Help me understand how a telescope work?

[AlexB67]

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?

Is the deeper theory of light being wave/particles really needed to explain this at some satisfactory level ?

On that topic, without any concept of photons you can also demonstrate why light travels in straight lines using wave theory because it annihilates all the neighbouring paths, an important point, like a minimum action principle or similar applied to light in explaining why geometric optics an using ray concepts work, so we just accept that.

Isn't using the geometric limit sufficient for this, as long as we are drawing ray diagrams that are optically and geometrically correct/sensible.

Unless there is something much deeper to what the OP is pointing at, which is why I also stated earlier and as you said in more detail, considering rays that are important in making a contribution to the final image is what matters, and as Umadog already said just now, it is all about angles, nothing more complex is needed IMHO to go largely towards explaining this.

[umadog]

Is the deeper theory of light being wave/particles really needed to explain this at some satisfactory level ?

You can explain the behavior of a telescope completely using ray diagrams. You can also explain it totally differently using quantum electrodynamics. You don't need QED to understand optics. What's important, however, is that it's possible to explain it both ways. It means that it makes sense to think of light as both a particle and a wave.

[NeilFawcett]

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...t/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.

I think you're agreeing that the FOV of the image captured by the raw telescope is as per my initial (noddy) diagrams? And I believe this is indeed backed up by the article you (& others have linked to). Ultimately the light (image) coming into the telescope is from a cone, and not just a tube of perfectly parallel light.

Now we (I think) agree the image/light gathered by the telescope is in a cone shape, defined by the aperture and focal length, my next question is why does placing an eye piece at 24mm, results in a magnification twice that of an eye piece placed at 12mm.

Clearly the lens at 24mm is discarding most of the light gathered and magnifying just the central area. (This in itself is interesting as straight away, most of mirror or lens is already wasted as it's capturing light that is of no use.)

Anyway, I'm hoping someone can explain, or point me to why if in the following image we move the view finder lens closer to the focal point, even more of the image is discarded so we zoom in even more on the center?

If we move than lens to the left, why would the house appear even bigger? Why would the lens in that position focus in on even less of the image?

I'd love to see a diagram showing the viewfinder lens in two places and showing why the "house " differs in size?

ie: Why does a lens at "A" zoom into more of the center of the image than at "B" (assuming this diagram isn't too mad):-

Surely there must be a simple diagram explaining why different amounts of the image is fed into the eye at the view piece?

This is really stumping me

ps: Hope this is annoying folks, but again, I like to understand how things work.

[AlexB67]

You can explain the behavior of a telescope completely using ray diagrams. You can also explain it totally differently using quantum electrodynamics. You don't need QED to understand optics. What's important, however, is that it's possible to explain it both ways. It means that it makes sense to think of light as both a particle and a wave.

I am aware of both approaches and agree completely with your point. I stated it to merely make the point that as you say, QED or any form of quantum mechanics is not needed for this IMO, trying to keep it simple

[AlexB67]

I think you're agreeing that the FOV of the image captured by the raw telescope is as per my initial (noddy) diagrams? And I believe this is indeed backed up by the article you (& others have linked to). Ultimately the light (image) coming into the telescope is from a cone, and not just a tube of perfectly parallel light.

Now we (I think) agree the image/light gathered by the telescope is in a cone shape, defined by the aperture and focal length, my next question is why does placing an eye piece at 24mm, results in a magnification twice that of an eye piece placed at 12mm.

Clearly the lens at 24mm is discarding most of the light gathered and magnifying just the central area. (This in itself is interesting as straight away, most of mirror or lens is already wasted as it's capturing light that is of no use.)

Anyway, I'm hoping someone can explain, or point me to why if in the following image we move the view finder lens closer to the focal point, even more of the image is discarded so we zoom in even more on the center?

If we move than lens to the left, why would the house appear even bigger? Why would the lens in that position focus in on even less of the image?

I'd love to see a diagram showing the viewfinder lens in two places and showing why the "house " differs in size?

ie: Why does a lens at "A" zoom into more of the center of the image than at "B" (assuming this diagram isn't too mad):-

Surely there must be a simple diagram explaining why different amounts of the image is fed into the eye at the view piece?

This is really stumping me

ps: Hope this is annoying folks, but again, I like to understand how things work.

Perhaps it would help to think about how magnification and FOV are defined mathematically in terms of each other, rather than trying to draw diagrams alone and rationalise it ? because in there lies the explanation. This paper

http://adsabs.harvard.edu/abs/1995JBAA..105..242G

that I recall using recently because it discusses something specific, but the background info in there is done nicely, with simple to understand diagrams and derivations. The article explains the concept of magnification and FOV using simple geometric arguments and diagrams, starting pretty much from first principle, with derivations and approximations discussed. Hope it helps.

[NeilFawcett]

Perhaps it would help to think about how magnification and FOV are defined mathematically in terms of each other, rather than trying to draw diagrams alone and rationalise it ? because in there lies the explanation. This paper

http://adsabs.harvar...JBAA..105..242G

that I recall using recently because it discusses something specific, but the background info in there is done nicely, with simple to understand diagrams and derivations. The article explains the concept of magnification and FOV using simple geometric arguments and diagrams, starting pretty much from first principle, with derivations and approximations discussed. Hope it helps.

