Why do we need bigger scopes?

[vlaiv]

This is not a click bait, but genuine question (although, it just occurred to me that including "this is not click bait" in title is guaranteed to be in fact click bait ).

Surface brightness depends on exit pupil, right?

I mean, take 8" F/6 scope and 4" F/6 scope and our regular John/Jane Doe galaxy with uniform surface brightness - like M33 or perhaps Fireworks?

If we choose EPs that in both scopes provide same exit pupil - we will in fact make views in both scopes equally bright.

8" has x4 larger aperture, and twice as long FL (if both are F/6 scopes), so using say Baader Morpheus 17.5mm in both scopes will make same exit pupil around 3mm.

Now, as 8" gathers x4 more photons - but due to twice as long FL - it will have twice as high magnification so galaxy will be double in size or x4 in surface. x4 more photons over x4 larger surface simply means same number of photons hitting same surface in our eye. We should see things the same (only bigger in bigger scope).

Yet, field experience says that we can easily see some faint fuzzies in 8" but struggle to see them in 4". How come?

[Ricochet]

2 minutes ago, vlaiv said:

Yet, field experience says that we can easily see some faint fuzzies in 8" but struggle to see them in 4". How come?

You've answered it in your question.

3 minutes ago, vlaiv said:

it will have twice as high magnification so galaxy will be double in size

Bigger things are easier to see. I can't give a direct quote/link but the idea that the brain picks out bigger things better than smaller things is a constant idea in texts on observational astronomy.

[vlaiv]

2 minutes ago, Ricochet said:

You've answered it in your question.

Bigger things are easier to see. I can't give a direct quote/link but the idea that the brain picks out bigger things better than smaller things is a constant idea in texts on observational astronomy.

If you mean this:

That is quite large range of sizes for small difference in contrast visibility. Take for example "Seen section" - there are at least 6-7 waves that are essentially the same in terms of contrast visibility - and largest is easily twice as large as smallest in spatial extent.

To me, above graph alone does not explain what is going on ...

[Knighty2112]

Not a scientific answer, but I think of the light as data, so with the larger scope your scooping up more data which means more detail seen. Smaller scope so less data, so less brightness. Thats my contribution anyway!  

Edited October 5, 2021 by Knighty2112

[Nik271]

My answer is that even though the light receptors in our eyes receive the same number of photon per unit for both scopes, there are 4 times more of them getting this stimulation with the bigger scope and our brain can easier register the object. It probably needs to hit a certain threshold at each light intensity, and once this is reached increasing the size of the image does not matter.

 

PS. Just another thought: this is defintitely connected with how the brain processes the image. If it was detecting the galaxies on a sensor, I believe that both scopes will do equally well in the same given time (except the larger scope will show smaller detail of course). The brain is probably not so efficient as a computer and just needs the bigger image to recognize it from noise and the background.

Edited October 5, 2021 by Nik271

[PeterW]

Yak my both eyes helps the brain average noise in the visual system for better  contrast… not directly linked, but another thing to consider. For pint sources (stars) the faintness you pick up purely depends on aperture as they are not extended sources, so aperture wins. Otherwise it’s the size/contrast thingy noted above. Filters can help improve contrast as well.

People like big… draws in them crowds…

peter

[vlaiv]

I'm not sure that I buy into that explanation for size.

That would mean that two galaxies of the same surface brightness would be seen differently - or rather one seen and other not, depending on their size?

There is easy test that we can do on that one - take EP that has smaller field stop so as to avoid vignetting issues - maybe Ortho with small AFOV will do. Take any galaxy that is close to threshold - just barely distinguished and observe it while it is whole in FOV vs - being only 1/4 of it in fov - a bit like this:

[JeremyS]

For me the answer is “we don’t “.

I’ve used scopes regularly in the 8 to 14 inch class over many many years. Yet my recent  purchases have been in the range 2.4 to a bit less than 5 inch (Tak FC 76DCU, TSA 120, FC 100 DZ and FOA 60Q in that order). And I’m enjoying my visual observing more than ever 👍🏻

[vlaiv]

4 minutes ago, JeremyS said:

For me the answer is “we don’t “.

