Exit pupil and AFOV

[andrew s]

16 minutes ago, vlaiv said:

Same here. Although I know what you mean - when you say image - I instantly think in terms of focused light (probably due to too much time spent contemplating imaging rather than observing).

Just draw a large X in felt tip on you primary then you will have a real  visible image at the exit pupil.🥴

If you were working in the IR your primary would produce an IR image at the exit pupil.

Regards Andrew 

Edited May 9, 2020 by andrew s

[vlaiv]

33 minutes ago, andrew s said:

Just draw a large X in felt tip on you primary then you will have a real  visible image at the exit pupil.🥴

If you were working in the IR your primary would produce an IR image at the exit pupil.

Regards Andrew 

Except you won't see it

Problem, like I said is wording and interpretation.

Secondary shadow appears in image in exit pupil - problem is - when you look at exit pupil, the way we think of looking at exit pupil - you won't see secondary - you'll see the moon for example.

Take a piece of tracing paper and put it where exit pupil is - it won't show the moon - it will show shape of primary with secondary shadow. I'm aware of that - problem is that I don't think of it as "image".

[andrew s]

1 minute ago, vlaiv said:

Except you won't see it

Problem, like I said is wording and interpretation.

Secondary shadow appears in image in exit pupil - problem is - when you look at exit pupil, the way we think of looking at exit pupil - you won't see secondary - you'll see the moon for example.

Take a piece of tracing paper and put it where exit pupil is - it won't show the moon - it will show shape of primary with secondary shadow. I'm aware of that - problem is that I don't think of it as "image".

Put your ccd there and you would! Just because it's  not the image your after does not stop it being an image. Looking in the mirror I wonder why it's my dad looking back at me rather than the young handsome youth I know I am.

Regards  Andrew 

PS I know what you mean only playing!

[jetstream]

12 hours ago, miguel87 said:

Thanks Vlaiv, I certainly have a better understanding than before.

Me too... I find the best approach is keeping an absolutely open mind, placing my own thoughts in the archives and building the archives up more. The end result is usually pretty good and thanks to those that help.

[bingevader]

On 09/05/2020 at 17:07, Don Pensack said:

Remember, every point on the focal plane of both eyepieces is illuminated by the entire primary (* see below)

I've always understood this.

On 09/05/2020 at 17:07, Don Pensack said:

A larger apparent field spreads the light farther into your peripheral vision, but does not brighten the image.

But why doesn't this then result in "wasted light" if the exit pupil is too big? That's where I need clarification.

[miguel87]

1 hour ago, bingevader said:

I've always understood this.

But why doesn't this then result in "wasted light" if the exit pupil is too big? That's where I need clarification.

Because you are maximising the brightness with ANY eyepiece over the max aperture of your pupil.

So why turn down an eyepiece if it's exit pupil is too big? You will still get the same FOV, magnification etc. And it will be as bright as any eyepiece can be for you.

Wasted light suggests that if you go smaller with the exit pupil you might not waste light and therefore get a brighter image. Which is wrong.

In essence a 'too big' exit pupil will not degrade your image in any way (unless we start talking about newtonian central obstructions).

Personally I dont see how you can be viewing the brightest possible image for your eye and say that there is wasted light?

That's my take anyway.

Edited May 10, 2020 by miguel87

[John]

I go by what has worked best for me under my skies over the years I've been observing.

My 21mm Ethos gets a lot more use in my F/5.3 12 inch dob than my 31mm Nagler does.

Whether that matches the theory I really don't know

 

[vlaiv]

2 minutes ago, miguel87 said:

Wasted light suggests that if you go smaller with the exit pupil you might not waste light and therefore get a brighter image. Which is wrong.

Let's say that your pupil dilates to 6mm and that exit pupil of a given eyepiece for 7" scope is 7mm.

Now take an aperture mask that reduces scope from 7" to 6" - so its just a flat ring 0.5" wide that you put on your aperture.

What will happen to your view? Absolutely nothing - image will not become dimmer. It will remain the same. In that sense - using that eyepiece that gives you 7mm exit pupil on 7" scope is like using 6" scope with eyepiece that gives same magnification and 6mm exit pupil.

This is not because of eyepiece and exit pupil - it is because your eye cant get wide enough.

[miguel87]

14 minutes ago, vlaiv said:

Let's say that your pupil dilates to 6mm and that exit pupil of a given eyepiece for 7" scope is 7mm.

Now take an aperture mask that reduces scope from 7" to 6" - so its just a flat ring 0.5" wide that you put on your aperture.

