Why do we need bigger scopes?

[Stu]

29 minutes ago, cloudsweeper said:

Not sure about that, Stu.  Does it not depend on the relative SBs?  A brighter fuzzy will dim less than the background, and this is why the exit pupil range of say 4 - 2mm is desirable for contrast.  Only at even lower exit pupil does the target dim a lot more.

Doug.  

That’s not my understanding Doug. I believe both object and background SB decrease at the same rate as magnification increases and exit pupil decreases.

I think the 2 to 4 mm range comes from the balance point between various factors. In favour of increasing mag/decreased exit pupil are the  increased image scale which gives better resolution of detail and an increased perception of contrast (though no actual increase), also the reduced exit pupil reduces abberations in your eye, so increases resolution and detail seen.

In favour of decreasing mag/increasing exit pupil are the fact that at some point the overall image dims to such a point that the ‘noise’ in your visual system starts to impact your ability to perceive the detail, and the reduced exit pupil shows floaters in your eye which start to interfere with the image.

These opposing factors ‘meet’ at different points depending on the object’s size and brightness, so there is not a one size fits all optimum exit pupil but a range which you need to find by trying different eyepieces.

This is much more eloquently and competently explained in this CN post….

https://www.cloudynights.com/topic/573724-for-extended-objects-magnification-does-not-improve-contrast/

[ollypenrice]

This is all very interesting because I have never found the standard arguments about surface brightness to agree with experience.

Please ignore me and carry on!

Olly

[vlaiv]

As far as surface brightness goes, I think following is true:

1. Contrast ratio in terms of photon counts between background and target is maintained as you increase magnification - They are dimmed per unit surface by same amount because both have fixed integrated brightness and increase in magnification - well magnifies them both by same amount (otherwise things would be very funny at the eyepiece).

So photon / physics wise - contrast ratio is maintained

2. Perception of that contrast changes.

It changes due to at least two things - one is already mentioned - as size of object changes so does our perception of contrast. I'm just having trouble accepting that this has magnitude that is ascribed to it.

Second is absolute level of light. We must not assume that we will perceive contrast ratio the same at high levels of light as at low levels of light.

For all those who maintain that integrated brightness is important - I'd say, what about the case when you move half or 3/4 of object outside of field stop. Does the rest change perceived contrast with respect to background?

If you move part of object outside field stop - you'll stop those photons from reaching your eye. Surface brightness will remain the same but total / integrated brightness of visible portion will fall.

 

[Stu]

18 minutes ago, ollypenrice said:

This is all very interesting because I have never found the standard arguments about surface brightness to agree with experience.

Please ignore me and carry on!

Olly

Don’t be coy Olly, tell us what you think!!

[Stu]

8 minutes ago, vlaiv said:

It changes due to at least two things - one is already mentioned - as size of object changes so does our perception of contrast. I'm just having trouble accepting that this has magnitude that is ascribed to it

Perhaps @faulksy can give some input here, about the impact of image scale on small object visibility eg galaxies, PNs etc.

[ollypenrice]

4 minutes ago, Stu said:

Don’t be coy Olly, tell us what you think!!

I'd be dragging the conversation backwards but my eye tells me that big scopes do make things brighter, even allowing for exit pupil similarities. I'm another who has been shot down in flames over this but I have never been convinced that the effect can be explained just by increased surface area. I've always been left with a lingering doubt so Vlaiv's original post came as something of a relief.

Olly

Edited October 6, 2021 by ollypenrice
Typo

[Stu]

43 minutes ago, vlaiv said:

For all those who maintain that integrated brightness is important - I'd say, what about the case when you move half or 3/4 of object outside of field stop. Does the rest change perceived contrast with respect to background?

Two things here. Firstly you seem to be accepting a concept called integrated brightness. Do we all agree this is a ‘thing’??? For the record, I think I do! 🤪

Your point about moving an object partly out of the fov is intriguing because my feeling is that it won’t affect visibility when it should 🤪🤪🤪

In summary, perhaps we should just look through our scopes, enjoy the views, and accept that perhaps we just don’t understand some of this stuff!! It’s highly complex and relates to the eye and brain’s functions as much as the scope/eyepiece and object.

[Stu]

2 minutes ago, ollypenrice said:

I'd be dragging the conversation backwards but my eye tells me that big scopes do make things brighter, even allowing for exit pupil similarities. I'm another who has been shot down in flames over this but I have never been convinced that the effect can be explained just by increased surface area. I've always been left with a lingering doubt so Vlaiv's original post came as something of a relief.

Olly

Not a problem. Personally I think this must be the case so I agree with you. Surely maintaining SB over a larger area results in more ‘overall brightness’, ‘integrated brightness’, whatever you want to call it; let’s just say it looks brighter! 🤪

[cloudsweeper]

21 minutes ago, ollypenrice said:

I'd be dragging the conversation backwards but my eye tells me that big scopes do make things brighter, even allowing for exit pupil similarities. 