I’ve used scopes regularly in the 8 to 14 inch class over many many years. Yet my recent  purchases have been in the range 2.4 to a bit less than 5 inch (Tak FC 76DCU, TSA 120, FC 100 DZ and FOA 60Q in that order). And I’m enjoying my visual observing more than ever 👍🏻

I hear what you say, but although I phrased question like that - what I really want to know is how come we see more in bigger scopes.

Yes, point sources are easy to explain. Planetary detail is easy to explain.

What I'm failing to explain is surface brightness.

Few years ago I had rather satisfying session with 8" scope next to 4" scope at dark(ish) location. There was clear distinction between views provided by two scopes - for example 8" scope showed all "members" of Markarian's chain, but 4" scope showed only 3 brightest ones. Regardless of magnification used. Scopes in question were 8" F/6 dob and 4" F/5 short frac.

[pjsmith_6198]

In addition to collecting 4x more light and magnification, the 8" has greater resolution so you can see more details.   On the other hand,  the 4" scope has a greater field of view and is much easier to carry around.  

 

Phil

 

[cloudsweeper]

For fuzzies, you need surface brightness, which means you need exit pupil.  But that alone is not enough - magnification is also needed, so that the overall integrated brightness is greater.  Now, for a given exit pupil, mag is proportional to aperture.  So - bigger aperture, more mag (for given exit pupil), more integrated brightness, brighter fuzzy.

This topic comes up from time to time, and can cause considerable divergence of opinion.  But this is my twopenn'orth.  😉

Doug.

[jetstream]

I know there is a link between object detection and magnification but we still need enough eye illumination. There is a massive difference between my 15" and 24" that I hope some members can explain. They are f4.8 and 4.7 (PCII) and Ive checked every possible combination using the Veil as a test subject and the 24" is just plain old brighter, with a PCII, without, using same eyepieces, filters, no filters, same exit pupil, same mag etc etc Mirrors made by same guy, same coatings and very similar specs.

[jetstream]

3 minutes ago, cloudsweeper said:

But that alone is not enough - magnification is also needed, so that the overall integrated brightness is greater. 

This implies increased mag increases object "brightness" .

[vlaiv]

2 minutes ago, cloudsweeper said:

For fuzzies, you need surface brightness, which means you need exit pupil.  But that alone is not enough - magnification is also needed, so that the overall integrated brightness is greater.  Now, for a given exit pupil, mag is proportional to aperture.  So - bigger aperture, more mag (for given exit pupil), more integrated brightness, brighter fuzzy.

This topic comes up from time to time, and can cause considerable divergence of opinion.  But this is my twopenn'orth.  😉

Doug.

Not sure what you mean by integrated brightness?

If you are referring to integrated brightness of the object - well that does not change. As surface brightness goes down with magnification and area increases - they cancel each other in the end. That is sort of obvious - for a given scope, we can only gather so much light from the object - changing magnification and exit pupil can't change that.

On the other hand - total amount of light gathered for any given object is proportional to aperture surface, and indeed 8" scope does collect more light than 4" scope, but look at my argument above about surface brightness and size - take either two different sized galaxies of same surface brightness or same galaxy placed so that only portion of it is visible in the FOV.

It will not change visibility of object - yet it is analog to using larger scope and smaller scope on same exit pupil - but these two differ in what they show.

[pjsmith_6198]

I think it's a combination of greater gathering, greater resolution and higher magnification

 

Phil

[jetstream]

Just now, pjsmith_6198 said:

of greater gathering

I've argued this in the past and have been shot down in flames

[vlaiv]

1 minute ago, pjsmith_6198 said:

I think it's a combination of greater gathering, greater resolution and higher magnification

 

Phil

Could you elaborate?

Resolution here is rather inconsequential, I think as we are talking about medium magnifications and extended faint objects - not planetary detail.

[JeremyS]

5 minutes ago, vlaiv said:

Not sure what you mean by integrated brightness?

If you are referring to integrated brightness of the object - well that does not change. As surface brightness goes down with magnification and area increases - they cancel each other in the end. That is sort of obvious - for a given scope, we can only gather so much light from the object - changing magnification and exit pupil can't change that.

