  The GREGOR Fabry-P\'erot Interferometer (GFPI) is one of three first-light
instruments of the German 1.5-meter GREGOR solar telescope at the Observatorio
del Teide, Tenerife, Spain. The GFPI allows fast narrow-band imaging and
post-factum image restoration. The retrieved physical parameters will be a
fundamental building block for understanding the dynamic Sun and its magnetic
field at spatial scales down to about 50 km on the solar surface. The GFPI is a
tunable dual-etalon system in a collimated mounting. It is designed for
spectrometric and spectropolarimetric observations between 530-860 nm and
580-660 nm, respectively, and possesses a theoretical spectral resolution R of
about 250,000. Large-format, high-cadence CCD detectors with sophisticated
computer hard- and software enable the scanning of spectral lines in time-spans
equivalent to the evolution time of solar features. The field-of-view (FOV) of
50" x 38" covers a significant fraction of the typical area of active regions
in the spectroscopic mode. In case of Stokes-vector spectropolarimetry, the FOV
reduces to 25" x 38". We present the main characteristics of the GFPI including
advanced and automated calibration and observing procedures. We discuss
improvements in the optical design of the instrument and show first
observational results. Finally, we lay out first concrete ideas for the
integration of a second FPI, the Blue Imaging Solar Spectrometer (BLISS), which
will explore the blue spectral region below 530 nm.

Instrumentation and Methods for Astrophysics;Solar and Stellar Astrophysics