Exit pupil and AFOV

[miguel87]

Probably getting out of my depth here but anyway...

I was  looking at exit pupil sizes and all that jazz and my largest is 6.5mm and with my eye snuggled up to the eyepiece I can only just see the entire FOV without moving my gaze. This fits with the idea that my pupil size is similar to the exit pupil image.

BUT, imagine that the exit pupil was 7.5mm and I had to 'look around' a little to see the edge of field. The exit pupil image is a resolved image, so not being able to see the edges would not dim any objects in the middle because my pupil is receiving all the light available for the object in the middle. This makes me think that a 7, 8 or 9mm exit pupil is not 'wasting light' like other people sometimes suggest. That's like suggesting that once an oil painting becomes so big that you cant see it all without moving your head, that it is somehow dimmer and the artist was wasting canvas.

Finally this got me thinking about the link between AFOV and exit pupil. If I have one of these expensive 100° eyepieces, the exit pupil must be bigger than 7mm, otherwise I could see the edges without moving my pupil around?

I know I must be wrong because AFOV is not a factor in calculating exit pupil size.

So can anybody explain where I am going wrong and how (if at all) exit pupil and AFOV are related?

Thanks!

Edited May 5, 2020 by miguel87

[bingevader]

Hi,

I don't think you are quite right with your middle paragraph. Using an EP with too large an exit pupil does result in 'wasted light' because you can't get all that light into your eye. You moving your eye around doesn't mean you are picking up any extra light, you are still only receiving the same amount of light on your retina. If the loss was, for example, 10%, it would still be a 10% loss regardless of where you looked. You can't physically get all that light into you eye so there is always 'wasted light'. With the 100° EPs (as long as they fall within the limits of your pupil size) you are not losing any light. Your retina is receiving 100% of the light leaving the EP regardless of the angle at which you observe through the EP.

I'm sure there must be some diagrams to explain, I'll have a look.

The 'wasted light' bit has never bothered me though, because if it is a pleasurable view, what difference does it make?  

[miguel87]

Thanks for your reply, the topic interests me.

And I understand what you say exactly, if you cant see the whole exit pupil 2d image, then some light is hitting your iris and not going into the pupi. However, the exit pupil is an in-focus image, so the light not entering your pupil is just resolved stars at the edge of the FOV. I cant see how this would dim any object in the centre of the view? For example a planet, the light from the planet is hitting the mirror, then secondary then eyepiece lenses, being resolved into an image of the planet at the exit pupil plane and into the human pupil, all of it. Ok so the light from a mag 12 star at the edge of the frame is lost, but the planet will not be any dimmer.

Perhaps we are agreeing with each other but in different ways?!

I dont think you can say that 10% too large an exit pupil results in 10% reduced brightness, because it depends how the light is distributed across the image. If the outer 10% of the exit pupil image has no stars then surely 0% light is lost?

Mike

Edited May 5, 2020 by miguel87

[bingevader]

1 minute ago, miguel87 said:

Ok so the light from a mag 12 star at the edge of the frame is lost, but the planet will not be any dimmer.

Nearly, but not quite.

The pictures are too complicated for me!

You aren't receiving all of the light from the planet, there is still an overall loss and so a dimmer planet.

Imagine a 5p coin as the ep and a 20p coin (okay, not round) as your pupil. The 5p fits nicely in the 20p, all the light is received.

If you now swap the 5p for a 2p, it's now bigger than the 20p. Not all of the light can enter the pupil, there is an overall reduction in the percentage of the light reaching the retina.

It's not that some bits are bright and others aren't, there's an overall reduction.

[miguel87]

6 minutes ago, bingevader said:

 

It's not that some bits are bright and others aren't, there's an overall reduction.

But some bits ARE brighter than others, or else there would not be a resolved picture to look at. Of course jupiter is brighter than an empty bit of sky.

I would agree if we were talking about the unresolved light cone. But the exit pupil is a resolved image and it's brightness by definition is not uniform.

