The Facility Infrared Spectrapolarimeter for the Dunn Solar Telescope is an advanced imaging spectropolarimeter developed by the Institute for Astronomy - University of Hawai'i (P.I. Haosheng Lin) and the National Solar Observatory. This instrument provides simultaneous spectral coverage at visible and infrared wavelengths through the use of a unique dual-armed spectrograph design. The geometry of the spectrograph has been specially designed to capture the Fe I 6302 Å and 15648 Å lines with maximum efficiency. In addition, the spectrograph operates in a multiple slit mode. By using narrow band filters, the spectra from four consecutive slit positions can be imaged at once on the same detector. This feature greatly reduces the time necessary to scan across a large area on the sun, making it an ideal instrument for the study of quickly developing active regions.

FIRS is an off-axis reflecting Littrow configuration spectrograph which can operate in an f/36 wide field or f/108 narrow field mode. The DST is a 76 cm aperture vacuum tower telescope and provides a beam which has been processed by the High Order Adaptive Optics system (Rimmele et al. 2004). A scanning and stationary mirror allow the beam to be shifted during a rester. Additional focusing optics form an image on the FIRS 4 slit unit. Light passes through the slits to an off-axis parabolic mirror which collimates the beam. The beam is then spectrally dispersed by a 31.6 line/mm echelle grating with a 63.5º blaze. For the standard alignment, 6302 Å has been selected from the 90th order and 15648 Å has been selected from the 36th order; the steep blaze angle ensures that there is more light at these widely dispersed, high orders. The dispersed beam is refocused by the parabolic mirror a second time and then travels to the respective pick-off and fold mirrors for the visible and infrared detectors and their optics. Following this point the layout of the essential optics for each arm is identical. Astigmatism is inherent in the spectrograph due to the second reflection from off-axis paraboloid. In order to correct this astigmatism the next focusing element is a cylindrical lens which refocuses the beam along the vertical axis. Following this there are two re-imaging lenses which readjust the size and focus of the image on the camera. Between these two lenses are two liquid crystal variable retarders (LCVRs) which modulate the beam polarization in an efficiency-balanced tuning scheme (and there is space for a linear polarizer and quarter-wave plate for calibration of the LCVRs). Following the polarizers is a DWDM filter, or dense wavelength-division multiplexing filter, which has been adapted from optical communications technology. This narrow filter has been made especially for each wavelength and ensures that the spectra from adjacent slits do not overlap at the focal plane. A Wollaston prism, or polarizing beam splitter, acts as the analyzer for the modulated beam and vertically separates linearly polarized light into its orthogonal components, and sits immediately before the final focal plane. The detector for the infrared side is a Raytheon Virgo 1024 x 1024 HgCdTe array. The detector for the visible side is a Kodak 2048 x 2048 CCD.

A raster of a solar region is produced by stepping the telescope image across the FIRS slit unit across using the motorized field scanning mirror. For a single slit position in the raster both the visible and infrared arms obtain spectral images of each of the four polarization states provided by the LCVRs. The Stokes vector is recreated from these four images in post-processing. This method produces observations of the visible and infrared spectrum which are coincident in space and time with little ambiguity (although differential refraction caused by the Earth's atmosphere can cause a slight shift between the focused telescope image at different wavelengths). Each spectral image consists of four spectra where the orthogonal polarization states have been vertically separated by the Wollaston prism. During a scan the four slits fully sample four adjacent regions and for this reason FIRS shows a significant performance advantage over traditional single slit imaging spectrographs. The result after post processing is a Stokes data cube for the visible and infrared which contains the Stokes spectrum for each of the consecutive slit positions (roughly 1000 spatial pixels x 600 scan steps x 400 spectral pixels x 4 Stokes components for the visible wavelength data, and 500 spatial pixels x 600 scan steps x 200 spectral pixels x 4 Stokes components for the infrared wavelength data).

The current configuration of FIRS with HOAO will deliver diffraction limited observations of the Fe I 6302 and Fe I 15648, or the Fe I 6302 and He I 10830 solar magnetic field diagnostics. An additional beamsplitter placed in the AO optical bench allows FIRS to share the light path with the Interferometric Bidimensional Spectrometer (IBIS) which provides coverage of the chromospheric Ca II 8542 line.