  High redshift star-forming galaxies are discovered routinely through a flux
excess in narrowband filters (NB) caused by an emission line. In most cases,
the width of such filters is broad compared to typical line widths, and the
throughput of the filters varies substantially within the bandpass. This leads
to substantial uncertainties in redshifts and fluxes that are derived from the
observations with one specific NB. In this work we demonstrate that the
uncertainty in measured line parameters can be sharply reduced by using
repeated observations of the same target field with filters that have slightly
different transmittance curves. Such data are routinely collected with some
large field imaging cameras that use multiple detectors and a separate filter
for each of the detectors. An example is the NB118 data from ESO's VISTA
InfraRed CAMera (VIRCAM). We carefully developed and characterized this method
to determine more accurate redshift and line flux estimates from the ratio of
apparent fluxes measured from observations in different narrowband filters and
several matching broadband filters. Then, we tested the obtainable quality of
parameter estimation both on simulated and actual observations for the example
of Ha in the VIRCAM NB118 filters combined with broadband data in Y, J, H. We
find that by using this method, the errors in the measured lines fluxes can be
reduced up to almost an order of magnitude and that an accuracy in wavelength
of better than 1nm can be achieved with the ~13nm wide NB118 filters.
