  We investigate the utility of the Tunable Filters (TFs) for obtaining flux
calibrated emission line maps of extended objects such as galactic nebulae and
nearby galaxies, using the OSIRIS instrument at the 10.4-m GTC. Despite a
relatively large field of view of OSIRIS (8'x8'), the change in the wavelength
across the field (~80 Ang) and the long-tail of Tunable Filter (TF) spectral
response function, are hindrances for obtaining accurate flux calibrated
emission-line maps of extended sources. The purpose of this article is to
demonstrate that emission-line maps useful for diagnostics of nebula can be
generated over the entire field of view of OSIRIS, if we make use of
theoretically well-understood characteristics of TFs. We have successfully
generated the flux-calibrated images of the nearby, large late-type spiral
galaxy M101 in the emission lines of Halpha, [NII]6583, [SII]6716 and
[SII]6731. We find that the present uncertainty in setting the central
wavelength of TFs (~1 Ang), is the biggest source of error in the emission-line
fluxes. By comparing the Halpha fluxes of HII regions in our images with the
fluxes derived from Halpha images obtained using narrow-band filters, we
estimate an error of ~11% in our fluxes. The flux calibration of the images was
carried out by fitting the SDSS griz magnitudes of in-frame stars with the
stellar spectra from the SDSS spectral database. This method resulted in an
accuracy of 3% in flux calibration of any narrow-band image, which is as good
as, if not better, to that is feasible using the observations of
spectrophotometric standard stars. Thus time-consuming calibration images need
not be taken. A user-friendly script under the IRAF environment was developed
and is available on request.