Edited May 5, 2020 by miguel87

[bingevader]

18 minutes ago, miguel87 said:

I dont think you can say that 10% too large an exit pupil results in 10% reduced brightness, because it depends how the light is distributed across the image. If the outer 10% of the exit pupil image has no stars then surely 0% light is lost?

I didn't. 

I made no correlation between exit pupil and brightness.

I said, that if you lost 10% (only as an example) of the light, then it is lost, regardless of where you are looking.

[John]

Exit pupil is calculated by dividing focal length of the eyepiece by the focal ratio of the scope. Apparrent field of view (AFoV) is not part of the calculation.

Where AFoV does come into play is to determine the amount of sky (true field) that will be visible.

With a very wide AFoV eyepiece, it is possible to have:

- reasonably high magnification

- see a good size chunk of sky

- maintain an exit pupil that is really effective

The above are some of the key reasons that ultra and hyper wide angle eyepieces are popular and effective with fast focal ratio scopes.

 

 

[bingevader]

6 minutes ago, miguel87 said:

But some bits ARE brighter than others, or else there would not be a resolved picture to look at. Of course jupiter is brighter than an empty bit of sky.

Yes, absolutely, but the exit pupil (amongst other things) determines how much of the light from Jupiter hits the retina.

If the exit pupil is too big, Jupiter will be dimmer. It's an overall effect, not just a centre and edges, or bright things and empty space effect.

You'll have to wait for someone who can explain the physics for a better answer, I just know that it's the way it is!

[markse68]

What determines the afov  then? The angle of the focused light cone?

[mikeDnight]

Simply, real field = apparent field divided by magnification.

[John]

18 minutes ago, markse68 said:

What determines the afov  then? The angle of the focused light cone?

The diameter of the field stop inside the eyepiece coupled with the focal length of the eyepiece define the AFoV. It can be as wide as the internal diameter of the eyepiece barrel but is frequently less because the optical quality of the lenses and overall optical system of the eyepiece would not support acceptable definition over their whole diameter.

 

 

[bingevader]

The AFOV is determined by the field stop (gives the edges of the FOV) and the focal length of the EP.

[Don Pensack]

On 05/05/2020 at 08:01, miguel87 said:

Probably getting out of my depth here but anyway...

I was  looking at exit pupil sizes and all that jazz and my largest is 6.5mm and with my eye snuggled up to the eyepiece I can only just see the entire FOV without moving my gaze. This fits with the idea that my pupil size is similar to the exit pupil image.

BUT, imagine that the exit pupil was 7.5mm and I had to 'look around' a little to see the edge of field. The exit pupil image is a resolved image, so not being able to see the edges would not dim any objects in the middle because my pupil is receiving all the light available for the object in the middle. This makes me think that a 7, 8 or 9mm exit pupil is not 'wasting light' like other people sometimes suggest. That's like suggesting that once an oil painting becomes so big that you cant see it all without moving your head, that it is somehow dimmer and the artist was wasting canvas.

Finally this got me thinking about the link between AFOV and exit pupil. If I have one of these expensive 100° eyepieces, the exit pupil must be bigger than 7mm, otherwise I could see the edges without moving my pupil around?

I know I must be wrong because AFOV is not a factor in calculating exit pupil size.

So can anybody explain where I am going wrong and how (if at all) exit pupil and AFOV are related?

Thanks!

You misunderstand the relationship between exit pupil and apparent field.

The exit pupil is the image of the primary mirror.  Essentially, every single point in the exit pupil is illuminated by the entire primary mirror.

When your iris blocks part of that exit pupil, you are blocking part of the primary mirror (an astute observer would note that so does the secondary mirror in both center and edge), but it doesn't reduce the apparent field of the eyepiece.

It reduces the field illumination in the eyepiece.

 

But you can simply move your eye laterally to see one edge of the field or the other since the field is being illuminated exactly as before, merely that your eye is not taking it all in.

Your supposition that a larger exit pupil is not wasted is correct.  The reduction in magnification brightens the image and the light loss exactly equals it, so for the field you see, it will be exactly as bright as the image when the exit pupil matches your pupil diameter.

