We expect polarimetry in K, H and J bands with no significant degradation of the optical quality with respect to the image mode in the FOV. We require a distortion of less than 2.5 pixels (~0.25''). Because most of the additional distortion is due to chromatic aberrations image polarimetry requires special attention. Particular attention is paid for longer wavelengths. Optical degradation in the spectropolarimetric mode shows as a geometrical distortion of the spectra on the detector. The main issue in this mode is the overlapping of the extraordinary and ordinary (e-o) rays at the edges of the spectra. In addition, the system throughput requires being greater than 75% at all the wavelength ranges. The optical tolerances of the polarization measurements must verify the constraints defined by the mechanical restrictions of the present design.
CIRCE is an asymmetric refractive system except for the entrance windows, filters, grisms and the polarimetric components. It is designed to be achromatic and encircle 80% of the incident energy in 2 pixels. Figure 1 shows the CIRCE optical design, composed of an entrance window before the focal plane, a collimator, a camera and a filter wheel. The collimator is composed of two mirrors C1 and C2. The flat mirrors (F1 and F2) before the collimator serve to fold the instrument to fit GTC envelope specifications. Camera mirrors M1, M2, M3 and M4 are situated after filter wheels. Wheels allow filters, grisms and other optical components to be switched during the night.
Images from the telescope are located in the focal plane P0 between the entrance window and F1. The HWP is inserted close to P0 where the beam size is minimal. The focal plane mechanism consists on two masks (see Annex A) that are translated parallel to the focal plane. The focal plane mask contains three long slits 0.4'', 0.7'' and 1'' width for regular spectroscopy, 9 slits for spectropolarimetry and 3 slots for image polarimetry. Three spectropolarimetric slits of the same size (0.4'', 0.7'' or 1'') can be selected simultaneously in order to extend the FOV to 2'. Each slit has a FOV of 12''. Similarly the FOV of each slot is 12''x50'' but the global field in image polarimetry extends to 50''x2'. One of the slots is situated in the center of the image in order to combine with high-speed readout. Each mode is selected by placing the required slit/slot in front of the assigned aperture in the Decker mask. This mask contains also the HWP that can be rotated along the whole range of angles. The HWP covers the central slot and slit allowing more accurate measurement of the polarimetry in the central field. The splitter is a cemented Wedged Double-Wollaston (WeDoWo). It is inserted in the Lyot wheel at the pupil plane P1 in the collimated beam. This configuration makes possible simultaneous measurements of the three linear STOKES parameters with a single observation. The WeDoWo (Oliva 1996) consists in two MgF2 27.5x55mm2 prisms dividing the pupil intensity in four beams with angles about +1.5deo/+0.5deo/-0.5deo /-1.5deo, polarization main axes at 0°/90°/+45°/-45° and width ~25mm. "Bernhard Halle Nachfolger GmbH" company offers both HWP and WeDoWo with these characteristics for $6470 and $9945 respectively.