On the other hand - total amount of light gathered for any given object is proportional to aperture surface, and indeed 8" scope does collect more light than 4" scope, but look at my argument above about surface brightness and size - take either two different sized galaxies of same surface brightness or same galaxy placed so that only portion of it is visible in the FOV.

It will not change visibility of object - yet it is analog to using larger scope and smaller scope on same exit pupil - but these two differ in what they show.

And don’t forget magnification. Roger N Clark has described how higher mags are needed for detecting extended faint sources like DSOs: https://clarkvision.com/visastro/omva1/index.html

 

 

[jetstream]

Just now, JeremyS said:

And don’t forget magnification. Roger N Clark has described how higher mags are needed for detecting extended faint sources like DSOs: https://clarkvision.com/visastro/omva1/index.html

 

 

Reviewed this lots starting many years ago- Clark and Blackwell.

Lets use Pickerings Triangle- the 24" not only shows it brighter but with more bright nebulosity appearing compared to my 15", tested side by side for years. The difference is huge.

My refractors show a "dim" image compared to these, same exit pupil, different mag obviously in this case. My refractors have better optical quality than my mirrors eventhough the mirrors are vg.

[mikeDnight]

17 minutes ago, jetstream said:

I've argued this in the past and have been shot down in flames

I believe in you. I bet you feel so much better now! 😀

[vlaiv]

11 minutes ago, JeremyS said:

And don’t forget magnification. Roger N Clark has described how higher mags are needed for detecting extended faint sources like DSOs: https://clarkvision.com/visastro/omva1/index.html

 

 

Although I agree with some of the points, I find it hard to follow when I see graph that is explained as : "This diagram shows that, for a given background (e.g. the night sky), less contrast is needed to see a larger object." yet graph shows brightness down to mag30 per arc second squared (contains data down to mag27) - when night sky is never darker than mag22 - which is natural brightness of dark sky.

Further, if magnification is the issue, given certain surface brightness vs background brightness in say 8" scope at certain exit pupil when object is seen and same exit pupil in 4" when object is not seen. We can conclude that it is down to magnification as contrast ratio and actual surface brightness of both sky and object is the same.

We can then increase magnification in 4" scope - two fold and that should keep contrast ratio the same because both target and sky will drop in brightness by same amount - and if we did not push target out of zone of perception (enough photons received) - we should now detect object in 4" scope as well, right?

Does that happen? I don't think so.

[JeremyS]

5 minutes ago, vlaiv said:

Although I agree with some of the points, I find it hard to follow when I see graph that is explained as : "This diagram shows that, for a given background (e.g. the night sky), less contrast is needed to see a larger object." yet graph shows brightness down to mag30 per arc second squared (contains data down to mag27) - when night sky is never darker than mag22 - which is natural brightness of dark sky.

Further, if magnification is the issue, given certain surface brightness vs background brightness in say 8" scope at certain exit pupil when object is seen and same exit pupil in 4" when object is not seen. We can conclude that it is down to magnification as contrast ratio and actual surface brightness of both sky and object is the same.

We can then increase magnification in 4" scope - two fold and that should keep contrast ratio the same because both target and sky will drop in brightness by same amount - and if we did not push target out of zone of perception (enough photons received) - we should now detect object in 4" scope as well, right?

Does that happen? I don't think so.

You’ll have to read his book Vlaiv. Long out of print. I’ve a copy in my library. It’s a classic.

[cloudsweeper]

23 minutes ago, jetstream said:

This implies increased mag increases object "brightness" .

Yes - but only at the same surface brightness (which is determined by exit pupil).  So - we have a given brightness per unit area (SB), then if the image is larger (magnified more), the brightness per unit area when integrated over a larger area will give more overall brightness.  

Some also say that a larger image with a given SB is perceived as being brighter.

Doug.

[pjsmith_6198]

26 minutes ago, vlaiv said:

Could you elaborate?

Resolution here is rather inconsequential, I think as we are talking about medium magnifications and extended faint objects - not planetary detail.

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

[vlaiv]

Well, I think that we need to experiment

There is rather nice way to conduct this sort of experiment - one just needs zoom eyepiece and variable aperture mask for their telescope. That way we can vary different parameters - like magnification and aperture size and see how visibility of object changes with these?