jc@saber.UUCP (John Cincotta) (11/13/85)
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I have a question for the experts
How many photons per second per square meter are recieved from a star of
say 10th magnitude on the surface of the earth??
Name: John Cincotta
Mail: Saber Technology, 2381 Bering Drive, San Jose, California 95131
AT&T: 945-9600
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...{amd,ihnp4,ittvax}!saber!jcjeff@utastro.UUCP (Jeff Brown the Scumbag) (11/14/85)
> How many photons per second per square meter are recieved from a star of > say 10th magnitude on the surface of the earth?? For V magnitude = 0.04 (Vega, that is) and a wavelength of 5556 Angstroms, there are 948 photons per square centimeter per second per Angstrom (of wavelength) at the top of the Earth's atmosphere. The absolute calibration is good to a few percent; relative values (i.e., differences in magnitudes) are more accurate. Going to other colors is more complex than scaling a blackbody curve since Vega (defined to have color index zero) is not a black body, but if you stay in the visual (longward of the Balmer jump at 3650A and short of the Paschen jump about 8200) you can probably get away with it. To get beyond this you need to do all kinds of horrible things (like response functions for photometric bands and flux curves of standard stars) which rapidly go to problems even the experts would prefer to avoid, and you really have to go to the literature to find out how to do it right. To convert your 10th-mag star to photons per whatever you need the bandpass response function convolved with the spectrum of the star. The V band is about 900A wide centered at about 5445A. I will let you go from there, but if you ignore the color slopes, call the bandpass function a box 900A wide, and remember that magnitudes are a logarithmic unit (5 magnitudes <----> a factor of 100) you ought to get a number that's the right order of magnitude (which is all a lot of us ask for).... Jeff Brown the Scumbag {allegra,ihnp4}!{noao,ut-sally}!utastro!jeff jeff@astro.UTEXAS.EDU Astronomy Department, U. of Texas, Austin
wls@astrovax.UUCP (William L. Sebok) (11/15/85)
In article <1863@saber.UUCP> jc@saber.UUCP (John Cincotta) writes: >How many photons per second per square meter are received from a star of >say 10th magnitude on the surface of the earth?? From Allen's "Astrophysical Quantities" (Chapter 10): log f (V) = -0.4m - 8.43 lambda V -2 -1 -1 where f (V) is visual flux in erg cm Angstrom sec outside the Earth's lambda atmosphere near 5500 Angstroms. "This relation is almost unchanged from B to M stars." Therefore (plugging in numbers), for m = 10 V -13 -2 -1 -1 f = 3.72 x 10 erg cm sec Angstrom lambda Now a photon at a wavelength of 5500 Angstrom has an energy of E = h(c/lambda) -27 18 = (6.626 x 10 erg-sec) (2.998 x 10 Angstrom/sec) / (5500 Angstrom) -12 = 3.612 x 10 ergs this yields -2 -1 Flux(5500 Angstrom) = 0.11 photons cm Angstrom -2 -1 = 110 photons meter Angstrom For a 1000 Angstrom bandpass (roughly since I am too lazy now to do the integral and the whole business is approximate anyway) this is about 110,000 photons / square meter for that 10th magnitude star. -- Bill Sebok Princeton University, Astrophysics {allegra,akgua,cbosgd,decvax,ihnp4,noao,philabs,princeton,vax135}!astrovax!wls