Yes - as I've also been saying - bigger ap, more mag for same exit pupil > greater integrated brightness, brighter image in EP.

No reason for us to crash and burn, Olly!

Doug.

[vlaiv]

50 minutes ago, Stu said:

In summary, perhaps we should just look through our scopes, enjoy the views!

Not sure how it's done - but I'm open to new experiences

50 minutes ago, Stu said:

Two things here. Firstly you seem to be accepting a concept called integrated brightness. Do we all agree this is a ‘thing’??? For the record, I think I do!

Indeed - integrated brightness is just total photons form target captured by scope. Larger aperture - more photons (per unit time). It stays constant for any given scope because aperture is fixed.

 

50 minutes ago, Stu said:

Your point about moving an object partly out of the fov is intriguing because my feeling is that it won’t affect visibility when it should 🤪🤪

That is my problem as well - I also think it won't affect anything, but it should (according to integrated brightness approach).

[vlaiv]

2 minutes ago, cloudsweeper said:

Yes - as I've also been saying - bigger ap, more mag for same exit pupil > greater integrated brightness, brighter image in EP.

No reason for us to crash and burn, Olly!

Doug.

Ok, so here is experiment.

You'll need two LED panels, or maybe two pieces of paper and something to illuminate them equally and a dark room.

Illuminate papers so that they turn grey. Try to get them equally illuminated. Now cover 90% of one paper with something dark. Did other paper turn white?

If we go by integrated brightness approach - total surface is important and reducing surface should reduce surface brightness as well. Any change in surface brightness will make one paper slightly darker and then another thing should kick in - we should perceive brightest "neutral" color in scene as white point - it should uncovered paper should become white - or at least lighter gray as it is brighter in intensity.

Perceiving it as having higher surface brightness will do as well.

Again, I have feeling that this will not happen - similarly to how you move half of object outside of FOV and it will not change brightness because of that.

[ollypenrice]

31 minutes ago, Stu said:

Not a problem. Personally I think this must be the case so I agree with you. Surely maintaining SB over a larger area results in more ‘overall brightness’, ‘integrated brightness’, whatever you want to call it; let’s just say it looks brighter! 🤪

Well yes, the statement, 'It looks brighter but isn't,' would not send it's author to the top of the class! 🤣

Olly

[vlaiv]

2 minutes ago, ollypenrice said:

Well yes, the statement, 'It looks brighter but isn't,' would not send it's author to the top of the class! 🤣

Olly

Actually - nothing wrong with that statement. As long as it "feels", "looks", "smells" or anything that has to do with our perception - it is quite possible that it differs from underlying physical reality.

I'm sure you've seen this before:

B looks brighter than A - but it isn't

[Stu]

26 minutes ago, vlaiv said:

Not sure how it's done - but I'm open to new experiences

I’ve now corrected the very confusing statement I made due to auto correct. I meant to say VIEWS, not the word which rhymes with that, just to avoid any religious discussion!! 😱😱🤪🤪🤣🤣

[johninderby]

Interestingly googling Brightness Perception  brings up many references in the field of psychology.🤔

[ollypenrice]

9 minutes ago, johninderby said:

Interestingly googling Brightness Perception  brings up many references in the field of psychology.🤔

I'm not surprized. I'm the son of a perception theorist and, as a result, have kept well away from the matter!  It is a minefield.

24 minutes ago, vlaiv said:

Actually - nothing wrong with that statement. As long as it "feels", "looks", "smells" or anything that has to do with our perception - it is quite possible that it differs from underlying physical reality.

I'm sure you've seen this before:

B looks brighter than A - but it isn't

I'm not sure about this. 'It looks brighter,' is all we need to know as observers if that means, 'It now looks bright enough to see whereas previously it didn't.'  It is perceived brightness in which we are all interested. If that is in conflict with some kind of measured brightness (and I rather doubt that it is) then we can happily ignore measured brightness.

I suppose another matter might be the darkness of the background sky in an optical system. The darker the better, so that might be an important player in comparing two instruments. (I scent the unsvoury whiff of the refractor-reflector debate in the air!)

Olly

[vlaiv]

1 minute ago, ollypenrice said:

I suppose another matter might be the darkness of the background sky in an optical system. The darker the better, so that might be an important player in comparing two instruments. (I scent the unsvoury whiff of the refractor-reflector debate in the air!)

I'm sure that someone with 8" refractor and 4" reflector would come to the same conclusion - bigger scope throws brighter image for some reason

 

[vlaiv]

Maybe we are just wrong in our assumptions?

Only difference between large and small scope operating at same F/ratio and using same eyepiece in both (same exit pupil) is - size of object / integrated brightness.

Contrast ratio is the same - surface brightness photon count is the same - so no difference in absolute values of light hitting sensors.

It can only be due to size of object / integrated brightness - and maybe we are simply wrong to assume that moving part of the object outside FOV will not impact perceived brightness of it / contrast with respect to background?

