[ATM] Extended Object Brightness

[Daniel Reinders <danreind@shaw.ca>]

Hi All,

So I've gone through the archives and read what I could about why you can't make an extended object brighter than would be seen by your eye (just bigger).  So you drop the magnification below a certain point and all that extra light you gathered just spills out onto the non-light receiving parts of the eyeball - got it.    

My newbie question then is: why is it so dangerous to look at the sun through a telescope (Note - please don't flame me with excited posts about not looking at the sun, I'm only raising it as a point of discussion, not an action plan).  As an extended object, the telescope should therefore make it no brighter (photons per retinal area) than with direct vision.  Now staring at the sun is a terrible idea even with the naked eye, but who hasn't looked up at the sun at some point in their lives for a second or two and not gone blind?  Or even just had it in your field of view?  Is it some subtle effect relating to the dark tube causing the iris to open up and then bang - you find the sun and your eye is not ready for it? Or is there some total amount of light received by the eye that cannot be surpassed?

Other question - when does an object transition between extended and point source (where telescopes can and cannot improve brightness).  Can I assume it is something to do with when the "disk" becomes smaller than the angular limit of resolution of the scope?  Along these lines, since a galaxy's diffuse light is a collection of point source points, it would seem to follow that there is some aperature of vast size (unimaginable?) where adding additional aperature does increase brightness since the stars become resolvable as point sources - true or false?  In fact, was not the resolution of variable stars in other galaxies the key to initially estimating inter-galaxy distances?

Thanks,

Dan Reinders

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