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We know that larger optics cannot produce an increase in surface brightness and I remember being told off, years ago, for suggesting that they did! OK, so let's agree that large optics cannot increase surface brightness but only the surface area of the object's image at its original brightness.

But, technicalities aside, what of it? Red supergiants have high absolute magnitudes despite low surface brightness because they have large surfaces.  Subjectively, we call Betelgeuse a bright star despite its low surface brightness because we can easily see it. It is subjectively bright enough to see. Since seeing is, by definition, a subjective experience I don't think we should be unduly influenced by the term 'surface brightness.' We certainly shouldn't be deterred from buying large scopes because of it.

I'm minded of this after reading of Stu's good fortune in this thread: 

 

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While above is true - it does not explain why do we see more with larger scopes for same exit pupil - even if surface brightness is the same.

Take two scopes of same F/ratio - one larger and one smaller aperture and use same eyepiece with both - you will find that you can see some objects in larger scope and not see them in smaller scope.

Same eyepiece in both scopes will yield same exit pupil and hence same surface brightness (for both sky and target - contrast ratio remains the same) - we should be able to see same things in both scopes.

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49 minutes ago, vlaiv said:

While above is true - it does not explain why do we see more with larger scopes for same exit pupil - even if surface brightness is the same.

Take two scopes of same F/ratio - one larger and one smaller aperture and use same eyepiece with both - you will find that you can see some objects in larger scope and not see them in smaller scope.

Same eyepiece in both scopes will yield same exit pupil and hence same surface brightness (for both sky and target - contrast ratio remains the same) - we should be able to see same things in both scopes.

Image scale is the answer I would give to that Vlad. At the same exit pupil, the larger scope gives a larger image, and the eye finds it easier to perceive the contrast. That’s my understanding anyway.

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2 minutes ago, Stu said:

Image scale is the answer I would give to that Vlad. At the same exit pupil, the larger scope gives a larger image, and the eye finds it easier to perceive the contrast. That’s my understanding anyway.

It's not going to be that much larger image, at least I think.

Take for example two popular scopes

F/6 8" and F/6 80mm refractor. Aside mirror reflectivity and central obstruction - let's imagine both are "refractor like".

Difference in object size for same exit pupil will be what - x2.5?

I'm sure that we can find two galaxies (now that galaxy season is upon us) that have same surface brightness - but one is x2.5 times larger than other.

If large galaxy can be seen in 8" but can't be seen in 80mm - that would suggest that smaller galaxy can't be seen in 8". I find that hard to believe.

Also - it is very easy to move galaxy so that about half of it (or rather 60% of it) is outside of the fov - that leaves only 1/2.5 of its size visible in 8" - according to size thing - it should disappear.

 

 

 

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13 minutes ago, vlaiv said:

It's not going to be that much larger image, at least I think.

Take for example two popular scopes

F/6 8" and F/6 80mm refractor. Aside mirror reflectivity and central obstruction - let's imagine both are "refractor like".

Difference in object size for same exit pupil will be what - x2.5?

I'm sure that we can find two galaxies (now that galaxy season is upon us) that have same surface brightness - but one is x2.5 times larger than other.

If large galaxy can be seen in 8" but can't be seen in 80mm - that would suggest that smaller galaxy can't be seen in 8". I find that hard to believe.

Also - it is very easy to move galaxy so that about half of it (or rather 60% of it) is outside of the fov - that leaves only 1/2.5 of its size visible in 8" - according to size thing - it should disappear.

 

 

 

Perhaps, but there are far cleverer people than me who suggest this is the reason 👍

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On the other hand….. this may be of interest. It discusses the concept of threshold contrast. Not heard of that before but a Google search on object visibility turned this up.

https://www.cloudynights.com/ubbthreads/attachments/5526435-threshold forCAA.pdf

Mel Bartels is the guru on this I believe, plenty of info and a calculator here, which seems to back up the image scale theory:

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

 

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16 minutes ago, Stu said:

On the other hand….. this may be of interest. It discusses the concept of threshold contrast. Not heard of that before but a Google search on object visibility turned this up.

https://www.cloudynights.com/ubbthreads/attachments/5526435-threshold forCAA.pdf

Interesting read - but here is important sentence:

Quote

Admittedly, I don’t know why threshold contrast is reduced in larger apertures. I suspect
this has something to do with the larger light packet delivered to the eye. Perhaps, this moves the
eye’s performance closer to that of daytime vision. But that’s just a hunch. I’m still exploring
that question...but not even close to throwing up my hands in despair over the lack—for the
moment—of an answer.

Since we don't understand why there is threshold contrast and why is it reduced in larger apertures - we did not answer anything really.

