Minor inconsistency with the `vcirc` output for a re-scaled, non-MWPotential2014 potential instance about galpy HOT 3 CLOSED

Thanks for the report! There a few things going on here:

1. turn_physical_on doesn't actually make it that the circular velocity at ro equals vo. It only does that if the potential is normalized such that vc(1.) = 1. in internal units. The latter is, for example, the case for MWPotential2014. In general, using turn_physical_on to change vo will re-scale all velocities by new_vo/old_vo. If you want to make the circular velocity a particular value at a particular radius, the best thing to do would be to compute the amplitude rescaling that makes this the case and then multiply the potential with that re-scaling.
2. Now, you might think that McMillan17 is such that vc(1.) = 1. in internal units, but the way it's defined that's not quite the case and in fact vc(1.) = 1.000424466056599. This slight difference from one leads to the issue that you see. This is, in fact, also an issue with the normal McMillan17 scaling, for which the circular velocity at R0 should be 233.1 km/s, but in galpy it's 233.2 km/s. This numerical inaccuracy results from how the McMillan17 potential is solved for in galpy (because it is a density-based model, we need to solve for the potential and that solution isn't perfect). The difference is much smaller than the uncertainty in the circular velocity, so shouldn't matter in practice, but of course there could be situations and tests where you need to account for it.

The only real bug here is that the McMillan17 circular velocity is in general ~0.1 km/s off, but as I explained above, I don't think that's an important issue (solving it would make using McMillan17 even more computationally expensive than it already is).

from galpy.

Thanks so much for the thoughtful response and clear explanation!

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