  The chromosphere is a highly complex and dynamic layer of the Sun, that
serves as a conduit for mass and energy supply between two, very distinct
regions of the solar atmosphere, namely, the photosphere and corona. Inferring
magnetic fields in the chromosphere, has thus become an important topic, that
can be addressed with large-aperture solar telescopes to carry out highly
sensitive polarimetric measurements. In this article, we present a design of a
polarimeter for investigating the chromospheric magnetic field. The instrument
consists of a number of lenses, two ferro-electric liquid crystals, a Wollaston
prism, and a CCD camera. The optical design is similar to that of a commercial
zoom lens which allows a variable f-number while maintaining focus and
aberrations well within the Airy disc. The optical design of the Adaptable
ChRomOspheric POLarimeter (ACROPOL) makes use of off-the-shelf components and
is described for the 70 cm Vacuum Tower Telescope and the 1.5 m GREGOR
telescope at Observatorio del Teide, Tenerife, Spain. Our design shows that the
optical train can be separated into two units where the first unit, consisting
of a single lens, has to be changed while going from the VTT to the GREGOR
configuration. We also discuss the tolerances within which, diffraction limited
performance can be achieved with our design.
