Potsdam Echelle Polarimetric and Spectroscopic Instrument

PEPSI is a high-resolution echelle spectrograph with two polarimeters for the Large Binocular Telescope (LBT) in Arizona.
Potsdam Echelle Polarimetric and Spectroscopic Instrument

PEPSI logo

PEPSI is a fibre-feed high-resolution Echelle spectrograph for the Large Binocular Telescope (LBT) in Arizona. It is designed to utilize the two 8.4m apertures of the LBT in a unique spectropolarimetric mode. With two identical but independent polarimeters, mounted in each of the direct F/15 Gregorian foci, this will allow the simultaneous observation of circularly and linearly polarized light with high spectral and temporal resolution. Besides the polarimeters, integral (non-polarized) light can be feed to the spectrograph via two permanently mounted focus stations, PFUs, thereby providing a high resolution standby spectrograph for the LBT Observatory. During the day, the spectrograph is fed with solar light from the SDI telescope.

The Spectrograph

The spectrograph is located on a 2mx6m optical table in the telescope pier, inside a pressure- and temperature-stabilized chamber. It receives light from the polarimeters and permanent focus stations via 45m long fibres, coupled with image slicers. The polarimetric mode provides a resolution of 120.000, while expected theoretical resolutions of 40.000, 120.000 and 300.000 can be obtained with the permanent focus stations through a choice of different fibres. A white-pupil two-arm design with a 200mm main beam diameter, utilizing the largest available monolithic echelle grating and VPH grisms for cross dispersion, is employed. The collimators are nominally off-axis Maksutov systems, allowing for a beam diameter reduction to 125mm in the blue and red transfer collimator arms. The two spectrograph cameras are equipped with a single monolithic 10.3kx10.3k CCD, with 9 micron pixels. For each spectral order, four spectra are recorded, i.e. two polarization states or object/sky for each telescope. The entire spectral range from 383 to 907 nm can be covered in three exposures.

The Polarimeters

The telescopes feed two identical but independent full Stokes IQUV polarimeters. The polarizing units will be based on interference-suppressed super-achromatic retarder plates and Foster prisms. A total of four fibers will simultaneously direct two ordinary and two extraordinary light beams to the spectrograph. Both polarimetric units are layed out in a modular design, each one optimized to the polarization state in which it is used. A number of observing modes can be chosen that are optimized to the type of polarization that is expected from the target, e.g. circularly and linearly polarized light simultaneously, or linearly polarized light in both polarimeters, or integral light from one and polarized light from the other telescope, a.s.o.. Calibration would be provided for each polarimeter separately.