Help me understand how a telescope work?

[NeilFawcett]

OK, I like understanding how things actually work.

Recently I realised although I understand the principles behind a telescope, and know how to calculate the magnification of a telescope with a given focal length and eye piece etc, I don't really understand how the mechanics really work.

So I'm hoping someone can walk me (us) through it?

OK, first of all, the field of view? If we have two telescopes, one with a focal length twice the other, then my understanding is it's FOV will be half as much? Is this simply because light enters as shown in the following diagram?

So is this a fair representation of how light gets into the telescope? And why with a longer focal length the FOV halves?

I'll assume so, so that would mean in the case of a reflector we end up with light entering something like this (& bouncing off the mirror?)

This may seem to be simple questions, but if I'm right I've never seen a diagram show this. Instead they show perfectly parallel light coming in, which doesn't explain the FOV of the telescope? eg: http://www.meade.com...howtelediag.gif

If I get a thumbs up to this, then I'll ask my next set of noddy questions which then get to my real confusion!

[rowan46]

light comes in parrallel its convergenge or divergence comes when it hits the objective so your images should be represented so

http://www.seeviewo.org/seeview/astronomy/articles/telescopes.htm

[NeilFawcett]

light comes in parrallel its convergenge or divergence comes when it hits the objective so your images should be represented so

http://www.seeviewo..../telescopes.htm

I totally do not get that then.

If only parallel light comes in (as your diagram shows) then you'd only see anything in a tube of vision with width of your telescope. eg: 100mm across, for infinity.

Clearly the telescope is seeing light from beyond this parallel to give you an X degree FOV for example, more akin to my diagram.

[AndyWB]

I might be wrong, but following the red line, wouldn't you need a convex mirror to get it to bounce away like that? And if it were, the other lines of light wouldn't hit the secondary.

I'm pretty sure the other diagrams are right - rays of light would enter the tube and strike the mirror running parallel, and then the concave mirror would reflect them inward towards the secondary. There's a good example of this on this page - http://www.garyseronik.com/?q=node/8 - as well as a fairly interesting discussion of the correct secondary size. Note the diagram shows a secondary that is too small.

[ronin]

The lines you show are of 2 light rays coming from different positions on an object, to demonstrate the operation correctly the rays need to start from the same point on the object.

The scope is just one part of the system the object being imaged is the other.

You are 2 rays short of the required minimum and the 2 shown will not come to a single point at the focal plane as they are from different locations on the object. They are from different points as they are diverging if you track back up from the objective.

[NeilFawcett]

The lines you show are of 2 light rays coming from different positions on an object, to demonstrate the operation correctly the rays need to start from the same point on the object.

OK, consider in my simple example(s), the two lines representing the light from the left of the moon, and the right of the moon. You're collecting light from an area greater than 3500km in your 10cm tube, so clearly light from beyond the parallel is coming in! Else you'd only see 10cm of the moon at a time!

With diagrams showing perfectly parallel light only entering the telescope it would be impossible to see anything wider than the width of the telescope no matter how close/far it was.

[NeilFawcett]

In short, if the diagrams showing perfectly parallel light only coming into the telescope are right, and my diagrams are wrong, then how would you ever see anything wider than your telescope?

eg: How would you be able to see the moon (3500km wide) in your 10cm diameter tube?

How can the following diagram not be right? Something wider than your telescope needs light beyond the parallel to be seen.

[floppygoose]

Your diagram is correct up to a point. If you consider the light from a distant galaxy, there is a good chance you will easily fit it into your field of view, and the chances are the galaxy would be bigger than a 100mm primary mirror, but the sheer distances involved would mean that the light from the galaxy would be entering the tube 'as near as damn it' parallel. The eyepieces (and focus tube) are then used to determine how much (or little) of 'the light' is delivered to the eye.

[umadog]

The rays are parallel because they come from infinity. What you're missing is that any one set of parallel rays constitute light from one direction only. Light from other directions comes into the objective at different angles. The correct diagram is this: http://www.telescope...t/eyepiece1.htm

Note that the dashed line (the object image that occurs before the eyepiece) is a plane. Light from each direction is focused to a point, but to together you get a plane. If you put a piece of paper there, you'll see an image on it. The angle epsilon is the field of view. Note that different directions are formed by different parallel ray bundles.