That is only safe with a refractor, though.  With a reflector, as the exit pupil grows larger, so does the shadow of the secondary.  At some point, the shadow becomes a large portion of your pupil diameter and you start noticing its presence.

Edited May 9, 2020 by Don Pensack

[miguel87]

19 minutes ago, bingevader said:

If the exit pupil is too big, Jupiter will be dimmer. It's an overall effect, not just a centre and edges, or bright things and empty space effect.

This assumes that some of Jupiter's light exists outside the image of the planet, which I dont think makes sense.

Image a 7mm diameter, circular camera sensor with the exit pupil being projected onto it. If the outer ring of pixels were removed, the image of Jupiter in the middle would not get any dimmer.

[John]

Just now, miguel87 said:

This assumes that some of Jupiter's light exists outside the image of the planet, which I dont think makes sense.

Image a 7mm diameter, circular camera sensor with the exit pupil being projected onto it. If the outer ring of pixels were removed, the image of Jupiter in the middle would not get any dimmer.

Have a look at Don Pensack's post above.

 

[miguel87]

7 minutes ago, Don Pensack said:

 

Your supposition that a larger exit pupil is not wasted is correct.  The reduction in magnification brightens the image and the light loss exactly equals it, so for the field you see, it will be exactly as bright as the image when the exit pupil matches your pupil diameter.

Thankyou for a great explanation. I think I have trouble visualising how AFOV works.

Glad you cleared up the wasted light issue tho. It had been troubling me to be honest!

Telescopes are so simple and so complicated at the same time.

Mike

[John]

This is Tele Vue's take on some of these issues. Myth #2 is relevant:

http://www.televue.com/engine/TV3b_page.asp?id=86

 

[bingevader]

13 minutes ago, Don Pensack said:

Your supposition that a larger exit pupil is not wasted is correct.  The reduction in magnification brightens the image and the light loss exactly equals it, so for the field you see, it will be exactly as bright as the image when the exit pupil matches your pupil diameter.

And this is still true if the magnification remains the same but the diameter of the EP increases (so no reduction in magnification to brighten the image)?

I will quite happily stand corrected, I've been living with a misconception for some years!

There was a vogue for larger and large EPs at one point and I remember the same conversation then.

Maybe I didn't remember it and that's the problem!

[miguel87]

2 minutes ago, bingevader said:

And this is still true if the magnification remains the same but the diameter of the EP increases (so no reduction in magnification to brighten the image)?

I will quite happily stand corrected, I've been living with a misconception for some years!

There was a vogue for larger and large EPs at one point and I remember the same conversation then.

Maybe I didn't remember it and that's the problem!

This is what I am wondering now because I was imagining the same magnification but a larger exit pupil. I guess I didnt think about the fact that you would have to use two telescopes to compare the effect.

👍

Edited May 5, 2020 by miguel87

[bingevader]

5 minutes ago, John said:

This is Tele Vue's take on some of these issues. Myth #2 is relevant:

http://www.televue.com/engine/TV3b_page.asp?id=86

 

Sums it up for me very nicely, thanks John.

At these magnifications the loss of light makes little difference.

[John]

Despite all the useful information, formulae etc, etc, I've found that there is no substitute for trying things for yourself to see what works best for you under your skies.

Thank goodness for the used astro equipment market !

 

[miguel87]

My problem is that I have always been a 'why' person. It's not good enough if it works, I want to know WHY and HOW it works 😬

Edited May 5, 2020 by miguel87

[John]

5 minutes ago, miguel87 said:

My problem is that I have always been a 'why' person. It's not good enough if it works, I want to know WHY and HOW it works 😬

Fair enough. I think quite a few on the forum have a similar approach, and why not ?

 

[miguel87]

Just now, John said:

Fair enough. I think quite a few on the forum have a similar approach, and why not ?

 

I wish I didnt ask so many questions! Life would be simpler haha. 

[John]

The only thing I would say is don't let the pondering of the theory get in the way of developing practical experience

I get a little lost by some of the optical and physics theory if I'm honest about it. I feel much more comfortable when I get behind a scope and getting on with the practical business of trying to see things.