  We present TransitFit, a package designed to fit exoplanetary transit
light-curves. TransitFit offers multi-epoch, multi-wavelength fitting of
multi-telescope transit data. TransitFit allows per-telescope detrending to be
performed simultaneously with transit parameter fitting, including custom
detrending. Host limb darkening can be fitted using prior conditioning from
stellar atmosphere models. We demonstrate TransitFit in a number of contexts.
We model multi-telescope broadband optical data from the ground-based SPEARNET
survey of the low-density hot-Neptune WASP-127b and compare results to a
previously published higher spectral resolution GTC/OSIRIS transmission
spectrum. Using TransitFit, we fit 26 transit epochs by TESS to recover
improved ephemeris of the hot-Jupiter WASP-91b and a transit depth determined
to a precision of 111 ppm. We use TransitFit to conduct an investigation into
the contested presence of TTV signatures in WASP-126b using 180 transits
observed by TESS, concluding that there is no statistically significant
evidence for such signatures from observations spanning 27 TESS sectors. We fit
HST observations of WASP-43b, demonstrating how TransitFit can use custom
detrending algorithms to remove complex baseline systematics. Lastly, we
present a transmission spectrum of the atmosphere of WASP-96b constructed from
simultaneous fitting of JWST NIRISS Early Release Observations and archive HST
WFC3 transit data. The transmission spectrum shows generally good
correspondence between spectral features present in both datasets, despite very
different detrending requirements.
