SPeaking as a reenactor I pretty much agree with Kimmers, above. Most reenactors cannot afford (nor would they want to risk damage in the field) true period instruments. We must adapt a bit to what is available. I take my banjo (nylon strung), and dulcimer to many events. Most of our playing is done after the camps are closed to the public. There have been occasions when we have been sitting around playing our instruments and spectators have asked the usual barrage of questions;
'Is that a real dulcimer?'
'Was that banjo around during the civil war?'

We usually cover a multitude of questions with a few simple answers.,
'Yes, it's a real dulcimer, no it probably would not have been found in a period camp.' 'Yes, the music we play is a close to period as we can produce it'
We take the time to explain the differences between the music of the period and that which groups such as Time-Life attempt to pass off as authentic. I feel we give the spectator enough knowledge that if they are truly interested they will go and research more on their own.