Help me understand how a telescope work?

[umadog]

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, I think we are in agreement.

[AlexB67]

Perhaps in future they should draw a thousand rays The original question was to define and explain FOV, all these multiple ray paths explain why and how FOV and magnification work, but you only need to drawn one case to explain the geometry and derive the rules/equations.

Perhaps unfortunate that the single case horizontal parallel lines drawn became a sort moot point because they are commonly used in such diagrams to make Neil think that is what restricts the view of the moon in the way he thinks about it ? Perhaps that caused some of the misconceptions .... would be my guess. That being said the original figure, let's quote it again

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

does show that.

[barkis]

Neil,

The full moon subtends an angle of 31 arc minutes in the sky. This a small area of sky, so not to difficult for the smallest of telescope objective to cover, and with room to spare.

Polaris is approximately 1 degree, or 60' from the NCP. In 24 hours, polaris will describe a circle in the sky 2 degrees. or 120' in diameter.

Almost 4 full moons edge to edge will fit into that circle.

If you look at an image of polar star trails, you will note how small that circle appears.

Not sure how useful this is, other that to explain what a telescopes objective is capable of covering in the sky.

When I and no doubt others observe the full moon, and althou it is fully suinlit, detail is visible, although a filter is desirable if you don't want a blind eye for a while.

Your analogy of extending the scopes aperture out to the moon itself, thinking that that is the only area you can see, is silly really.

If that was the case, the telescope would suffer from vignetting, resulting in a drop in brightness, which could be noticable, but never is.

A good book on Geometric Optics will explain everything, but perhaps you have one already.

Ron.

[rowan46]

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

All the light that you can see is parallel it has to be or it will not be focussed. the light that is not parallel will end up bouncing off your tube and some of that may reach the eye causing interference in the image but it will not be part of the focussed image. The focussed image consists entirely of parallel lines converging to a focal point.

[Moonshane]

if you use a 32mm plossl with a 50 degree field in a scope with a focal length to provide a magnification of 1 degree true field (TFOV) and another scope a TFOV of 0.5 degrees, the moon will fill half the field of the first scope and the full field of the second scope. the moon produces any number of light waves and the things in the space around it do the same. only a few / possibly one wave in theory? of these points will be 'on axis' of the optical system of the scope and the rest will come in at an angle. the steeper the light cone of the scope (shorter the focal length) the harder it is to get all these waves / wavelengths in sharp focus and hence the reason why slower scopes work better with a wider range of varying quality eyepieces than a faster scope.

[NeilFawcett]

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!

So, I think we're agreeing with my diagram then? I am not intending to show anything in my initial diagrams other than what dictates the FOV of the telescope itself. And why a telescope with twice the tocal length as another (all things equal) will have a different (smaller) FOV. Diagrams simply showing a row of perfectly parallel lines dropping down a telescope imply absolutely no difference in behaviour according to focal lengh. Where as mine surely does?

This explains why you can see the moon and surrounding area, and why the FOV is different depending on focal length:-

An image like this does not (follow those two lines out, they will not encompass the entire moon):-

That said though, the reflection of the rear mirror I suggest for such a telescope, although it looks right to me (to explain the FOV), sort of feels wrong because light coming down at right angles (like traditional diagrams) will not be collected It's in complete contradition to normal diagrams:-

Normal diagrams suggest this behaviour (which to me looks wrong as it does not account for a true FOV):-

[NeilFawcett]

thinking that that is the only area you can see, is silly really.

But this is what diagrams suggest.

This diagram sugests, if I were to look at the moon, only light from a 10cm (or so) patch would reach my mirror.

Where we know of course a far far cone of the sky is in the FOV. Without considering an eyepiece my gut instinct tells me the FOV is as per my diagrams. And if you consider light coming in at any angle beyond parallel (a 10cm tube for infinity) in the above diagram, it simply cannot be collected.

[AlexB67]

For sure your red line in the Newtonian case is incorrect, angle of incidence =/= angle of reflection there very clearly in accordance with the shape of the mirror anyway, for that you must draw a line or tangent to the mirror as guide, then draw a line perpendicular to that ( a so called normal ) , hopefully then you will more clearly where the reflected ray should go, as it is you are breaking optical rules.

Also the first diagram is incorrect IMO. The middle one is okay to sort of explain things for that case. compare with where the light goes as in the original diagrams quoted.

