NavList:

   

Reply

I think we're largely talking at cross purposes here, and I may try to deal 
with that further tomorrow, but for now I just want to talk about this 
business of constant "surface brightness". I remember when I first learned 
about it mathematically, in Astrophysics 201 or whatever it was called, over 
25 years ago. And my first thought was, "huh? surface brightness can't be 
changed by a telescope? Surely not." It's quite counter-intuitive, but the 
physics is undeniable. As food for thought, here's a couple of quotes from 
some astronomy and astrophysics textbooks that may be of interest for some of 
you:

[note: the correct astronomical/astrophysical term is "specific intensity" or just "intensity"]

From Novotny's "Introduction to Stellar Atmospheres and Interiors":
"No optical instrument can make a radiating surface appear more intense, since 
it cannot increase the amount of energy passing along a ray. A telescope 
gathers more light than the eye, but spreads it over an enlarged image. 
Hence, the unresolved continuum of stars in the Andromeda Galaxy appears just 
as disappointingly faint when viewed through a telescope as it does to the 
naked eye."

From Bradt's "Astronomy Methods":
"Interesting implications of this equality B=I surround us, for example 
starting a fire with a magnifying glass by focusing sunlight. Another 
ramification is that radiation passing through an optical system has the same 
brightness before and after its passage. Thus the moon has the same 
brightness when viewed with bionuclars as when it is viewed with the naked 
eyes. The image is bigger but the energy received per pixel of the retina, or 
per square arcmin, is the same. This is true of the sun also. Thus the filter 
material safe for naked eye viewing of the sun is also safe for viewing the 
sun with binoculars as long as it fully covers both objective lenses."

[the above should really mention that this refers to non-electronic optical 
systems --those which do not add energy to the light-- so of course "night 
vision" systems are not limited in this fashion]

I looked around a bit hoping to find a nice web article on this topic, but 
haven't had much luck. There's one from an NRAO astrophysics class here:
http://www.cv.nrao.edu/course/astr534/Brightness.html
which includes the following:
"The conservation of brightness also applies to any lossless optical system, a 
system of lenses and mirrors for example, that can change the direction of a 
ray.  No passive optical system can increase the specific intensity or total 
intensity of radiation.  If you look at the Moon through a large telescope, 
the Moon will appear bigger (in angular size) but not brighter."

-FER




--~--~---------~--~----~------------~-------~--~----~
Navigation List archive: www.fer3.com/arc
To post, email NavList@fer3.com
To , email NavList-@fer3.com
-~----------~----~----~----~------~----~------~--~---

   

Reply