5. Optical system

• The lens system

  The collimator, one for all the scales and the observing modes, is a detached doublet (BaF2 -IRG2) with all spherical surfaces that transforms the f/11 TNG beam into a 22 mm parallel beam (f/# larger than about 2000) with a total level of aberrations less than 0.4 mrad. The blur of the entrance pupil image has a maximum size of 0.21 mm at 90% encircled energy; this is most important to guarantee an accurate background rejection with a Lyot stop. The level of total aberrations of the pupil image can be lowered by a factor 2 by means of a mildly aspheric entrance window, that is designed to leave unchanged the pupil plane positioning.

  The three optical trains for the cameras, and the pupil reimaging system, are mounted on a wheel, accurately driven by an external motor. These optics are based on a detached doublet (BaF2 -IRG2) followed by one or two lenses made of the same materials.

  The wide field camera (0.25"/pix) consists of 4 lenses. Its layout is shown in Figure 1. The spot blur on the detector is good (90% of the energy within a pixel inside a ~ 120" radius, 75% at corner), as reported in Table 5 and as shown in Figure 2 (spot diagrams). The exit pupil is at -88 mm, implying that the system is not completely telecentric; this introduces some vignetting losses ( 4.5% ) at the corners of the field.

  The small field camera (0.13"/pix) consists of 3 spherical lenses, and its layout is shown in Figure 3 . Its image quality is much better than wide field camera performance: the spot blur is smaller, distortion is basically absent (< 0.16% ), and the exit pupil is at about -350 mm, that makes the system almost telecentric. The data on the spot blur are reported in Table 5 and the spot diagrams are shown in Figure 4 .

  Once coupled with the adaptive optic f/35 module the scales of the two cameras are reduced by a factor of ~ 3, i.e. 0.08"/pix and 0.04"/pix, that are suitable for imaging at the diffraction limit of the telescope (0.1" FWHM and 0.14" FWHM in H and K band respectively).

  The low-resolution spectroscopic mode uses the wide field camera and a suitable set of grisms and order sorting filters that are inserted in the parallel beam after the pupil. Further details on this mode are given in the next section. The high-resolution (echelle) spectroscopic system should consist of a Si-grism and four lens, all mounted on the same wheel that hosts the other cameras, while the cross dispersors should be mounted on the second filter wheel. The lens system is designed to handle the large anamorphic magnification introduced by the grism. Details on a preliminary version of the optical system are given in [11],[12].





• Filters, grisms and polarizers

  The filters will be mounted on two wheels located in the parallel beam right after the pupil. The list of filters in Table 2 is still preliminary, and it might be modified depending on budget, feasibility and available space on the filter wheels. Filters will be tilted by 6° with respect to the optical axis to minimize ghosts effects. They should have an average transmission > 70% in the passed band. The narrow band filters should have a shift in central wavelength smaller than 0.25% at the corner of the field of view. The filter wheel will also hosts filters aimed at order sorting for the grisms.

  The low resolution grisms are resin replicated Milton-Roy gratings on IRgn6 prisms. They will be mounted on the second filter wheel, and they will be tilted with respect to the optical axis to achieve their best efficiency. Laboratory measurements, aimed at quantifying the total efficiency of the grisms after delivery, gave precise information about the required tilt angle. Grisms efficiencies and spectral resolution (1" slit) are shown in Figure 5 at the optimum tilt angle. One of the five grisms can operate at four different orders, depending on the filter selected on the first filter wheel that acts as order sorter. The spectral resolutions listed in Table 3 refer to the case of 1" slit. Wider slits cause the spectral resolution to drop proportionally, while with the 0.5" slit the grating anamorphism reduces the spectral resolving element in less than 2 pixels, and it is therefore undersampled.

  The silicon high resolution grism, is chemically etched and will have a blaze angle of 76°. It will be able to cover the whole near-IR range, at a resolution of ~ 9600, by using three low-angle silicon grisms as order sorters located on the second filter wheel. The large anamorphic magnification of the grism will squeeze the 1" slit into two pixel in the direction. The efficiency of the Si-grism and deviations in the periodicity of the groves pattern, that might produce spectral ghosts, are still matter of concern; further information should be available in the near future. A more detailed description on the silicon grisms can be found in [10].

  The second filter wheel will host a wedged double Wollaston [13], made of LiYF4 or MgF2 , that will deviate the rays of the parallel beam into four different beams corresponding to the polarization angles of 0, 45, 90 and 135 degrees; as a consequence, the input field of view will be splitted into four images on the detector, one for each of the four polarization angles. This system will enable to perform simultaneous measurements of the polarized flux at angles 0, 45, 90 and 135 degrees, that are required to derive the first 3 elements of the Stokes vector. A suitable field stop right after the focal plane will limit the field of view to about 1/4 of the total field, to avoid overlapping of the four polarized images on the detector plane.

  Another wedged double Wollaston, made of LiNbO3 and thinner than the former, will be located on the first filter wheel. This will allow to perform spectro-polarimetry by inserting in the beam one of the grisms that are located on the second filter wheel. The polarizer works with the same principle as the imaging mode, simultaneously delivering four polarized long-slit spectra at angles rotated by 45 degrees. The slit normally used in the spectroscopic mode will be masked to have an equivalent length of about 50" , to avoid the four polarized spectra to overlap on the detector.

Table 5: Optical quality at image and pupil plane



Figure 1: Layout of the collimator + large field camera.



Figure 2: Spot diagrams and encircled energy for the large field camera.



Figure 3: Layout of the collimator + small field camera.



Figure 4: Spot diagrams and encircled energy for the small field camera.



Figure 5: Efficiencies and spectral resolution of the low resolution grisms.



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