  This paper presents the concept of a community-accessible stratospheric
balloon-based observatory that is currently under preparation by a consortium
of European research institutes and industry. The planned European
Stratospheric Balloon Observatory (ESBO) aims at complementing the current
landscape of scientific ballooning activities by providing a service-centered
infrastructure tailored towards broad astronomical use. In particular, the
concept focuses on reusable platforms with exchangeable instruments and
telescopes performing regular flights and an operations concept that provides
researchers with options to test and operate own instruments, but later on also
a proposal-based access to observations. It thereby aims at providing a
complement to ground-, space-based, and airborne observatories in terms of
access to wavelength regimes - particularly the ultraviolet (UV) and far
infrared (FIR) regimes -, spatial resolution capability, and photometric
stability. Within the currently ongoing ESBO Design Study (ESBO DS), financed
within the European Union's Horizon 2020 Programme, a prototype platform
carrying a 0.5-m telescope for UV and visible light observations is being built
and concepts for larger following platforms, leading up to a next-generation
FIR telescope are being studied. A flight of the UV/visible prototype platform
is currently foreseen for 2021. We present the technical motivation, science
case, instrumentation, and a two-stage image stabilization approach of the
0.5-m UV/visible platform. In addition, we briefly describe the novel mid-sized
stabilized balloon gondola under design to carry telescopes in the 0.5 to 0.6 m
range as well as the currently considered flight option for this platform.
Secondly, we outline the scientific and technical motivation for a large
balloon-based FIR telescope and the ESBO DS approach towards such an
infrastructure.
