  Two independent investigations of the atmosphere of the hot Jupiter HAT-P-12b
by two different groups resulted in discrepant solutions. Using broad-band
photometry from the ground, one study found a flat and featureless transmission
spectrum which was interpreted as a gray absorption by dense cloud coverage.
The second study made use of the Hubble Space Telescope (HST) observations and
found Rayleigh scattering at optical wavelengths caused by haze. The main
purpose of this work is to find the source of this inconsistency and provide
feedback to prevent similar discrepancies in future analyses of other
exoplanetary atmospheres. We studied the observed discrepancy via two methods.
With further broad-band observations in the optical wavelength regions, we
strengthened the previous measurements in precision and with a homogeneous
reanalysis of the published data, we managed to assess the systematic errors
and the independent analyses of the two different groups. Repeating the
analysis steps of both works, we found that deviating values for the orbital
parameters are the reason for the aforementioned discrepancy. Our work showed a
degeneracy of the planetary spectral slope with these parameters. In a
homogeneous reanalysis of all data, the two literature data sets and the new
observations converge to a consistent transmission spectrum, showing a
low-amplitude spectral slope and a tentative detection of potassium absorption.