[NeilFawcett]

For sure your red line in the Newtonian case is incorrect, angle of incidence =/= angle of reflection there very clearly in accordance with the shape of the mirror anyway. Also the first diagram is incorrect IMO. The middle one is okay to sort of explain things for that case. compare with where the light goes as in the original diagrams quoted.

This is absolutely doign my head in

I believe this accounts correctly for the FOV on a reflector (And a couple of linked documents seem to back it up I believe):-

Can we agree this explains the FOV collected by a refractor?

No if we have a reflector, with exactly the same focal length (& diameter) clearly is collected FOV should be basically the same to. Can we produce a similar diagram explaining that?

[Andrew*]

Okay, well, the diagram with the converging lines that illlustrate FOV are showing light from three seperate points. Of course, they're hugely exaggerated to make a point, but it's exactly that which is causing confusion!

I've thrown this diagram together in two seconds, showing the light rays from TWO POINTS at "infinity". One is red, the other is green, and they are at opposite ends of the telescope's MAXimum field of view. Note how they both have a parallel beam that hits opposite sides of the mirror (and every point on the mirror in between), but arrive at the focal PLANE at the same point. The green rays converge at one end of the focal plane and the red at the other. The red rays are all parallel until they hit the mirror, at which point the mirror strives to converge all parallel rays to a point.

[Andrew*]

Also note the very shallow curve of the primary mirror! It's not a great big semi-circle! and note the small distance between the edge of the mirror and the walls of the tube. If this gap were any smaller on this particular telecope, the field of view would darken at the edges as the parallel beams from the source point would hit the outside of the tube instead of the mirror. Only a portion of the mirror would receive those beams.

[NeilFawcett]

Okay, well, the diagram with the converging lines that illlustrate FOV are showing light from three seperate points. Of course, they're hugely exaggerated to make a point, but it's exactly that which is causing confusion!

I've thrown this diagram together in two seconds, showing the light rays from TWO POINTS at "infinity". One is red, the other is green, and they are at opposite ends of the telescope's MAXimum field of view. Note how they both have a parallel beam that hits opposite sides of the mirror (and every point on the mirror in between), but arrive at the focal PLANE at the same point. The green rays converge at one end of the focal plane and the red at the other. The red rays are all parallel until they hit the mirror, at which point the mirror strives to converge all parallel rays to a point.

My friend! That is brilliant. I was on the toilet a second ago and had one of my urika moments, and thought maybe the problem with the reflector is that the mirror is not infect the entire width of the "tube", so it collects at a greater angle, thus giving us the cone. And this is EXACLY what you have drawn too!

So now with a refractor and reflector, of the same focal length and diameter, I bet basical this FOV cone spreading out is identical!

NICE!

[NeilFawcett]

....and all it took was FOUR pages

[Andrew*]

Yay, glad we got somewhere. Check out the comparison with the refractor, which is actually much simpler!

[rowan46]

edited

[NeilFawcett]

Yay, glad we got somewhere. Check out the comparison with the refractor, which is actually much simpler!

That's it, or certainly close enough! Voila!

Thanks! So I'd basically guessed the refractor right, but had got confused with the reflector due to diagrams showing the mirror basically the same size as the tube so not allowing light to come in from a greater angle for the FOV. eg this is slightly misleading (really the mirror should be a bit smaller to allow for the cone of FOV):-

[Andrew*]

Yes, that's about right, but in real terms this gap is not very large.

For example, a telescope with an 80cm long tube and a maximum field of view of 2.5 degrees will have a frontal aperture 3.5cm larger in diameter than the mirror. The calculations for this are dead simple and I can talk you through them if you like

[umadog]

Yes, exactly. However, with the Newt you show in your image you will get angles other than the those from straight ahead but the front aperture will clip them so they won't use all of the mirror. In other words, the front aperture of that scope will vignette the image.

[Andrew*]

Yes, that's about right, but in real terms this gap is not very large.

For example, a telescope with an 80cm long tube and a maximum field of view of 2.5 degrees will have a frontal aperture 3.5cm larger in diameter than the mirror. The calculations for this are dead simple and I can talk you through them if you like

And even that scope could get a larger possible field of view, but it will not be served by the entire mirror and will darken (vignette) at the edges.

(too slow! what he said ^)

[NeilFawcett]

Yes, that's about right, but in real terms this gap is not very large.

For example, a telescope with an 80cm long tube and a maximum field of view of 2.5 degrees will have a frontal aperture 3.5cm larger in diameter than the mirror. The calculations for this are dead simple and I can talk you through them if you like

No! I think I've got to my necessary point of understanding now. Which was understanding the FOV (cone) of light coming into the telescope. This was a stepping stone to...

