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Homework Help: Unit Conversion - Flux Densities

  1. Mar 10, 2014 #1
    1. The problem statement, all variables and given/known data
    I need to convert from a fv in units of Jy to fλ in units of erg s^-1 cm ^-2 A^-1

    fv = 1.0254e-2 Jy

    2. Relevant equations
    fv dv = fλ dλ
    fλ = fv dv/dλ
    and because v = c/λ...

    fλ = fv*c / λ^2

    Also 1 Jy = 10^-23 erg cm^-2 s^-1 Hz ^-1
    3. The attempt at a solution

    fλ = [(1.0254e-2 Jy)(10^-23 erg cm^-2 s^-1 Hz ^-1) (3e10 cm/s)(1A/10^-8 cm)] / [1 A^2]

    *note: A = angstrom units

    my final answer was fλ = 3.0762e-7 erg s^-1 cm^-2 A^-1

    But I checked my answer on an online converter and it was wrong, but I don't know why because all my units cancelled out correctly.
  2. jcsd
  3. Mar 10, 2014 #2


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    Science Advisor

    You have done the conversion as though your given fv was measured at a wavelength of 1A. I doubt this is the case. When you put in the c/λ^2 factor, you need to do it at the wavelength (frequency) of the measurement. Was this given?
  4. Mar 10, 2014 #3
    The first part of the question had us convert a magnitude (12) into fv given a zero point for the magnitude system (fv=647 Jy). So I used m-m0 = -2.5 (f/f0) to solve for f where m0 = 0 and f0 = 647 Jy and m =12. This gave a value of fv = 1.0254e-2 Jy. Then we just have to convert that value into fλ

    Here's the question as it appears on the assignment:
    Given that the zeropoint of the magnitude system is fv=647 Jy. Find the flux density of a 12th magnitude star in Janskeys (which is in fv) and then convert it to fλ (in units of ergs^-1cm^-2A^-1)(ignore any minus signs in the conversion).

    That's all the information I have.
  5. Mar 10, 2014 #4


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    Science Advisor

    Well, I may be wrong, but I think you have to assume some wavelength to do the conversion. If they are visible magnitudes, I would use the center of the V band, which is at 0.55 μm.
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