FRIDA will be placed on the Nasmyth B platform of the Gran Telescopio de Canarias, athrough a support structure that will have sufficient degrees of freedom to align it with respect to the adaptive optics system (GTCAO).

Its design contemplates:

A Cryostat: body where all the subsystems are contained.

Several Subsystems: elements that make it possible for FRIDA to provide its two modes of operation.

Criostato

The cryostat is a 2.5 m³ airtight dewar that withstands (and maintains) liquid nitrogen temperatures, i.e., of the order of –196 °C (77 K).

It has a rectangular shape with a cylindrical front part, called extension, where the entrance window is located, allowing the beam coming from the telescope, already corrected by GTCAO, to coincide with the first focal plane of FRIDA.

Inside, there is a tank that can be filled with liquid nitrogen, whose cover serves as an optical bench where all the cryogenic subsystems are located.

On its exterior, it has all the necessary ports to attach vacuum pumps, closed-cycle coolers, cables, etc.

The operating conditions of the instrument are obtained by first achieving high vacuum conditions inside the cryostat, of the order of 10^-7 mbar, with the help of a turbo molecular pump and a mechanical pump, and inside the liquid nitrogen tank, of the order 10^-3 mbar, using another mechanical pump. Then, it is cooled with liquid nitrogen and with the two closed-cycle coolers.

In addition, the cryostat has cold and adiabatic shields inside it that allow the subsystems to be isolated from the outside and cooled by radiation.

Subsystems

FRIDA has a classical collimator-camera refractive system, a mechanism that provides the different spatial scales, a mechanism that contains a large variety of filters within the operating range of the instrument, and an integral field unit (IFU) that, together with a spectrograph with a double-pass optical system and several diffraction gratings, makes integral field spectroscopy (IFS) possible.

All mechanisms using wheels have a gear wheel geared to a worm gear that is driven by a cryogenic stepper motor. The starting reference positions are defined by a high-precision microswitch.

All refractive components, collimator, cameras and spectrograph are based on achromatic lens arrays: dobletes de CaF2–S–FTM16, postitive or negative doublets, and CaF2–Infrasil301 doublet.

The cryogenic mounts used were designed to maintain the centering and spacing of the lenses, since, due to the materials used in the doublets, it was impossible to glue them together. In addition, they control the mechanical stress that the cells exert on the lenses, mitigate possible thermal shocks due to temperature changes and guarantee alignment even after the numerous thermal cyclings that the instrument will undergo, as well as possible shocks and accelerations during transport

Focal plane wheel (FW)

This is the mechanism that provides the field limiters for both modes of operation, as well as the calibration masks and the masking masks that will allow the acquisition of high contrast images.

Pupil Mechanism (PM)

This mechanism has three wheels, two of which house a total of 19 filters each, and the third has the pupil masks with different apertures.

Classic refractive system Collimator-Camera and Cameras wheel (CW)

It has the function of:

Place the GTC pupil images in the pupil plane of the FRIDA pre-optics

Place the output image of GTCAO at the second focal plane, where the detector is located, in the case of imaging mode, or at the entrance of the spectrograph optics, in the case of integral field spectroscopy mode.

To achieve the spatial scales offered by FRIDA, four cameras providing linear amplification are used, one of them fixed, Medium Camera (CM) and the other three interchangeable, Coarse (CC), Fine (FC) and Pupillary (PC) cameras, mounted on a camera wheel (CW).

Mechanism switching (MS)

After the beam coming from the GTC-AO passes through the focal plane wheel, the pupil wheel and through the collimator-camera system it reaches the mode switching mechanism, which sends the beam:

directly to the detector, if the image mode is chosen.

directly to the second focal plane position, so that it passes through the IFU and then to the optics of the spectrograph, if the integral field spectroscopy (IFS) mode is chosen

Integrated Field Unit (IFU)

The integral field spectroscopy technique used by FRIDA consists of an image slicer, thus avoiding loss of information as with other IFS methods.

The IFU has a slicer capable of dividing the image into 30 slices, three mirror blocks with 30 spherical mirrors each and a Schwarzschild relay. All these elements allow to slice the image, amplify it linearly and arrange it, without losing its quality, in a known order forming a pseudo slit, ready to be diffracted after passing through the double pass collimator-camera system of the spectrograph optics.

Spectrograph (OB)

The spectrograph has a double-pass optical system that receives the image sliced by the integral field unit and arranged in the pseudo-slit, which it sends to the grating carousel, and then receives the scattered light that it redirects to the detector.

Grating carousel (GC)

The grating carousel (GC) is a circular mechanism with two mapping mirrors for the IFU and seven diffraction gratings in "quasi-Littrow" configuration, i.e. one low resolution, four medium resolution and two high resolution gratings. All gratings work in first order with the exception of the low resolution grating which works in second order. To achieve this, the design places the different gratings in such a way that the quasi-Littrow angle is different for each of them. Due to the repeatability requirements of this mechanism it was necessary to include a closed-loop absolute encoder

Detector and Focus mechanism (FM)

The detector to be used by FRIDA is a 2048x2048 pixel HAWAII-2RG™ dwhich is attached to a focusing mechanism (FM) that will allow selecting the correct position to acquire the images or spectra according to the observation mode configuration chosen.

Calibration Unit (CU)

A calibration unit located outside the cryostat between FRIDA and GTC-AO is available for calibrations.