Imaging circle.

[Alien 13]

I have been looking at the topic regarding "ES Reid checked scopes" and there was a discussion about the corrected image circle of certain scopes and this had me thinking about why its so tiny.

I have often played around with binocular objectives producing a projected image on a wall from a window and the resultant image is at least a foot across, its similar with my short FL scope so whats going on, are we throwing 95% of the light away by only having an image circle of 44mm or so or should we be all using much bigger curved sensors?

Alan

[Davey-T]

I think it's something to do arc tangents and such, can't remember as it's well over 50 years since I was at school 

Mind a DSLR image is pretty big

Dave

[JamesF]

I'd guess that the images you project on the wall are not in really sharp focus however.

If you think about light leaving feature on the Moon, say, it spreads out in every possible direction.  Some of that spreading light will end up hitting your (simple refractor) telescope or binocular objective.  Not in one place, but all over it.  The job of the objective is then to gather all that light hitting its surface from a distant point and organise it all to arrive at the same place (one particular spot on the focal plane) so you can look at it with an eyepiece (or a camera sensor).  The only place that happens is a spot at the focal length away from the objective.  Projecting the image on the wall will work, but the light from that single point on the Moon is now spread out over a larger area and no longer in focus.

Also, because the light on your wall is spread out more, the image will not be as bright (less light/fewer photons per unit area).

So no, it's not really that light is being "wasted", it's just concentrated in a much smaller, brighter, in-focus (hopefully :) area.

Curved sensors to account for the fact that the focal plane isn't actually, err, planar might be quite cool, but I'd guess it's a lot cheaper to add optics to correct the field to be flat than it is to try to manufacture a curved sensor that by its nature would have to be specific to a small number of telescopes.

James