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gorann

Traveling close to M42

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53 minutes ago, Stu said:

No, they would not be the same brightness. At the same magnification, the 12" would give a brighter image (not higher surface brightness than naked eye though). At the same exit pupil, the surface brightness would be the same but obviously the magnification would be less in the smaller scope.

With say a 21mm eyepiece, the f4 scope gives a 5.25mm exit pupil, the f8 scope gives 2.625mm so the brightness perceived would be lower at the same image scale.

You would need a 42mm eyepiece in the f8 scope to give the same surface brightness, but obviously the image scale would then drop.

To summarise, the f4 scope can give the same surface brightness at twice the magnification.

Let's be clear that I'm referring to surface brightness. The larger the image in the eyepiece, the easier it is to perceive, hence the attraction of large dobs. You could magnify the same image to the same degree in a 4" scope but it may not be visible as the surface brightness will have decreased too far.

I do not have a strong enough grasp of the maths to debate what happens when you hypothetically move your scope closer to the target. As said, @acey would be able to address these issues so hopefully he will comment.

All interesting and healthy debate ??

 

You are exactly right. IMHO.

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4 minutes ago, jetstream said:

How many paths has the light taken that you see as the object you are viewing? If you place your hand over the front of your scope and leave a small opening will you still see the object "as the object"? How is the information transmitted for the source through your scope to your eyes and brain? Will one photon do it?

I suggest you read Feynmans QED book to get a basis of understanding.

I'm sorry, you have lost me there. I'm familiar with the photon problem but what has that got to do with it.

Feynman is always a good read though.

cheers

gaj

 

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The past couple of pages of posts appear to have concentrated on debating the circumstances under which an object would appear brighter. Can I take this back to the point I was trying to make in my first post which is that I do not believe that there is any vantage point at which any of us could stand and see the Orion Nebula as anything resembling the beautifully dense coloured images that are produced through photography and processing.

Does anyone actually believe that the nebula would appear this way to the human eye from anywhere in the galaxy?

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6 minutes ago, DRT said:

The past couple of pages of posts appear to have concentrated on debating the circumstances under which an object would appear brighter. Can I take this back to the point I was trying to make in my first post which is that I do not believe that there is any vantage point at which any of us could stand and see the Orion Nebula as anything resembling the beautifully dense coloured images that are produced through photography and processing.

Does anyone actually believe that the nebula would appear this way to the human eye from anywhere in the galaxy?

Probably not. But it was a nice idea.

I actually like the image.

 

cheers

gaj

 

 

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2 minutes ago, gajjer said:

I actually like the image.

So do I :smile:

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50 minutes ago, gajjer said:

I think that is the source of your problem. It does gather more light. Take that to an extreme. A star is small in the eyepiece. Now move much much closer and it becomes like our own sun. ie Very bright. The angle subtended is much much larger. With a distant star you are looking at light coming from a very small solid angle. As you get closer that angle gets bigger. Hence more light. The amount of light from the original angle would be the same when viewed closer.

Ooooh. This is fun. I've not had to think like this for a long time. :-)

cheers

gaj

But we are talking about extended light sources here, NOT stars. I totally agree with you regarding stars, but for extended sources the situation is very different.

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1 hour ago, gajjer said:

Hey Stu. At least we haven't resorted to calling each other names yet.!

I just did a Wiki on surface brightness and I think that is where I dis agree.

I agree if you use a higher magnification with a scope it does not get brighter. The  amount of light available is the same. However, if you use a larger aperture then there is more light getting to the eye.

I can imagine people believing a higher magnification would make things brighter.

Also if you get closer the amount of light available to get to the eye has increased - not the same a when you magnify.

Our own sun is a clear example of this.

It is interesting to debate and I'm happy to part of the discussion.

 

cheers

gaj

Absolutely gaj, no place for name calling here, just healthy, friendly debate. It all helps build better understanding which can only be good.

Please can we agree on one thing though. Point sources and extended sources behave very differently. A point source does get brighter with aperture because by definition it doesn't have a surface area. The surface brightness of an extended object does not get brighter, but it can be enlarged to a greater degree whilst maintaining enough surface brightness to be visible.

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1 minute ago, Stu said:

But we are talking about extended light sources here, NOT stars. I totally agree with you regarding stars, but for extended sources the situation is very different.

Yes. I agree it is different for an extended light source.

We are in a galaxy but it doesn't look like one. Apparently when there are no clouds you can see the milky way. As you move away that would start to look more like a galaxy. I'm guessing that would look brighter at some point.