If you want to understand why a telescope magnifies, you need to digest this image: http://cnx.org/content/m42493/latest/Figure_27_05_02.jpg Magnification is all about apparent angles.

[rowan46]

http://www.youtube.com/watch?v=8vbd3E6tK2U

:grin:seriously though, light isn't a ray its a wave I forget the wave length of visible light and my physics is so rusty and rudimentary I hesitate to try but here goes. light travels in waves when it radiates or reflects off an object not just one point reflects but every point reflects this produces expanding wave fronts which gets bigger the further away from the object it travels. The eye /scope intersects the wave front the lens focusses it to a coherent image and voila. If the moon was right on top of your scope you would see only the bit of the wave front radiating from within the area your scope was focussed The moon is at a distance whereby all of the wave fronts from the near side are capable of being seen. Interesting question i look forward to an answer that makes this clear

[NeilFawcett]

The rays are parallel because they come from infinity. What you're missing is that any one set of parallel rays constitute light from one direction only. Light from other directions comes into the objective at different angles. The correct diagram is this: http://www.telescope...t/eyepiece1.htm

Note that the dashed line (the object image that occurs before the eyepiece) is a plane. Light from each direction is focused to a point, but to together you get a plane. If you put a piece of paper there, you'll see an image on it. The angle epsilon is the field of view. Note that different directions are formed by different parallel ray bundles.

If you want to understand why a telescope magnifies, you need to digest this image: http://cnx.org/conte...re_27_05_02.jpg Magnification is all about apparent angles.

Ahh! There you go! So I am technically right! The FOV is defined as I suggested! The field of view is defined (drawn) pretty much as I showed.

[NeilFawcett]

:grin:seriously though, light isn't a ray its a wave I forget the wave length of visible light and my physics is so rusty and rudimentary I hesitate to try but here goes. light travels in waves when it radiates or reflects off an object not just one point reflects but every point reflects this produces expanding wave fronts which gets bigger the further away from the object it travels. The eye /scope intersects the wave front the lens focusses it to a coherent image and voila. If the moon was right on top of your scope you would see only the bit of the wave front radiating from within the area your scope was focussed The moon is at a distance whereby all of the wave fronts from the near side are capable of being seen. Interesting question i look forward to an answer that makes this clear

You're missing my point I think.

If my diagram has no element of truth, then why does the length of a telescope tube (focal length) affect anything? Look at my diagrams and note how it explains the changes in FOV according to focal length.

Only considering perfectly parallel light coming perfectly down into the telescope (as all/most diagrams seem to show) don't explain this!? Two diagrams of difference focal length telescope would look identical, yet we know this can't be the case!

[NeilFawcett]

So the more accurate diagram of the refractor type setup to explain the change in FOV would be:-

Note: greater FOV of short focal length!

[rowan46]

So the more accurate diagram of the refractor type setup to explain the change in FOV would be:-

Note: greater FOV of short focal length!

Thats more like it, now i think i understand yes you are right it is the steepness of the light cone that determines fov

[AlexB67]

IMHO Umadog hit it on the head, the rays come from infinity and are parallel is what matters, and it is what you look at, and this makes the dominant contribution. Now my suspicion, what you are talking about is why you can get stray and extra light in a scope and unwanted side effects, though I may be misunderstanding some of what you say.

Consider for example a Flextube or open tube design, you will see extra light coming in, the image contrast is lost to some degree, but the bulk of that you looking at comes from infinity ( in all practical purposes ).

Imagine a completely open Newtonian with no tube at all, but just the mirrors arranged, it does not have a larger FOV compared to the closed design. If what you say is true everyone could widen the FOV by not having a tube at all.

[NeilFawcett]

IMHO Umadog hit it on the head, the rays come from infinity and are parallel is what matters, and it is what you look at, and this makes the dominant contribution. Now my suspicion, what you are talking about is why you can get stray and extra light in a scope and unwanted side effects, though I may be misunderstanding some of what you say.

