willner@cfa.HARVARD.EDU (Steve Willner, OIR) (01/18/90)
> In article <1730001@hpislx.HP.COM> gvg@hpislx.HP.COM (Greg Goebel) writes: >>"Spectral Resolution" for FIRAS: >> 0.2 cm^-1 From article <1990Jan14.233158.9704@utzoo.uucp>, by henry@utzoo.uucp (Henry Spencer): > Looks fine to me. The spectroscopists have this curious habit of using > waves per unit length ("wavenumber") rather than length per wave (normal, > conventional "wavelength"). Right so far; wavenumber is useful because it is essentially a frequency unit. It just has the speed of light divided out. The unit even has an official name ("kayser"), which is seldom used. > A wavenumber resolution of 0.2 cm^-1 is a wavelength resolution of 5cm. This part was a bit too hasty. Wavenumber resolution of 0.2 cm^-1 is a frequency resolution of 6x10^9 Hz (in vacuum; very slightly different in air). The corresponding wavelength resolution depends on the wavelength (or frequency) being observed. The correct formula can be found by setting dw/w = dW/W, which leads to dW = dw/w^2, where dW is wavelength resolution, dw is wavenumber resolution, and w is wavenumber and W is the wavelength. (It's really easy to get confused, though; I had to write the conversion formulas down even though I work with these units all the time.) I forget exactly what the wavelength for FIRAS is, but I think it is about 1 mm or 10 cm^-1. The spectral resolution stated would then correspond to 0.02 mm. ------------------------------------------------------------------------- Steve Willner Phone 617-495-7123 Bitnet: willner@cfa 60 Garden St. FTS: 830-7123 UUCP: willner@cfa Cambridge, MA 02138 USA Internet: willner@cfa.harvard.edu