I guess it's an energy density thing.

Perhaps I should contact Mr Cox.

 

cheers

gaj

 

 

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If the hand is placed over the front of the scope as described, will the object still have the same brightness? Yes. But because we have effectively reduced the aperture while maintaining the same focal length the exit pupil will shorten, and if enough this is what will dim the image to our eyes/brain.

Does the eye perceive time as a camera does? with respect to photons gained by sub length etc. Time is a factor in imaging but not visual.

I now view through tree branches a lot (unfortunately) and am amazed at the images seen visually, M42,NGC 2237 etc etc- amazing how light find the paths available to present the image at the SAME brightness...I adjust my EP focal length when experimenting. Will a larger aperture gain more light to give a brighter image. No. How many photons are needed to transmit the object as a complete image?

 

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3 minutes ago, Stu said:

Absolutely gaj, no place for name calling here, just healthy, friendly debate. It all helps build better understanding which can only be good.

Please can we agree on one thing though. Point sources and extended sources behave very differently. A point source does get brighter with aperture because by definition it doesn't have a surface area. The surface brightness of an extended object does not get brighter, but it can be enlarged to a greater degree whilst maintaining enough surface brightness to be visible.

This has really made me think and question my own thoughts. As you say, it helps to better understand.

Totally agree that the point source is different. This reminds me of the question of why the sky is not white at night because of the infinite number of stars.

And I can see that there isn't the density to make that happen.

I did watch the links by the way. But the second one seemed to be talking about really close up. And this is where I have the difficulty.

At one extreme it's not bright and at the other extreme it's not bright. So that suggests that somewhere in between there is an optimum position. And are we there or is there somewhere better.

It's as simple as that really.

And thank you for an interesting discussion.

cheers

gaj

 

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12 minutes ago, jetstream said:

If the hand is placed over the front of the scope as described, will the object still have the same brightness? Yes. But because we have effectively reduced the aperture while maintaining the same focal length the exit pupil will shorten, and if enough this is what will dim the image to our eyes/brain.

Does the eye perceive time as a camera does? with respect to photons gained by sub length etc. Time is a factor in imaging but not visual.

I now view through tree branches a lot (unfortunately) and am amazed at the images seen visually, M42,NGC 2237 etc etc- amazing how light find the paths available to present the image at the SAME brightness...I adjust my EP focal length when experimenting. Will a larger aperture gain more light to give a brighter image. No. How many photons are needed to transmit the object as a complete image?

 

Sorry Jerry but I totally disagree with that. I think you will find that if you place the lens cap over the front of your telescope you will see nothing. If you obscure parts of the light path sufficient light can still reach the camera to form a complete image. After all, the Newtonian telescopes do that. The trees are only obscuring part of your image. I think you will find as the branches thicken up they will become a problem.

I have had a 6" Newt and an 8" Newt and I can assure you I got a better image with the 8" for the same exposure.

Larger aperture definitely gives more light. And regardless of the tricks photons play on getting to your camera , then at least one is required for it to register.

cheers

gaj

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Woow,

I have been processing images the whole day so I had no idea how interesting (and sometimes confusing) this discussion has evolved. It seems that most agree that point sources (stars) will get brighter while surfaces will not when we move closer. I thought of a good analogy: Professional studio photographers  use their light meters by walking up to the object and check the light close by. That would suggest that they know that light intensity would be the same when they step back to the camera. It is a bit disappointing to accept that using a camera to catch photons over a long time is not the same thing as getting close to the object with our naked eyes, so M42 will never be as grandiose as in our AP images (but it may look best at some optimal distance as someone suggested).

So, should we tell the politicians to stop funding space flight and buy telescopes to the people instead?

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30 minutes ago, Stu said:

But we are talking about extended light sources here, NOT stars. I totally agree with you regarding stars, but for extended sources the situation is very different.

Just one last comment Stu. I still don't know what the answer is but you really made me think and I did enjoy that.

Thanks buddy.

 

cheers

gaj

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3 minutes ago, gajjer said:

Just one last comment Stu. I still don't know what the answer is but you really made me think and I did enjoy that.

Thanks buddy.

 

cheers

gaj

Well I'm really glad you enjoyed the discussion. I by no means claim to be an expert with scientific knowledge of this, but try to educate myself, largely by discussion with others with greater knowledge on the forum here.

Keep thinking!! ??

