  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--1um. 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 in first order of approx. 1.5, 3.2,
and 7 A, depending on the groove density (1200, 600, 300 gr/mm). While the
standard integral field unit (IFU) uses a 16x16 element lens array, which
provides seeing-limited sampling in a relatively small field-of-view (FOV) in
one of three magnifications (8x8, 12x12, or 16x16 arcsec^2, respectively), a
recently retrofitted bare fiber bundle IFU (PPak) expands the FOV to a
hexagonal area with a footprint of 65x74 arcsec^2. 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-Perot 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 3.5m, ESO-VLT, LBT). It is
offered as a common user instrument at Calar Alto under contract with MPIA
Heidelberg since 2002.
