Quote:
Originally Posted by davewaldo
A larger telescope captures more light and produces a brighter image. Whether you decide to use higher magnification and therefore spread that light out again I think is up to the user. When your eye, a small scope and a large scope are all compared on equal terms, where all variables are the same ie. magnification, FOV etc. The images are brighter as you increase aperature.
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Thanks for considering and allowing me to rephrase things perhaps it will be clearer.
Yes the images *are* brighter as you increase aperture, but so does the minimum magnification needed increase, to get all of the light of the mirror into your eye. Lowering the magnification increases the exit pupil diameter, and beyond 6mm or so the light is just wasted. So its a kind of relationship you can't escape unless you are doing astrophotograghy. A good understanding of the concept of the exit pupil in a telescope then perhaps best to brush up on that or you won't follow the concepts I offer here.
So to recap : Given the limitations of our eye pupil ( lets say 6mm diameter for example) , every telescope has a magnification at which the image appears *just as bright* as the naked eye. This magnification is ( aperture in MM ) / Exit pupil ie 6mm. In the example I gave below, this is 100X for a 600mm aperture scope. if we use more magnification , then the exit pupil pupil and view becomes *dimmer* than the naked eye view. So every telescope will need a miniumum magnification of around 4X per inch to get all its light into the human eye.
The reason that the larger telescope doesn't give us a * brighter* image is that exit pupil and magnification are inextricably linked to the aperture. In order to get the light of a 600mm mirror into our 6mm pupil we must use 100X magnification, an action which reduces the surface brightness of the image by 10,000 times at the aperture of the naked eye, but we win back the light intensity by pumping the light of a mirror with 10,000 X the surface area of our pupil into the pupil ( a 600mm diameter mirror ! ). It all goes back Newton's inverse square law of light.
Heres another simple thought experiment. Imagine we put a star diagonal into the newtonian and angle it so we can look at the rising full moon both with one unaided eye and through the scope. Now pulling our eye back so the angular field is more and more reduced, until all we can see of the full moon through the scope is a disc subtending about 1/2 degree in aparent diameter. We now find that regardless of the telescope aperture , the Moon will look the same brightness when we compare the naked eye view to the small piece of the lunar surface we can see with the head pulled way back from the eyepiece
And yet the Moon 'seems' brighter when we put our eye up to the eyepiece, simply becasue there is som much more surface area to dazzle our eye becasue the telescope's magnification has given the Moon a much larger apparent angular size in our field of view !
Telescopes used for photograpghy , don't have the exit pupil limitation and for a given focal length you can pump in as much light as you wish depending on how much money you have to spend
The very reason this topic came up is because I know how the eye must have X amount of light hitting the cones ( ?) for colour sensitivity to activate, and it was only when I bumped my scopes pupil up to 5mm that th e colour intensity matched the naked eye view , so I know the brightness * per unit of angular view* must have approached the same between the naked eye view and telescopically.
I brought this subject up as it is a fascinating one, that is so counter -intuitive and will change the whole way you think about your telescope ;-) Obviously the increased aperture gives you increased resolution but that is a different subject altogether .
Limiting magnitude of stars ( point sources ) is something that varies with the seeing and has more to do with resolving power I think, this is what I would like to delve into understanding further.
