  We describe techniques concerning wavelength calibration and sky subtraction
to maximise the scientific utility of data from tunable filter instruments.
While we specifically address data from the Optical System for Imaging and low
Resolution Integrated Spectroscopy instrument (OSIRIS) on the 10.4~m Gran
Telescopio Canarias telescope, our discussion is generalisable to data from
other tunable filter instruments. A key aspect of our methodology is a
coordinate transformation to polar coordinates, which simplifies matters when
the tunable filter data is circularly symmetric around the optical centre.
First, we present a method for rectifying inaccuracies in the wavelength
calibration using OH sky emission rings. Using this technique, we improve the
absolute wavelength calibration from an accuracy of 5 Angstroms to 1 Angstrom,
equivalent to ~7% of our instrumental resolution, for 95% of our data. Then, we
discuss a new way to estimate the background sky emission by median filtering
in polar coordinates. This method suppresses contributions to the sky
background from the outer envelopes of distant galaxies, maximising the fluxes
of sources measured in the corresponding sky-subtracted images. We demonstrate
for data tuned to a central wavelength of 7615~$\rm\AA$ that galaxy fluxes in
the new sky-subtracted image are ~37% higher, versus a sky-subtracted image
from existing methods for OSIRIS tunable filter data.