I understand at least the basics of the maths behind FOV and magnifications etc, but it's a bit impersonal and I really would like to just see a simply diagram explaining:-

a) Why an eyepiece (eg: 24mm) drops huge amounts of the FOV captured by the telescope and magnifies in on the center of the image.

Why an eyepiece (eg:12mm) drops twice as much/magnifies twice as much as the 24mm.

[AlexB67]

I understand at least the basics of the maths behind FOV and magnifications etc, but it's a bit impersonal and I really would like to just see a simply diagram explaining:-

a) Why an eyepiece (eg: 24mm) drops huge amounts of the FOV captured by the telescope and magnifies in on the center of the image.

Why an eyepiece (eg:12mm) drops twice as much/magnifies twice as much as the 24mm.

I would encourage you to have a good read of that paper and take your time on it, because it does both approaches very nicely IMO, staring with a geometrical diagram first and foremost, and not just putting down the end result equations and skipping a lot of info. I hope it will make sense for you then.

[umadog]

You can make a simple two lens telescope and point it an object using this applet: http://www.mtholyoke.edu/~mpeterso/classes/phys301/geomopti/twolenses.html

You can change the focal length of the eyepiece lens and watch how the angles alter to result in image magnification.

A switching from a 24 mm to a 12mm eyepiece will double the magnification but it doesn't mean the FOV will halve. The field of view depends on the field stop. For practical reasons, within a particular eyepiece series (e.g. Plossls) field stop size scales with focal length. This keeps apparent field the same size. But field stop size doesn't have to change. e.g. a 5 mm Ethos should have about the same field of view as a 10 mm Plossl.

[AlexB67]

The field stop is an important point. I should have added that the paper I quoted ignores the effect of field stop and how it affects resultant FOV, but to understand the concept geometrically to a first approximation where that is not the case, it explains the concepts very clearly.

[Peter Drew]

The telescope objective forms an image at its focal plane. An eyepiece magnifies this image and can be focused. If an eyepiece of half the focal length of the first eyepiece is used then you are able to view the focal plane image half again nearer to the focal plane resulting in the image appearing twice the size. You can produce a similar effect by looking at your finger at 12" distance and then at 6" distance, it will appear twice the size.

[Scott]

if this has been covered in earlier posts,i'm sorry, but i can't see how the light is parallel. if i'm using say a 10mm lens with a 50mm aperture on a camera (i assume this is the same principal) then I can see objescts in the sky that are (at a guess) 50 degrees apart. even without a scope it is clear that the light from these objects can't be travelling parallel to reach my eye.

I'm probably missing something really basic here

Edit:- SORRY I think this is already covered

[umadog]

Light is from infinity. It's parallel. I have two posts above on the matter: scroll back up.

[Scott]

Light is from infinity. It's parallel. I have two posts above on the matter: scroll back up.

Light from polaris and vega will both reach me in my back garden. I don't think that the light from these two objects travel parallel to each other to reach me. I also don't think that anything we look at is at infinity as we know the distance to many of them. we just call it infinity 'coz it's a long way away. It probably sound like I'm splitting hair, but I'm not trying to be arguementive, I'm just trying to explain myself.

I'll try and explain myself better

the light from a star we'll call parallel, it's not but it's so close it's impossible to notice

The light from the various stars of Lyra are very close and also probably to hard to measure

Now if we move to Hydra The stars in the head an tail are quite a distance apart and as such I'd say the angle is definately measureable.

ok, I'm ready to be shot down now

[AlexB67]

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.

In all that discussion I forgot something as well and good point, even though it has been said indirectly to get back to basics. To sum up, angle of incidence equal angle of reflection. I cannot see that the rays in Neil has drawn in some cases follow that rule correctly as is the case in this post, it is optically incorrect.

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!

[NeilFawcett]

Light from polaris and vega will both reach me in my back garden. I don't think that the light from these two objects travel parallel to each other to reach me. I also don't think that anything we look at is at infinity as we know the distance to many of them. we just call it infinity 'coz it's a long way away. It probably sound like I'm splitting hair, but I'm not trying to be arguementive, I'm just trying to explain myself.

I'll try and explain myself better

the light from a star we'll call parallel, it's not but it's so close it's impossible to notice

The light from the various stars of Lyra are very close and also probably to hard to measure

Now if we move to Hydra The stars in the head an tail are quite a distance apart and as such I'd say the angle is definately measureable.

ok, I'm ready to be shot down now

I concur (with my noddy understanding). The light from the left of the moon and right of the moon all enters a telescope, but clearly to do so they cannot be parallel to each other as they start 3500km apart and end up only 10cm apart going into a telescope. They come in as part of the cone of light permitted into the telescope as defined by its diameter ahd focal length (basically).

