Optical Design

The EMIR optical design is based on a fully refractive system with axial symmetry, using a classical collimator-camera configuration, which provides a nominal pupil of 100 mm in diameter.

The EMIR collimator becomes a refractive system and is made up by four lenses (one single lens, CO1, and one splitted triplet, CO2, CO3 and CO4). A periscope is located between the collimator lenses CO1 and CO2 to fold the optical path in order to fit the instrument to GTC envelopes and achieve a more compact design. The dimensions and shape of the periscope mirrors shall be calculated not to vignette or obstruct the beam of a 6 x 6 arcmin FOV.

A circular IR grade Fused Silica window with optical power, having a physical diameter of 500 mm, is used as the light entrance port to the instrument. The use of this window moves the telescope image plane by 35,32 mm from its nominal location. This position of the entrance focal plane (at 420.32 mm from the telescope rotator flange) is where the field stop (for imaging), or the slit system (for spectroscopy), are located. This EMIR focal surface is not a plane but a sphere with a radius of curvature of 1801.57 mm and concave to the telescope.

An all-refractive camera is used to focus the collimated beam on a detector. The camera consists of 6 lenses (named CA1 to CA6), all spherical surfaces except one aspheric surface on the rear side of lens CA6. The camera EFL (effective focal length) of 190.8 mm allows to obtain the required detector scale (~0.2 arcsec/pixel) on a flat focal plane. With the present element apertures, the camera does not vignette a telescope FOV of 6x6 arcmin.


A main feature in the optical design of an infrared camera is its cold aperture stop to reduce the background noise. The GTC entrance pupil or aperture stop is the secondary mirror, as in any infrared telescope. The size of the cold stop is mainly defined by the required spectral resolution (R » 4000) using grisms between 0.90 and 2.50 mm. We choose a nominal collimated beam of 100 mm diameter, to achieve a resolution of 4250 with a 40º ZnSe (n » 2.45) grism, with a slit width of 0.6 arcsec. The dispersion component has a novel design and is based on a pseudo grism proposed by LAM whose diffractive component has been developed by Jobin Yvon (France). It consist in a high efficiency diffractive pattern, which is engraved on a fused silica substrate. To manufacture the grating first a mask is recorded holographically in photoresist and then the mask is transferred into the substrate by ion etching techniques. The difficulty arises from the fact that it is important the modulation depth to get a high efficiency in transmission mode. The transmission grating is sandwiched by two symmetric prisms to tune the undeviated wavelength and to set the spectral resolution in the instrument.

The change between the imaging and spectroscopic observing modes is made inserting or removing the diffractive element in the pupil area of the instrument (using wheels).