Why do we need bigger scopes?

[vlaiv]

1 minute ago, pjsmith_6198 said:

The larger the objective, the fainter the objects that can be seen.

That is empirical fact.

I'm trying to understand why this happens because logic / science says following:

If you have object with certain surface brightness and you decrease aperture size and decrease magnification equally - apparent surface brightness of object does not change.

Same number of photons hit back of our eye - per unit surface or per receptor cell (as we have definite density of receptor cells).

How come that with same number of photons hitting our receptor cells - in one case we see something and in another we don't? That is the question.

[mikeDnight]

Im not sureI'm following the reasonings so far, but as far as a 4" is concerned I've found that magnification needs to be tailored to suit the deep sky target as they do differ. Sky background darkens with increased magnification while the target nebula to some extent becomes more obvious. This can be used to advantage providing the magnification threshold isn't over stretched as contrast increases. If it is over stretched the nebula is lost. Of course the visual acuity of different observers is an aspect of the observability equation  that is hard if not impossible to calculate.

Edited October 6, 2021 by mikeDnight

[pjsmith_6198]

OK. I understand what you are asking now.     Maybe it's just bigger barely perceptible objects are easier to see than smaller barely perceptible objects.

 

Phil

[cloudsweeper]

3 minutes ago, vlaiv said:

4 minutes ago, vlaiv said:

 

If you have object with certain surface brightness and you decrease aperture size and decrease magnification equally - apparent surface brightness of object does not change.

For constant SB, there is more overall (integrated) brightness with a larger image.  A 1-LED torch is not as bright as a 9-LED torch, yet they have the same SB figures (brightness per unit area).

Doug.

[mikeDnight]

If photons are packets of energy, then the greater aperture will gather more photons, and as the rods in our eyes are photo receptors, the more photons  hitting those rods the greater the chance our rods will fire. Hence larger apertures enable us to see fainter objects.

[vlaiv]

I just thought of another point that we have not considered.

We take that JND is constant - but maybe it is not.

JND stands for Just Noticeable Difference. In discussion so far we referred to it as contrast ratio - or rather we referred to physical aspect rather than perceptual aspect.

Say we have two sources of light - how much different they need to be in order for us to be able to tell that they are different. I've found somewhere that this threshold for vision is about 7%.

People doing variable star observations (without computer) - can chip in here - what is magnitude difference that can be seen between two stars? Above 7% is 0.07 and that is equal to 0.073 mags of difference.

Question is - is this 7% (or what the actual value is) - constant or does it change when we approach low light limit?

Maybe in low light scenario we need larger JND and that is part of the problem?

[vlaiv]

1 minute ago, mikeDnight said:

If photons are packets of energy, then the greater aperture will gather more photons, and as the rods in our eyes are photo receptors, the more photons  hitting those rods the greater the chance our rods will fire. Hence larger apertures enable us to see fainter objects.

If we keep exit pupil constant we have constant ratio of photons per eye receptor.

Our eye receptors are fixed in sense of how many there are per mm2 of back of our eye.

If we keep exit pupil the same - larger telescope will gather more photons but at the same time it will magnify image more and projection of object on the back of our eye will be of larger size (eye's focal length is fixed) - covering more millimeters squared.

These more photons will be spread over more receptors and on average - each receptor will gather the same amount of photons (for same exit pupil - increase in aperture and increase in magnification just cancel each other out).

 

[Knighty2112]

Turns out (after extensive research via Google!  ) that the answer is super complicated. So rather than trying to wrap my puny human  brain around these mysteries I’m just going to enjoy the views in my telescopes and EP’s as best as my old eyes and telescope optics allow!  

[vlaiv]

14 minutes ago, cloudsweeper said:

For constant SB, there is more overall (integrated) brightness with a larger image.  A 1-LED torch is not as bright as a 9-LED torch, yet they have the same SB figures (brightness per unit area).

Doug.

Indeed - that is equivalent of saying - larger scope gathers more light. Problem is - that light is spread over more photo receptors and each photo receptor gathers same number of photons as before.

[vlaiv]

1 minute ago, Knighty2112 said:

Turns out (after extensive research via Google!  ) that the answer is super complicated. So rather than trying to wrap my puny human  brain around these mysteries I’m just going to enjoy the views in my telescopes and EP’s as best as my old eyes and telescope optics allow!  

Care to share what you've found? Simple link will be enough.

[Knighty2112]

5 minutes ago, vlaiv said:

Care to share what you've found? Simple link will be enough.

Well, I may have over exaggerated the extensive part in my research, but this certainly did my head in!  

https://medium.com/@phpdevster/how-telescope-aperture-affects-your-view-24507147d7fc

Edited October 5, 2021 by Knighty2112

[Stu]

2 hours ago, PeterW said:

Yak my both eyes

You have Yak eyes? That’s cheating, surely?

[JeremyS]

33 minutes ago, vlaiv said:

I just thought of another point that we have not considered.

We take that JND is constant - but maybe it is not.

JND stands for Just Noticeable Difference. In discussion so far we referred to it as contrast ratio - or rather we referred to physical aspect rather than perceptual aspect.

Say we have two sources of light - how much different they need to be in order for us to be able to tell that they are different. I've found somewhere that this threshold for vision is about 7%.

People doing variable star observations (without computer) - can chip in here - what is magnitude difference that can be seen between two stars? Above 7% is 0.07 and that is equal to 0.073 mags of difference.

Question is - is this 7% (or what the actual value is) - constant or does it change when we approach low light limit?

