PMAS: The Potsdam Multi-Aperture Spectrophotometer. I. Design, Manufacture, and Performance

We describe the design, manufacture, commissioning, and performance of PMAS, the Potsdam Multi-Aperture Spectrophotometer. PMAS is a dedicated integral field spectrophotometer optimized to cover the optical wavelength regime of 0.35-1 μm. It is based on the lens array-fiber bundle principle of operation. The instrument employs an all-refractive fiber spectrograph, built with CaF₂ optics, to provide good transmission and high image quality over the entire nominal wavelength range. A set of user-selectable reflective gratings provides low to medium spectral resolution of approximately 1.5, 3.2, and 7 Å in first order, depending on the groove density (1200, 600, 300 grooves mm^-1). While the standard integral field unit (IFU) uses a 16×16 element lens array, which provides seeing-limited sampling in a relatively small field of view (FOV) in one of three magnifications (8" × 8", 12" × 12", or 16" × 16"), a recently retrofitted bare fiber bundle IFU (PPak: PMAS fiber pack) expands the FOV to a hexagonal area with a footprint of 65" × 74". Other special features include a cryogenic CCD camera for field acquisition and guiding, a nod-shuffle mode for beam switching and improved sky background subtraction, and a scanning Fabry-Pérot etalon in combination with the standard IFU (PYTHEAS mode). PMAS was initially designed and built as an experimental traveling instrument with optical interfaces to various telescopes (Calar Alto 3.5 m, ESO VLT, LBT). It is offered as a common-user instrument at Calar Alto under contract to MPIA Heidelberg since 2002.