  We demonstrate spectrally-tunable Fabry-Perot bandpass filters operating
across the MWIR by utilizing the phase-change material GeSbTe (GST) as a
tunable cavity medium between two (Ge:Si) distributed Bragg reflectors. The
induced refractive index modulation of GST increases the cavity's optical path
length, red-shifting the passband. Our filters have spectral-tunability of ~300
nm, transmission efficiencies of 60-75% and narrowband FWHMs of 50-65 nm
(Q-factor ~70-90). We further show multispectral thermal imaging and gas
sensing. By matching the filter's initial passband to a CO$_{2}$
vibrational-absorption mode (~4.25 $\mu$m), tunable atmospheric CO$_{2}$
sensing and dynamic plume visualization of added CO$_{2}$ is realized.