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2 minutes ago, gorann said:

Woow,

I have been processing images the whole day so I had no idea how interesting (and sometimes confusing) this discussion has evolved. It seems that most agree that point sources (stars) will get brighter while surfaces will not when we move closer. I thought of a good analogy: Professional studio photographers  use their light meters by walking up to the object and check the light close by. That would suggest that they know that light intensity would be the same when they step back to the camera. It is a bit disappointing to accept that using a camera to catch photons over a long time is not the same thing as getting close to the object with our naked eyes, so M42 will never be as grandiose as in our AP images (but it may look best at some optimal distance as someone suggested).

So, should we tell the politicians to stop funding space flight and buy telescopes to the people instead?

That's it, turn up when the shows all over! Ha. Only kidding.

I think you'll find that the meter reading is being taken off the source not the object.

And wouldn't it be fantastic if we could see all the nebula etc with our naked eyes.

There would still be clouds here though!

cheers

gaj

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1 hour ago, gajjer said:

Sorry Jerry but I totally disagree with that. I think you will find that if you place the lens cap over the front of your telescope you will see nothing. If you obscure parts of the light path sufficient light can still reach the camera to form a complete image. After all, the Newtonian telescopes do that. The trees are only obscuring part of your image. I think you will find as the branches thicken up they will become a problem.

I have had a 6" Newt and an 8" Newt and I can assure you I got a better image with the 8" for the same exposure.

Larger aperture definitely gives more light. And regardless of the tricks photons play on getting to your camera , then at least one is required for it to register.

cheers

gaj

I think you missed the as described part, as in my other post.

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21 minutes ago, jetstream said:

I think you missed the as described part, as in my other post.

Ah. I think I see what you are saying. Correct me if I'm wrong. I'm not sure where the hand comes from but you are saying that the reduction in light level is compensated for by the eye.

I do imaging so I'm not that familiar with visual. I'd have to take your word for that.

However, the argument really revolves around whether an extended source gets brighter as you get closer - as recorded by a camera.

And agreed, that time has no influence on visual.

cheers

gaj

 

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3 hours ago, jetstream said:

You are exactly right. IMHO.

Stu, jetsteam - what is the difference between surface brightness and brightness of the image? Stu said the 12" will give a brighter image but not higher surface brightness than the naked eye. I am not really clear on this I guess.

No luck on taking the scope out - had to help my daughter with a last minute life or death school assignment... getting late here so I will tuck in!

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6 hours ago, gajjer said:

Ah. I think I see what you are saying. Correct me if I'm wrong. I'm not sure where the hand comes from but you are saying that the reduction in light level is compensated for by the eye.

I do imaging so I'm not that familiar with visual. I'd have to take your word for that.

However, the argument really revolves around whether an extended source gets brighter as you get closer - as recorded by a camera.

And agreed, that time has no influence on visual.

cheers

gaj

 

Well the hand is just an easy way to reduce aperture and is not of uniform shape really so it illustrates 2 points.

The dimming effect will come from the reduced exit pupil that reducing the aperture causes at the same focal length of telescope. ie my 250mm dia 1200mm fl = f4.8 vs the masked 127mm dia fl 1200 = f9.4.

If we view using a 1mm exit pupil in the 250mm f4.8, this is a 4.8mm fl eyepiece.

Using the same eyepiece in the masked 127mm (250mm masked to 5") this results in an exit pupil of 4.8/9.4= .51mm exit pupil, and this will dim the image visually for sure.

Second- I find it intriguing that reducing aperture with different shaped masks (ie the hand) still results in the image form being retained at focus.

 

I would think that it matters which extended source you are talking about as to whether it appears brighter the closer we get.... galaxies are made up of many point sources and also diffuse nebula and dust and....

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5 hours ago, beka said:

Stu, jetsteam - what is the difference between surface brightness and brightness of the image? Stu said the 12" will give a brighter image but not higher surface brightness than the naked eye. I am not really clear on this I guess.

No luck on taking the scope out - had to help my daughter with a last minute life or death school assignment... getting late here so I will tuck in!

Brightness...would an object have brightness if no one was there to observe it?...The question can be what is brightness as we perceive it through a telescope...

