Stargazing Telescope Illumination: Mirror Diameter & Focal Length

[CaptainMarvel]

Hi there,

Could somebody explain how illumination - by which I mean flux per unit area - depends on the mirror diameter of a telescope and/or its focal length?

Is this different for point objects like stars and for extended objects like nebulae?

Thanks in advance,

Nick :-)

 

[mgb_phys]

In simple terms the larger the aperture, the more light you can capture.
It's a terrible shame to be a little photon thqat has made it across the universe and then hits the ground just outside your telescope lens!
The effect of focal length and magnification is a little more complicated, assumign you are talking about a viewing telescope (ie with an eyepiece rather than a camera)
This article is a good introduction http://www.skyandtelescope.com/equipment/basics/3077091.html?page=1&c=y Sky and Telescope is a good source of articles on amateur astronomy.