jay@npois.UUCP (Anton Winteroak) (12/03/85)
A few weeks ago, someone asked how many photons per square meter per second did we receive for objects of various stellar magnitudes. I have computed that from the following facts: 1. The sun is at apparent magnitude -26 to us. 2. The sun is 1.5 e11 meters away from us. 3. The sun gives off 4.0 e33 ergs per second. 4. The average energy of a photon from the sun is 2.1 eV. 4a. Therefore there are 2.9 e11 photons per erg. 5. I counted photons of all wavelengths (including ir, uv, etc.) 5a. For visable light, divide by about two. Apparent Mag. Photons/meter square per sec comments. -26 4e21 this is what we get from the sun. -25 2e21 -20 2e19 -15 2e17 as bright as a half moon. -10 2e15 Comet Kohoutek was supposed to get this bright. -5 2e13 Venus gets this bright now and then. Halley's Comet was this bright in 867. 0 2e11 Several Stars look this bright. 5 2,000,000,000 dimmest naked eye stars. 10 20,000,000 dim stars in big binoculars. 15 200,000 the planet Pluto, on a good night. 20 2,000 visable on photographs and image intensifiers only. 25 20 near the limit of what can be photographed from through our atmosphere. remember to divide by two to get visable photons, and by two again for atmospheric absorbtion.
jay@npois.UUCP (Anton Winteroak) (12/04/85)
The magnitude of the sun is -26.8, so divide by 2 again. The magnitude of Pluto is 13.8, not fifteen. So basically you could expect a one meter dish to collect 1 or 2 visable photons per second from an object of magnitude 26.
c20@nmtvax.UUCP (12/19/85)
> > A few weeks ago, someone asked how many photons per square meter > per second did we receive for objects of various stellar magnitudes. ... > Mag. Photons/sq. m./sec. Notes > 20 2,000 visible on photographs and > image intensifiers only. > 25 20 near the limit of what can > be photographed through > our atmosphere. It is interesting to note in regard to these numbers that under the right circumstances (full dark adaptation), the human brain will perceive a flash when the eye is presented with as few as seven photons (I can find the reference for this, if anyone wants). There is a certain necessary dis- tribution of photons on retinal rods in order for this minimum number to be seen. This is, as far as I know, the only case in which humans can perceive things at the elementary particle level; we are otherwise limited to the macroscopic universe. greg -- Greg Titus ..!ucbvax!unmvax!nmtvax!c20 (uucp) NM Tech Computer Center ..!cmcl2!lanl!nmtvax!c20 (uucp) Box W209 C/S c20@nmt (CSnet) Socorro, NM 87801 c20.nmt@csnet-relay (arpa) (505) 835-5735 ======================================================================