[umadog]

The confusion is occurring because you misunderstand what is being referred to as "parallel" and because you're not taking in the diagram I have linked to. The diagram illustrates the very situation you're confused about. Light from a single point is what is parallel. From the perspective of a fixed observer light from different directions in space arrive at different angles (what you're talking about). But from any single, infinitely point-like, direction the light is parallel (what I'm talking about). Look at the diagram: http://www.telescope-optics.net/eyepiece1.htm There are two sets of parallel rays in that diagram. Each set originates from a different direction and so arrive at the objective at different angles. But within each set of rays the light is parallel. Does this answer your question?

[NeilFawcett]

The confusion is occurring because you misunderstand what is being referred to as "parallel" and because you're not taking in the diagram I have linked to. The diagram illustrates the very situation you're confused about. Light from a single point is what is parallel. From the perspective of a fixed observer light from different directions in space arrive at different angles (what you're talking about). But from any single, infinitely point-like, direction the light is parallel (what I'm talking about). Look at the diagram: http://www.telescope...t/eyepiece1.htm There are two sets of parallel rays in that diagram. Each set originates from a different direction and so arrive at the objective at different angles. But within each set of rays the light is parallel. Does this answer your question?

Let's use a traditional diagram then...

Now, lets point that scope and the center of the moon and extend those two lines out. When they read the moon they encompass a area with diameter of the scope. How can we the see the entire moon if the capture from the telescope is infact not a cone then going beyond the implied (parallel) lines in these diagrams?

Indeed, diagrams and even the java applet a few posts above, suggest no light from beyond those angles can reach the focal point so we could not see the entire moon?

[NeilFawcett]

TYPO: "when they read the moon" = "then they reach the moon"

ps: Why can't we amend posts

[umadog]

Let's use a traditional diagram then...

Now, lets point that scope and the center of the moon and extend those two lines out. When they read the moon they encompass a area with diameter of the scope. How can we the see the entire moon if the capture from the telescope is infact not a cone then going beyond the implied (parallel) lines in these diagrams?

--> BECAUSE THOSE LINES COME FROM ONE DIRECTION (ONE POINT ONLY) <--

You are missing ALL of the rest of the rays.

http://www.telescope-optics.net/images/46e.PNG

[rowan46]

There is a hole in your logic, the light must be parallel or close to it or it would not be able to be focussed and hence seen. Take a stick say 12"(object) tie a 12" piece of string to each end and tie the other ends to a stick 2" long(objective). Note the steep angle as the string is pulled taught. If you lengthen the string still keeping it taught eventually the 2 strings will be so close to parallel as to be immeasurable. Try it and see if the angle doesnt change I can't be asked to do the maths and I am not sure if i remember how but it is possible to calculate using trigonometry exactly how long the string would need to be to achieve an apparently parallel line.

I would guess a mile would do it

[Scott]

No, I understand that the light from a single point is parallel. my point is that a single point of light (star) is not your fov. it is only one of millions that make up your field of view. the image I can see on the view finder of a camera with a 10mm lens surely isn't entering the lens parallel....also, I have looked at the diagram. To this end, I am still not convinced that all light entering a scope/camera is parallel

Edit :- Or to take it to the nth degree, a night sky camera where the lens takes in light from 180 degrees

[umadog]

To this end, I am still not convinced that all light entering a scope/camera is parallel

EXACTLY.

NOT all light is entering the scope in parallel. ONLY light from one point enters in parallel. You looking at diagrams that show ONE ray, from one point, and assuming these rays come from multiple points. I think you are not realising that to form an image from one point you use ALL of the mirror. That is what is shown in your diagram.

[AlexB67]

To keep it friendly I hope, getting a bit heated I think/suspect that you are thinking about a specific set of rays to convince yourself that you cannot see the entire moon. Will this diagram help ? I was just about to draw something simple but this page is quite informative. It shows additional rays for different scopes, the principle holds anyway and is the same. Note the additional ray traces from various angles.

http://en.wikipedia.org/wiki/Refracting_telescope

edit: as Umadog already said but got there before this post, but shows it in pictures instead of words.

[Scott]

EXACTLY.

NOT all light is entering the scope in parallel. ONLY light from one point enters in parallel. You looking at diagrams that show ONE ray, from one point, and assuming these rays come from multiple points. I think you are not realising that to form an image from one point you use ALL of the mirror. That is what is shown in your diagram.

Ahhh, Now I understand the confusion.... I did not realise that you were explaining the reason why diagrams are drawn the way they are. So basically we've been discussing this for half a page when infact we are in agreement......I think

[Andrew*]

Ahhh, Now I understand the confusion.... I did not realise that you were explaining the reason why diagrams are drawn the way they are. So basically we've been discussing this for half a page when infact we are in agreement......I think

Yes. Diagrams are hugely simplified. They show light coming from a single point. A diagram showing all the light coming from all the available angles would be very busy indeed.

In the previous example, there are two light rays coming from the same source to different points of the mirror. This is happening right across the mirror, which explains why larger apertures are brighter - more of those parallel rays from that single point are collected and focused to the same point in the eyepiece. However, you'll notice that the light is not shown to collect at a single point at the eyepiece, so it's not in fact a correct diagram. Either the entry light must be non-parallel (coming from different directions) or the light at the eyepiece should collect at a single point, or the telescope is rubbish!