Consider for example a Flextube or open tube design, you will see extra light coming in, the image contrast is lost to some degree, but the bulk of that you looking at comes from infinity ( in all practical purposes ).

Imagine a completely open Newtonian with no tube at all, but just the mirrors arranged, it does not have a larger FOV compared to the closed design. If what you say is true everyone could widen the FOV by not having a tube at all.

No, no and no

As per my OP, I'm simply trying to understand how change the length of a telescope (focal length) changes the FOV (& magnification). This is (I'm now certain) absolutely down the the exact reasons shown in my simple diagrams. They're of course not 100% accurate, but in principle I'm certain they explain perfectly the FOV and magnification change when a telescopes length is changed.

[AlexB67]

Sorry, In that case I think we are on a completely different line of thought, I must be misunderstanding you completely . Check out this interactive demo.

http://demonstrations.wolfram.com/NewtonianTelescope/

To me the optical diagrams Umadog pointed to and this demo explains everything there is to explain. For the demo you do need to download the wolfram player, which is free. There is a refractor version as well somewhere on that site, probably there will be some more animations knocking around the web that ray trace these things accurately.

I still believe/suspect that where your thought comes from, is that you are thinking about rays that are not important and/or invalid in making a contribution to the final image that you see in a telescope.

[Moonshane]

I don't think it matters where the light is coming from, whether at infinity or not. assuming you have two telescopes, same aperture and one has double the focal length of the other and both focus at 5m. if you look at a bird (or maybe a streetlamp) 5m away (light is certainly not from infinity in this case) then the bird will occupy half the field in the shorter scope with the same eyepiece.

look down a kitchen roll centre and a toilet roll centre with just your eye and the effect is similar with no glass involved at all. for that matter make a hole with finger and thumb and move this back and forth and the field changes with 'focal length'.

it's the shape of the light cone that matters with regard to field of view and this (and magnification with the same focal length of eyepiece) is determined by focal length of the telescope for the same aperture.

[AlexB67]

I never tried to focus my scope on a toilet roll, not sure of it has that short range to focus on, I must try it some day but even a chimney or a bird in a tree is for all practical purposes already an infinity situation for this discussion, ( and the parallel rays consideration )

[NeilFawcett]

I don't think it matters where the light is coming from, whether at infinity or not. assuming you have two telescopes, same aperture and one has double the focal length of the other and both focus at 5m. if you look at a bird (or maybe a streetlamp) 5m away (light is certainly not from infinity in this case) then the bird will occupy half the field in the shorter scope with the same eyepiece.

look down a kitchen roll centre and a toilet roll centre with just your eye and the effect is similar with no glass involved at all. for that matter make a hole with finger and thumb and move this back and forth and the field changes with 'focal length'.

it's the shape of the light cone that matters with regard to field of view and this (and magnification with the same focal length of eyepiece) is determined by focal length of the telescope for the same aperture.

Indeed, and put on a (theoretical) eyepiece that matches the focal length of the telescopes (so 1x magnification) and I suspect what you see in the eye piece would be the field of view pretty much what my diagrams prescribed.

[ollypenrice]

Traditional ray 'top and bottom' ray diagrams are indeed confusing and the simple ones inadequate for a complete understanding. The first thing is that all parts of the objective contribute to all parts of the image. (If they didn't, you'd have a black hole in the middle of the image in a reflector with a secondary.) In the traditianal ray diagram you could be forgiven for thinking that the bottom part of the objective provided a view of an observed pedestrian's head and the top part of his feet. (We are too polite to look at ladies through telescopes on SGL!) This is partly why the OP's focal length comparison diagram is incorrect. In the original drawing light from the 'top' of the subject could not get to all of the focal plane. Ditto light from the 'bottom' of the subject. So the entire subject apparently depicted by the diagram could not form an image. And here's a thing; how, would these diagrams depict rays from the subject's middle??

Anyway now that the thread has corrected the ray diagrams shouldn't we also ditch this nonsense about photons bouncing off mirrors? You mean they don't? No they don't. Quantum theory put that explanation to bed a long time ago. What happens is that the incident photon is absorbed in the aluminium and a new one is realeased which will probably go in the same direction it would have gone had it been a little billiard ball. bouncing off the mirror's surface. When we say probably we really do mean very probably.