What will happen to your view? Absolutely nothing - image will not become dimmer. It will remain the same. In that sense - using that eyepiece that gives you 7mm exit pupil on 7" scope is like using 6" scope with eyepiece that gives same magnification and 6mm exit pupil.

This is not because of eyepiece and exit pupil - it is because your eye cant get wide enough.

I cant tell if you are agreeing or disagreeing with me 😂

[vlaiv]

21 minutes ago, miguel87 said:

I cant tell if you are agreeing or disagreeing with me 😂

Both

I'm explaining why people say it is wasted light - using 7" scope for something 6" scope would do as well - so you are not using full benefit of 7" aperture and on the other hand - that is what such magnification provides with 7" scope - take it or leave it

[mikeDnight]

59 minutes ago, miguel87 said:

Because you are maximising the brightness with ANY eyepiece over the max aperture of your pupil.

So why turn down an eyepiece if it's exit pupil is too big? You will still get the same FOV, magnification etc. And it will be as bright as any eyepiece can be for you.

Wasted light suggests that if you go smaller with the exit pupil you might not waste light and therefore get a brighter image. Which is wrong.

In essence a 'too big' exit pupil will not degrade your image in any way (unless we start talking about newtonian central obstructions).

Personally I dont see how you can be viewing the brightest possible image for your eye and say that there is wasted light?

That's my take anyway.

By using an eyepiece that gives you say an 8mm exit pupil, an iris that has a maximum 7mm aperture will be cutting  into the light path, effectively reducing the aperture of the scope. However, you can still use an oversized exit pupil if the eyepiece frames the picture well, you just won't be using your full aperture. But you can only do this with unobstructed systems such as refractors, as the secondary obstruction becomes problematic with reflectors. Al Nagler explains it perfectly if you fancy searching Televue's site. The real problem with very low powers, even if the exit pupil doesn't cut exceed 7mm, is that the sky brightness lacks contrast. It's better to increase the power in most cases so as to darken the sky background, and there by increasing contrast.

[miguel87]

23 minutes ago, mikeDnight said:

effectively reducing the aperture of the scope.

I have an issue with this phrase.

How can the brightest possible image that YOU can see through the scope be considered reducing the aperture.

"Effectively reducing the aperture", people hear that and think wow, I'm not gonna do that, I dont want to dim my telescope. Truth is, Give me any example with the same telescope and same eyes of an image that is brighter? That is the brightest that your telescope can perform so how can it be considered reducing the aperture?

Just because a smaller telescope is capable of showing an image just as bright, doesnt mean that you are somehow degrading the bigger telescope.

"Reducing" implies that you could somehow not reduce. Brightness is maximised, not reduced.

Anyway, we all know what is happening I guess it's just a question of perspective/opinion 

Edited May 10, 2020 by miguel87

[mikeDnight]

A telescope of say 8" will always gather the same amount of light no matter what eyepiece is used. If however you fit an adjustable diaphragm at any point throughout the system, thereby cutting into the light path, you are effectively reducing the aperture, even though the true aperture of the instrument remains 8". Because of the diaphragm it is unable to work at its full aperture. An eyepiece that has a 10mm exit pupil will still deliver a full aperture view, its just that the human eye can't accept that exit pupil size. In the same way, the human iris is just such a diaphragm, which can cut into the light path if the max 7mm limit is exceeded. 

 

[miguel87]

Exactly, so if I am using an eyepiece that somebody describes as wasting light.

The only ways to brighten the view are 1, new eyes, 2 new telescope.

If there is no eyepiece you can buy that will produce a brighter view, how can it be a light waster? It is surely a light maximiser!

I use an eyepiece with a 6.5mm exit pupil on a relatively fast newt. It is likely that my pupil is not often this large but the view is bright, beautiful and there is no sign of a secondary shadow even on the full moon. I certainly dont feel like I am wasting light.

If I replaced it with an eyepiece that matched my dilated pupil size perfectly, what would I be gaining? A bit more mag? I have that in spades in other pieces.

Edited May 10, 2020 by miguel87

[scarp15]

Pupil dilation, I do not know, having never measured my exit pupil, to what extent my pupil will become fully dilated when my eye has achieved complete dark adaption at a dark sky location. I do understand that in visual adjustment, as the cones, that are colour sensitive, run out of light, the pupils will become fully dilated and the rods respond taking over more. This winter season, I had the opportunity to employ using a new eyepiece that provided an exit pupil of 7.73mm, with a Paracorr attached to my 14" F4.6 Dob. Selectively there were particular subjects I aimed to apply this to. Highlights included Andromeda, The Pleiades but two subjects in particular, that in the former were greatly enhanced and in the later revealed with certainty than previous attempts and much more extensively. They were The California Nebula and Barnard's Loop. Each required using my traditional 2" Lumicon H-beta filter. The aggressive nature of the filter complementary to the brighter contrast set by the eyepiece. The eyepiece is a 41mm Panoptic, which I took a punt on last year and has now become a go to for both my dobs and refractor. 

