- the right side of this equation, dxx, is unitless regardless of the units of x;
- therefore the left side, dlnx, must also be unitless;
- dlnx must have the same units as lnx;
- therefore lnx must also be unitless, regardless of the units of x.
The fact that dlnx specifies a fractional change in x has further repercussions in astronomy, because it is traditional to quote the measurement of a flux f in the magnitude system: m=−2.5log10ff0 where f0 is some reference flux. This means that a quoted uncertainty in the magnitude of a star or galaxy, dm, specifies a fractional uncertainty in the flux. Let's work out the details: log10x is the same as lnxln10 so dm=−2.5ln10dlnff0 dm=−2.5ln10dff Because ln10≈2.30, we get dm≈−1.086dff. For quick estimation purposes, the magnitude uncertainty is about the same as the fractional uncertainty in flux.
This explains why a 0.1 mag uncertainty is about a 10% flux uncertainty, regardless of the magnitude. One should not say that a 0.1 mag uncertainty is a 1% uncertainty in an m=10 star, nor a 0.5% uncertainty in an m=20 galaxy. For the quantity that matters---the flux of the object---a 0.1 mag uncertainty implies about a 10% uncertainty regardless of the flux.
No comments:
Post a Comment