What I now want to understand is how the eyepiece (eg: 12mm or 24mm) magnifies just a portion of that FOV captured to the eye.

I did post that question earlier but it's been lost in the wash of FOV discussion I fear

So if someone can take something like Andrews diagrams, and then explain how a 12mm or 24mm eye piece magnifies different portions of the FOV captured I would be very greatful! Why does a 12mm eyepiece only display half of what the 24mm one does? I can't seem to find any diagrams clearly explaining it

[Andrew*]

Righto, bear with me as I'm kind of working this out as I go along. I've never thought about this in depth, so I'm furthering my own understanding in the process. It's quite fun!

So, for first thoughts on magnification, look at this post:

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.

Field of view is a different matter, because some eyepieces will show more TRUE (sky) field of view (TFOV) at the same magnification. That means they have a larger apparent field of view (AFOV).

Look at this close up diagram of the focal plane of light rays coming from an objective at the top. The red and green lines represent the edges of the field of view as per my other diagrams. The orange lines represent a smaller, circular, TFOV and the blue lines represent a TFOV of half that of the orange lines.

We can give them diameters in mm and degrees. So the outer (red and green) TFOV is the scope's max. Let's call this 5°, and when it arrives at the focal plane, we can call it 50mm. An eyepiece will need to have a field stop diameter of 50mm to accommodate this TFOV. It can either magnify this a little bit and show a small AFOV, or it can magnify it a lot and show a large AFOV, for exampe, a 50mm focal length eyepiece with 50° AFOV or a 30mm focal length with an 80° AFOV.

The orange TFOV is 3° and so has a diameter at the focal plane of 30mm. This could be a 30mm 50° AFOV eyepiece or a 15mm 100° AFOV eyepiece.

The blue field of view is 1.5° and has a diameter of 15mm. Again, this could be a 15mm 50° AFOV or a 30mm 25° AFOV or a 7.5mm 100° AFOV.

Please be aware I haven't done these calculations - I am just making approximations

[tingting44]

interesting read but way over my head lol i point my scope and look, then ahhhh wow lol and thats technical enough for mwah lol

[Andrew*]

interesting read but way over my head lol i point my scope and look, then ahhhh wow lol and thats technical enough for mwah lol

Excellent! There's a lot to be said for just ignoring all the techy bits and getting on with enjoying the night sky. I often get myself wound up in it all, but it does give me something to do on cloudy nights, and enhances my understanding of my hobby

[AndyWB]

We can give them diameters in mm and degrees. So the outer (red and green) TFOV is the scope's max. Let's call this 5°, and when it arrives at the focal plane, we can call it 50mm. An eyepiece will need to have a field stop diameter of 50mm to accommodate this TFOV. It can either magnify this a little bit and show a small AFOV, or it can magnify it a lot and show a large AFOV, for exampe, a 50mm focal length eyepiece with 50° AFOV or a 30mm focal length with an 80° AFOV.

Okay, so let's see if I've got this right:

• The telescope projects an image onto the focal plane.
  
• An eyepiece takes some of that image and focuses it into your Mk1 eyeball.
  
• A high magnification eyepiece might only take a small part of the focal plane - this is why such eyepieces seen to have smaller lenses at the scope-ward side? (not sure what this lens would be called). This would be represented by blue.
  
• A low magnification eyepiece might take lots of the image on the focal plane - which is why such eyepieces have bigger lenses on the scope ward side. This would be the yellow lines.
  
• Presumably unused light at the focal plane is rejected (Should I be flocking my eyepieces? )
  
• The light gathered at the focal plane can then be magnified a little - giving a smaller apparent field of view - or a lot, giving a larger field of view. However, the light gathered at the focal plane could remain the same.
  
• Would this explain why higher magnification eyepieces produce a dimmer image - they're only using small part of the image at the focal plane, and then spreading it out lots for the same Apparent FOV - and if you increased the AFOV too, you'd make it even dimmer still?
  

Assuming that I've understood that correctly, this raises a question - what does the focal length of the eyepiece actually mean (I've wondered this since noticing that my 18mm is shorter than my 12mm)?

[NeilFawcett]

What I'm having trouble understanding is how placing a lens (eye piece) at 12mm... and then at 24mm will result in the magnifcation halving? I think I need another "Andrew" type diagram to help me... And then I'll shut up honest!

So if the lens is moved to the right, why is the magnifcation reduced?