Field Lens, Prism/Grism/VPH Wheel, Filter Wheel, Collimator, Camera, Folder Mirrors, Shutter and Cryostat with the detector). Both sections are connected through a hexapod type structure. The optical path is bent twice with the two folder mirrors providing a compact system.
The design phase of the ELMER Structure and Mechanisms finished on November 2002. Procurement and manufacturing covered from December 2002 to June 2003. Mechanical and electrical integration was accomplished on September 2003. Test campaign at factory covered from the end of September to mid November. Critical performance of the mechanics has been carefully tested during this period: positional tolerances of optical interfaces, repeatability of the 5 mechanisms (4 rotating wheels and collimator linear stage) and deflections of the instrument due to gravity. Results from the tests are widely within the specified values, providing a top performance instrument.