In the old school ray diagram we have a horizontal mirror and an incident photon at a 45 degree angle. It hits the middle of the mirror and bounces off at a 45 degree angle. Obviously the rest of the mirror plays no part in any of this? But it does! Some extra mirror is needed to generate the probability that the incident photon will leave at 45 degrees. Without a bit of extra mirror an incident photon at 45 degrees will go... absolutely anywhere it likes! All of this is deliciously explained in QED by the incomparable Richard Feynman and I've probably mangled it somewhat so I urge you to read the original. It is so exotic that the intrigue of ray diagrams can't hold a candle to it.

Olly

Edit; If the rays incident to a telescope's optics were not parallel how would it know what to do to them? How would it differentiate between the variety of incident angles it was imbibing?

[AlexB67]

I like these kind of brain twister type discussions. I often do it myself, think about stuff and try to rationalise it, have some internal brain fights. Wish I could help more but I do not get the issue. For me the ray diagrams clarify it for me in the previously quoted.

Hopefully someone will give a satisfactory answer that will settle it for you . Perhaps only an accurate ray tracer using and examining optical theory on a level such as this would be more convincing

http://bagira.iit.bme.hu/~szirmay/gascon/vnt/vnt_paper/

Not sure where it is available though or something similar.

[NeilFawcett]

Sorry, In that case I think we are on a completely different line of thought, I must be misunderstanding you completely . Check out this interactive demo.

http://demonstration...onianTelescope/

To me the optical diagrams Umadog pointed to and this demo explains everything there is to explain. For the demo you do need to download the wolfram player, which is free. There is a refractor version as well somewhere on that site, probably there will be some more animations knocking around the web that ray trace these things accurately.

I still believe/suspect that where your thought comes from, is that you are thinking about rays that are not important and/or invalid in making a contribution to the final image that you see in a telescope.

Sorry, but that demo seems to suffer from the same issue as the other diagrams I've mentioned. It shows light only coming in from basically 90 degrees.

Diagrams like this seem to suggest that such a telescope couldn't see both sides of something more than the diameter of the mirror. I mean how would light from the left of the moon and right of the moon get into the scope, as they are not parallel.

Follow those two lines out to the moon and can they encompass the whole width of it? No... They'd encompass 10cm or so.

If you point at the middle of the moon, the light is coming on from a cone not a tube, as per my diagrams

Argh!

[umadog]

I don't think it matters where the light is coming from, whether at infinity or not. assuming you have two telescopes, same aperture and one has double the focal length of the other and both focus at 5m. if you look at a bird (or maybe a streetlamp) 5m away (light is certainly not from infinity in this case) then the bird will occupy half the field in the shorter scope with the same eyepiece.

Yes, the only difference is that when an object is at infinity the image plane is coincident with the the focal length. i.e. the image is formed at one focal length. When an object is closer than infinity, the rays diverge and the image plane shifts (I guess outwards, but I'd have draw the diagram to be sure). So that's why you have re-focus during terrestrial observing.

Also, I think I was wrong above: The angle epsilon is the apparent FOV. Alpha is the true field of view.

[umadog]

Sorry, but that demo seems to suffer from the same issue as the other diagrams I've mentioned. It shows light only coming in from basically 90 degrees.

Diagrams like this seem to suggest that such a telescope couldn't see both sides of something more than the diameter of the mirror. I mean how would light from the left of the moon and right of the moon get into the scope, as they are not parallel.

Again, yes, those are just the rays rays from straight ahead. The image is just missing the other rays. These are the other rays: http://www.telescope-optics.net/eyepiece1.htm The image on that page is the answer to your problem. The rays from other parts of the moon are also parallel. They're just coming in at different angles. Optics is all about angles.

The entire mirror contributes to forming each on the image; I don't think you've completely digested that point. i.e. the left part of the mirror doesn't form the left part of the object. All of the mirror forms each point in the object. You can easily prove this to yourself: look at the moon and then slowly stop down the objective. The image will become dimmer as the exit pupil becomes smaller but the size of the true or apparent FOV won't change.