  We present the results of optical panoramic and long-slit spectroscopy of the
nebula MF16 associated with the Ultraluminous X-ray Source NGC6946 ULX-1. More
than 20 new emission lines are identified in the spectra. Using characteristic
line ratios we find the electron density n_e ~ 600cm^{-3}, electron temperature
in the range from ~9000K to ~20 000K (for different diagnostic lines) and the
total emitting gas mass M ~ 900 Msolar. We also estimate the interstellar
extinction towards the nebula as A_V = 1.m54 somewhat higher than the Galactic
absorption. Observed line luminosities and ratios appear to be inconsistent
with excitation and ionization by shock waves so we propose the central object
responsible for powering the nebula. We estimate the parameters of the ionizing
source using photon number estimates and Cloudy modelling. Required EUV
luminosity ($\sim 10^{40}$\ergl) is high even if compared with the X-ray
luminosity. We argue that independently of their physical nature ULXs are
likely to be bright UV and EUV sources. It is shown that the UV flux expected
in the GALEX spectral range (1000-3000Angstroms) is quite reachable for UV
photometry. Measuring the luminosities and spectral slopes in the UV range may
help to distinguish between the two most popular ULX models.