I'm sure this can easily be checked - although I'm not sure when I'll be able to do it - it seems that I ran out of my clear skies observing budget for this year when I had that one session last week

 

[iantaylor2uk]

I've probably come in late to this conversation, but I feel an essential element is missing. 

If the aperture is twice as large, the light gathered will be 4x as great - this is accepted as fact.

However, I think the discussion about the magnification and field of view is a red herring. The fact is that the retina at the back of the eye is the "receiver" and it's area is unchanged whatever the field of view. 

If 4 times as many photons are gathered by the telescope, then surely 4 times as many photons will enter the eye. Let's consider a faint star that is not visible in the smaller scope but is in the bigger scope. Surely the explanation for larger telescopes being able to see fainter objects is that there are not enough photons received by the eye for the smaller telescope to trigger a response, whereas in the example I gave, where there would be 4x as many photons (in the larger telescope) and that is then enough to trigger a response. 

Then if we consider a galaxy, treat it as many individual stars - the only logical conclusion (for the example I give) is that there 4 times as many photons at each point in the galaxy - that is why it appears brighter - the fact that it is bigger (due to a greater magnification and smaller field of view in the eyepiece) is surely irrelevant. 

Edited October 6, 2021 by iantaylor2uk

[vlaiv]

18 minutes ago, iantaylor2uk said:

the fact that it is bigger (due to a greater magnification and smaller field of view in the eyepiece) is surely irrelevant. 

It does not work quite like that - and you can easily test this on any planet / moon,

Take Jupiter - pump up magnification and the planet will be less bright.

When we increase magnification we spread light over larger surface - either angular or actual on the back of our eye. We have fixed number of sensing cells there and if we spread photons over lager area - each sensing cell will get less photons as total photon number is unchanged.

Cells can't tell if this is due to dimming of the object (reduced number of photons) or because of increased magnification - object being bigger. In both cases - we will see equal dimming.

[neil phillips]

Thought i had a lot of time on my hands. Back in a week to find the answer.  Meanwhile i will continue to use my larger scope for brighter more vivid, and higher detailed images. 

[iantaylor2uk]

Your example is where the object you are viewing is very bright already and extended over many receiving cells in the retina. 

However, my understanding is that telescopes of larger aperture allow you to see fainter objects and I think if you consider pointlike objects such as stars, my explanation would allow you to understand why the larger aperture allow you to see fainter objects. 

[vlaiv]

3 minutes ago, iantaylor2uk said:

However, my understanding is that telescopes of larger aperture allow you to see fainter objects and I think if you consider pointlike objects such as stars, my explanation would allow you to understand why the larger aperture allow you to see fainter objects. 

It is everyone's understanding and experience that larger telescopes allow to see fainter objects - we are just trying to understand why is that.

For point like sources - that is sort of easy to understand. When you zoom in on point source - well, it stays point like - star will be point like in both large and small scope. Point like star covers only very limited number of detector cells and that number does not change between large and small scope. Total photon count does change - and when you divide larger total photon count - over same number of receptors - each receptor gets more photons - hence image is brighter.

With extended sources - photons spread around with magnification - that is why they are different to point sources and called extended sources - they behave differently.

You don't need planet that is bright to see this - just look at background sky. It has some brightness to it and is extended source itself. Increase magnification and it will get darker.

[iantaylor2uk]

I think you need to distinguish between the light gathering properties of the telescope - which is determined by the scope size, and the focal length, which effectively is the rate at which photons arrive. 

If you "zoom in" to a planet or galaxy, for example with a zoom lens, the focal length will be increased, and the f ratio will be higher. So even though the light gathering capacity of a scope may be larger, if the f ratio is increased (for example when you zoom in) then of course it will appear dimmer (it is well known in photography that low f ratios are "fast" and high f ratios are "slow" - so with a fast lens you can go to shorter exposures). When you increase magnfication of course things will get darker since the f ratio is higher. 

I don't buy your argument between point-like sources and extended objects. For example, take a galaxy, this can be considered as a large number of point-like objects, and so my argument would still apply - at each point in the galaxy there will be more photons hitting the back of the eye. 

[vlaiv]

3 minutes ago, iantaylor2uk said:

If you "zoom in" to a planet or galaxy, for example with a zoom lens, the focal length will be increased, and the f ratio will be higher.

F/ratio is ratio of aperture to focal length of telescope - that does not change irrespective of eyepiece used in telescope.

4 minutes ago, iantaylor2uk said:

When you increase magnfication of course things will get darker since the f ratio is higher. 

No, F/ratio remains the same and has nothing to do with things getting darker.

4 minutes ago, iantaylor2uk said:

I don't buy your argument between point-like sources and extended objects. For example, take a galaxy, this can be considered as a large number of point-like objects, and so my argument would still apply - at each point in the galaxy there will be more photons hitting the back of the eye. 

You really don't have to by into my argument - it is empirical fact - next time when you are at the telescope - observe background sky brightness at two different magnifications. Observe star under two different magnifications (low enough so you don't start to resolve airy disk as that is the point where point source stops being point source).