As someone who is not of much understanding regarding the science, some of the in-depth information on this thread has been of interest, yet actually applied 'in the field' at a dark sky location, yields interesting and successful results.

[bingevader]

2 hours ago, miguel87 said:

If I replaced it with an eyepiece that matched my dilated pupil size perfectly, what would I be gaining? A bit more mag? I have that in spades in other pieces.

Then why not use them?

[John]

A bit more magnification is what I find works well at teasing faint DSO's out of my moderately light polluted sky. If I can get that AND have a large true field, well that just seems ideal to me

 

[jetstream]

AFOV...

I find that 100 deg eyepieces give me better results on large diffuse nebula that smaller AFOV eyepieces, regardless of TFOV. Some say that the larger, full engagement of the eye, with no visible field stop can provide better views on certain things and in my experience this holds true. Please note I usually observe these objects at an exit pupil smaller than my 7mmish pupil dilation. Dilation measure by the "drill bit" method.

There is a sweet spot mag wise for every object in each telescope IMHO. It might be possible that photon concentration as dictated by the exit pupil (field stop) plays a role. I get better views around 4.5mm-5.5mm that at say 6.25mm, still below the 7mm dilation. Ive been testing this for years.

[Don Pensack]

Say you use and eyepiece that yields a 7mm exit pupil and your pupil size is 7mm.  That will be the maximum brightness you can see in the scope.

Use an eyepiece yielding a 3.5mm exit pupil and the image will be 1/4 as bright due to the smaller exit pupil.

You can also look at it another way--the doubling of the magnification results in the brightness being 1/4 as great.

Now, use an eyepiece that yields a 14mm exit pupil (refractor in this illustration).  You have effective stopped down the scope by only admitting 1/4 of the light in the telescope's exit pupil.

Why is the image the same brightness as the eyepiece that yields a 7mm exit pupil?

Because the magnification is only 1/2 as much, which results in a 4X increase in brightness.  1/4 x 4 = 1.

That's why using a larger exit pupil than your eye doesn't result in a dimming of the image, and why using a smaller exit pupil than your pupil does.

[Ags]

1 hour ago, Don Pensack said:

Use an eyepiece yielding a 3.5mm exit pupil and the image will be 1/4 as bright

Except stars - their brightness remain constant regardless of exit pupil, except when exit pupil exceeds eye pupil, in which case stars get dimmer.

[miguel87]

5 minutes ago, Ags said:

 

Nothing gets dimmer if the exit pupil is too big. It just fails to get brighter beyond that point.

Edited May 11, 2020 by miguel87

[John]

I guess this may or may not be related to all this but I find that using high magnification (very high sometimes) and obviously very small exit pupils helps me pick dim point sources out, eg: super novae, quasars, faint planetary moons etc. These don't seem to be as apparent at lower magnifications.

Or is that something different at work ?

 

[Ags]

3 minutes ago, miguel87 said:

Nothing gets dimmer if the exit pupil is too big. It just fails to get brighter beyond that point.

I suggest you try this out with some eyepieces. Also refer back to the helpful diagrams posted earlier by @vlaiv

[miguel87]

27 minutes ago, Ags said:

I suggest you try this out with some eyepieces. Also refer back to the helpful diagrams posted earlier by @vlaiv

Have done, plus the maths behind it explains it anyway.

You would be right if a larger exit pupil spread the same brightness over a larger area (dimmer per unit) but it doesnt. We know that as magnification decreases, the image gets brighter. So increase exit pupil incrementally from 1mm up to your pupil size. As you go past your pupil size, the section 'stepped down' by your iris doesnt get any dimmer, you just miss out on the extra light of the larger pupil.

No dimming, just a stable maximum based on your pupil size.

Vlaiv was extremely generous and helped me to understand how an exit pupil functions. I never really got it before but he has a way of explaining things!

Edited May 11, 2020 by miguel87

[Ags]

If I have an F4 scope and I use a 40mm plossl, the exit pupil will be 10 mm. This means most of the light from the star will not fit in your eye pupil. So the star will be fainter than if I had used a 4 mm eyepiece giving a 1 mm exit pupil. This only applies to point sources, extended object do get fainter at smaller exit pupils and reach a maximum brightness at exit pupil = eye pupil.