Telescope collecting light

[Sconesbie (Scott)]

Over the last couple of nights, I've been pondering something. It may seem weird and I'm certainly no maths expert nor am I a scientist but I often think of weird and wonderful things.

This question is purely hypothetical and random.

If I have a telescope that is 6" diameter (hypothetically), and add a funnel type thing on the end that the diameter at the opening is 10" and tapers down to the 6" diameter to the tube of the telescope, would this allow more light to be collected into the tube itself?

If not, where would the excess light go? Does it just sit in the funnel? Does it make a difference to viewing?

If the scenario was to be a tube filled with water and a funnel added to the end of the tube, it could hold more water so why wouldn't it hold more light?

As I said, this is purely hypothetical and not a serious question by any means so have a bit of fun but I'm curious to know if it would make a difference at all.

[multiweb (Marc)]

The aperture of your scope is still 6". Star light is parallel to the axis of your OTA tube. Anything that falls outside of the aperture doesn't land on the mirror or the lens. The only advantage of your system depending on its length would be to cut off street lights, etc... to some extent or act as a dewshield to limit condensation.

[julianh72 (Julian)]

But in theory, if your "funnel type thing" had a suitable lens at the far (big) end that was designed to focus the light from the 10" aperture in such a way that it hits the 6" mirror (or objective lens) as parallel rays, then yes, it could increase the aperture and light-gathering power of your telescope - but what you have really done is made a bigger 10" compound telescope!

I suspect the optics of such a telescope with a large aperture objective lens and an intermediate aperture mirror / objective are not practical - or somebody would surely have already built such a beast! You can have a go at designing one using an optics simulator like this:
http://physics.bu.edu/~duffy/java/Opticsa1.html

[raymo]

If the inner surface of the funnel was actually a tapered mirror, so the light that would normally have missed the scope's primary mirror was deflected onto it, I imagine you'd end up with a strange image. I bet it'd
be a bugger to collimate.
raymo

[Shiraz (Ray)]

light does not behave like water. if you pour water into something, it stays there. Light only travels in straight lines and will either bounce straight out of whatever you put it in or will be absorbed somewhere in the container - it doesn't "stay" anywhere. For example a photon that was near you only 2 seconds ago could now be out the other side of the moon and heading for some far off galaxy - and that same photon will probably get there.

Adding a funnel on the end of a telescope will certainly increase the total amount of light which is collected, but none of it will be coming from the right directions to contribute to an image - it will be scattered around and back out, or it will be absorbed inside the scope to heat it very slightly.

The idea of adding a funnel has application in solar heating, where a funnel shaped parabolic concentrator can be used to direct light onto a non-directional absorber.

[Meru (Michael)]

+1 for Ray's comment!