And the answer might be this:

CSF.jpg

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Just now, vlaiv said:

Interesting read - but here is important sentence:

Since we don't understand why there is threshold contrast and why is it reduced in larger apertures - we did not answer anything really.

And the answer might be this:

CSF.jpg

I think you are saying image scale is the answer, but it’s a bit cryptic still for my pea sized brain…

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16 minutes ago, Stu said:

I think you are saying image scale is the answer, but it’s a bit cryptic still for my pea sized brain…

Indeed - but I can't really understand how they are related either.

It's not just simple size of things, I think that how contrast varies with space that is important.

image.png.3e9550734eb2045ef383794baa2e997b.png

Here is a crop from above image - we see some variations in contrast in the middle of the image - but we start to loose it to the left and to the right.

So it's related to size - but too much magnification hurts as well according to this - or at least if we equate feature size with wavelength in image.

I'm not sure that is how it works - I think it has to do with size of transition rather than size of feature.

How big is transition between light and dark area versus how big is actual object.

This means that visibility of object will depend on how "soft" its edges are.

To confirm this - there is another image on the same page I found previous one:

CSF-square-seen-unseen.png

Here instead of soft change - we have sudden change - so "change zone" does not depend on the size of feature - it is steep for thin as well as thick lines. Yellow line marks what should be seen - but we can clearly see contrast change past yellow line in this case.

 

 

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On 23/03/2022 at 10:35, vlaiv said:

Also - it is very easy to move galaxy so that about half of it (or rather 60% of it) is outside of the fov - that leaves only 1/2.5 of its size visible in 8" - according to size thing - it should disappear.

I like this one :grin:

Also it is a complete myth that some objects will not show brighter in large aperture, espc fast telescopes, IMHO.

I think the effects are multifaceted, some of which might not be understood and I do believe that the eye/brain plays a role in this. M42 is much brighter in the 15" than the TSA120 at the same exit pupil and the 24" is much brighter than them both on it.Oddly enough it is the already "bright" objects that appear to be greatly enhanced by aperture eventhough there are more threshold objects to be seen in the larger apertures.

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6 minutes ago, jetstream said:

@vlaiv any more thoughts, conclusions or questions on this subject?

I just love the fact that letting an object drift out of the FOV doesnt make it dimmer then disappear.

Question…..would it be any different drifting INTO the field of view? ie is it a perception thing??

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9 minutes ago, Stu said:

Question…..would it be any different drifting INTO the field of view? ie is it a perception thing??

Ive never seen an object drift into view getting larger as it goes and then get brighter. If this worked a threshold object should "just appear" as it gets larger. I have no idea about how this all works but I firmly think that the eye and brain play a role.

I also dont understand how an ortho will best a more complex eyepiece when transmissions are very close- we are told that even 5% difference in transmission should not be noticeable and the differences between orthos and most widefields are smaller than this.

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19 minutes ago, jetstream said:

@vlaiv any more thoughts, conclusions or questions on this subject?

I just love the fact that letting an object drift out of the FOV doesnt make it dimmer then disappear.

I don't have any more questions - but I am very interested in your observation that objects appear brighter in larger aperture telescope for the same exit pupil.

When you say brighter - do you mean brighter than surrounding sky or brighter in general?

Does the contrast somehow increase in large aperture scope or do both sky and target seem to be brighter in larger aperture scope (say M42 for example)?

Brightness on is own is very hard to asses - it's like saying that one sound was louder than the other - without comparing them side by side. Very difficult to tell for small intensity changes but very obvious on for very large changes (say whispering and shouting - you can't really miss that one is louder than the other - even if you don't hear them side by side).

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16 minutes ago, vlaiv said:

When you say brighter - do you mean brighter than surrounding sky or brighter in general?

I'm like a bat :D when I'm dark adapted, the sky, through the eyepiece is not black, its grey and I see mottled structure in places that I believe could be galactic dust, very faint nebs etc. For reference we'll say my sky is 21.6 mag or darker- which it is most of the time.

 

16 minutes ago, vlaiv said:

When you say brighter - do you mean brighter than surrounding sky or brighter in general?

This depends on the object but my eyes "see the background" as a constant once adapted and under stable conditions. So for the objects- we'll use the Veil to start. A bunch of things happen here with large aperture, one of them is pulling out features because the mag at the exit pupil pulls them out, invisible in smaller scopes. And then existing features are seen as "brighter", not just bigger.

All this is in reference to viewing the object itself and not looking at the edges (sky). However to pull out very faint nebula I do use the sky edge to notice a different shade or shape.

16 minutes ago, vlaiv said:

Does the contrast somehow increase in large aperture scope or do both sky and target seem to be brighter in larger aperture scope (say M42 for example)?

The sky looks the same to me, once adapted using any telescope.

Edited by jetstream
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