Bar Parameters from Hα Observations
The very first results of the application of the Tremaine-Weinberg (hereafter TW) method on Hα velocity fields are presented to find pattern speeds of galaxies. The technique is used for a sample of four barred galaxies and M51. For two of these, our results are very similar to those obtained using the same technique but based on CO observations. For the three other galaxies, a comparison is provided between this observational method and numerical ones.