Re: [APML] Image Brightness (was Color of Objects (was Astronomy Magazine M8))

[Gene Horr <genehorr@texas.net>]

Kevin Wigell wrote:
 
> Stosh, I'm having a very hard time believing your statement that a telescope
> does not increase the brightness of an image to the eye. My 10" scope allows
> me to see to at least 14th magnitude from my backyard, while naked eye
> limiting magnitude from there is about 5. So if what you say is true, why
> can't I walk out into my backyard, look up, and see millions of stars to
> 14th magnitude?

His statement was imcomplete.  It should have included "... for extended
objects."  It is true, though somewhat misleading.  A telescope cannot
increase the photon flux/area for an extended object.  But it does increase
total flux for the object, but at the same time magnifying the object so
that this increased light is spread across a larger area.  There is more
total energy (and information) reaching your eye, but the light/apparent
area for the object is the same.

Point sources work somewhat differently.  A star of a given magnitude
through a f/6 of one design will theoretically be the exact same size
as a star looked at through the same design but larger aperture.  So
you have more photons focused on an area of the same size, hence making
it brighter over the same area.

I know it sounds counter-intuitive at first, but once you think about it
it starts to make sense.

Gene Horr
genehorr@texas.net

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