Maybe in low light scenario we need larger JND and that is part of the problem?

VS observers can routinely, and accurately, discriminate 0.1 mag. Sometimes even a bit better.

The precise discrimination depends on a few factors. One observer reports that with a telescope that can detect ca mag 16 stars, there is an optimum range where discrimination is greatest, at around mag 14 to 15.  

[globular]

20 minutes ago, vlaiv said:

Indeed - that is equivalent of saying - larger scope gathers more light. Problem is - that light is spread over more photo receptors and each photo receptor gathers same number of photons as before.

a few receptors slightly activated..... brain says "nah it's probably noise, ignore it"

lots more receptors with same very low activation... brains says "too many to ignore this time, see what you make of it"

[jetstream]

1 hour ago, cloudsweeper said:

the brightness per unit area when integrated over a larger area will give more overall brightness.  

The brightness adds up?

[cloudsweeper]

29 minutes ago, vlaiv said:

Indeed - that is equivalent of saying - larger scope gathers more light. Problem is - that light is spread over more photo receptors and each photo receptor gathers same number of photons as before.

Yes........more light flux enters the 'scope, so more light flux enters the eye.  Not sure I follow the point that it then spreads over more receptors - I would have expected receptors to gather more flux/photons.  That would be consistent with the empirical assertion that we started out with, namely that a bigger aperture gives brighter fuzzies.  (Because the SB exists across a larger area/image size.)

Doug.

[Stu]

@vlaiv, my understanding is much as has been mentioned here already. A larger scope can maintain the same surface brightness as a smaller acope at higher magnification . Therefore it shows the objects at a larger image scale, same surface brightness so higher overall (integrated??) brightness.

Your eye perceives contrast more easily on larger objects, so the larger scope makes faint, smaller galaxies etc easier to detect. This, to me, explains the conundrum you posed in the last paragraph of your original post.

Mel Bartels has some great information and calculators on his site about object visibility:

https://www.bbastrodesigns.com/NewtDesigner.html#visual

https://www.bbastrodesigns.com/ObjectContrastCalculator.htm

[cloudsweeper]

3 minutes ago, jetstream said:

The brightness adds up?

Is not an array of 9 LEDs brighter than a single LED?  Or is it just perceived to be so?  

As I said earlier, this topic can be relied on to stimulate interesting exchange!  😉

Doug.

[vlaiv]

15 minutes ago, globular said:

a few receptors slightly activated..... brain says "nah it's probably noise, ignore it"

lots more receptors with same very low activation... brains says "too many to ignore this time, see what you make of it"

A human rod cell is about 2 microns in diameter (quote from wiki: https://en.wikipedia.org/wiki/Rod_cell)

Focal length of human eye is 17mm - 22mm (depending on source) - but let's take average value of 20mm

Single rod cell thus covers - 20" or 20 arc seconds.

When we talk about galaxies - we usually talk about object that are about few degrees in AFOV - maybe smallest is half a degree - size of full Moon to the naked eye.

That is x60 - x90 larger than single cell in diameter - and if we compare by surface - it contains about 3000 rod cells.

I would not call that "few receptors". I think that in cases we are discussing - enough receptors are stimulated that size of object does not make difference with respect to number of stimulated cells.

[jetstream]

4 minutes ago, cloudsweeper said:

Is not an array of 9 LEDs brighter than a single LED?  Or is it just perceived to be so?  

As I said earlier, this topic can be relied on to stimulate interesting exchange!  😉

Doug.

So the "volume" of light through the EP matters? not simply just exit pupil at a mag?

[cloudsweeper]

10 minutes ago, Stu said:

 Therefore it shows the objects at a larger image scale, same surface brightness so higher overall (integrated??) brightness.

I agree.

 

12 minutes ago, Stu said:

 Your eye perceives contrast more easily on larger objects, so the larger scope makes faint, smaller galaxies etc easier to detect. This, to me, explains the conundrum you posed in the last paragraph of your original post.

 

Yes.  It's a combination of size perception along with the "contrast effect".

Doug.

[globular]

1 minute ago, vlaiv said:

A human rod cell is about 2 microns in diameter (quote from wiki: https://en.wikipedia.org/wiki/Rod_cell)

Focal length of human eye is 17mm - 22mm (depending on source) - but let's take average value of 20mm

Single rod cell thus covers - 20" or 20 arc seconds.

When we talk about galaxies - we usually talk about object that are about few degrees in AFOV - maybe smallest is half a degree - size of full Moon to the naked eye.

That is x60 - x90 larger than single cell in diameter - and if we compare by surface - it contains about 3000 rod cells.

I would not call that "few receptors". I think that in cases we are discussing - enough receptors are stimulated that size of object does not make difference with respect to number of stimulated cells.

However many there are, there are still 4x as many when you double the aperture... a change bigger than the 7% JND.... so just might be noticed.

[jetstream]

9 minutes ago, cloudsweeper said:

Is not an array of 9 LEDs brighter than a single LED? 

Thing is the light coming from the extended object does not change like using more LEDs in a flashlight.

[vlaiv]

1 minute ago, globular said:

However many there are, there are still 4x as many when you double the aperture... a change bigger than the 7% JND.... so just might be noticed.

Don't forget that those rod cells also pick up light from background at the same time - regardless if object is there or not.

[globular]

Just now, vlaiv said:

Don't forget that those rod cells also pick up light from background at the same time - regardless if object is there or not.

Indeed.  But by the same argument there are now fewer that look like the background and more that look slightly different to the background.  So me might take more notice of the slight difference.