This link will answer some questions. Also a quick answer...if you make an extended object bigger while maintaining an eye friendly exit pupil, the extended object will appear   brighter, as Stu rightly says. There are many many factors at play here, including the sky brightness in mag/arcsec2 and how the eye/brain perceives contrast ie MTF, etc.

https://dept.astro.lsa.umich.edu/ugactivities/Labs/brightness/

This image is an avg of about 200 human eyes MTF at differing exit pupils, possibly another factor in things

 

 

eye exit pupil mtf.jpeg

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Here is some interesting info. Apparently when the 8.3 meter Subaru telescope was inaugurated in 1999 an eyepiece was put in place so that Princess Sayako of Japan could actually look through the telescope (good to be a princess). Anyway here is a link to a National Geographic article that quotes what staffers that looked through the telescope at the time said. 

http://ngm.nationalgeographic.com/print/2009/07/telescopes/ferris-text

I think getting up close to M42 must be similar - at least till till you are actually inside! 

I need to keep another browser tab open for when the bosses are nearby :-)

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8 hours ago, jetstream said:

Brightness...would an object have brightness if no one was there to observe it?...The question can be what is brightness as we perceive it through a telescope...

This link will answer some questions. Also a quick answer...if you make an extended object bigger while maintaining an eye friendly exit pupil, the extended object will appear   brighter, as Stu rightly says. There are many many factors at play here, including the sky brightness in mag/arcsec2 and how the eye/brain perceives contrast ie MTF, etc.

https://dept.astro.lsa.umich.edu/ugactivities/Labs/brightness/

This image is an avg of about 200 human eyes MTF at differing exit pupils, possibly another factor in things

 

 

eye exit pupil mtf.jpeg

Hi jetstream,

I took a look at the link and the distinction is now clear.  However the conditions described with the surface brightness not changing with distance does not apply to very distant objects. Looking again at the formula for image or plate scale, we see that it has no provision for the distance of the object. Thus the size of the image of Orion Nebula  on our retina or at the image plane of the telescope will be almost the same if we are 1500 or 500 light years from it. On the other hand the amount of light gathered will be nine fold -  the image size will not scale in proportion with distance as illustrated in the link, therefor it will "appear brighter" in the same way that it appears brighter in the 12" with the same focal length as a 6".  

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Interesting link Beka. I would be interested to hear your comments on the links I posted previously, in particular the mathematical proof regarding the face that the surface brightness seen in telescopes is no brighter than with the naked eye. Please do let me know your thoughts, this is not my info but from someone who knows!

Back to your link, let's look at this a little more carefully. First off, the Subaru scope is at an altitude of 4139m. This puts it above the most dense parts of the atmosphere, and in a position few of us ever get to experience.

In addition, it is on Mauna Kea in Hawai with very little/no light pollution and at around 19 degrees North. This means M42 gets to 65 degrees altitude, vs 33 degrees for me for instance. These facts in themselves hugely reduce the atmospheric extinction seen, so basically M42 will look pretty amazing in any scope!

Next, let's move onto the scope itself. This is an 8.3m reflector, with an effective aperture of 8.2m and a focal ratio of 1.83 according to Wiki i.e. 15m.

I have no idea what sort of eyepiece you might use in a scope like this, but let's assume for the purposes of this exercise that it's a 3" ES 100 degree afov 30mm. I suspect in reality it may have been longer focal length than that, but we can try that too.

For a 30mm eyepiece in a 15000mm focal length scope, the magnification is x500, and the exit pupil is 16.39mm with a field of view of 0.2 degrees.

If instead we had a hypothetical 60mm eyepiece, we would have x250 magnification, a 0.4 degree field of view, and a somewhat oversized exit pupil of 32.78mm.

Now, unless you are a bush baby, or perhaps a bizarre alien, I hope you will agree that with this sort of scope, your maximum 7mm pupil just cannot use the light from the scope. In terms of area, a 7mm pupil has an area of 153.9mm2, a 32.78mm exit pupil has an area of 3375.7mm2, so you are only using 4.56% of the area of the 8.2m mirror, somewhat inefficient I think you will agree? If you do the maths, this reduces your 8.2m scope to around 1.75m, or 70" approx.

If you genuinely want to use all the light from the scope efficiently, then you need to limit yourself to a 7mm exit pupil (or less depending on your age, but let's stick to 7mm). This means using a 12.81mm eyepiece, let's call it 12.5mm. Fine, that sounds normal, let's use a Docter 12.5mm 84 degree afov as I have one of them. Put it in my 8.2m, 15000mm focal length scope and I get x1200 magnification and a 0.07 degree afov.

This totally confirms my point about larger dobs giving the image scale whilst maintaining image (surface) brightness. You effectively get the image brightness associated with a large exit pupil, under the best sky in the world, at altitude and looking up not so far from the zenith, but at crazy high mag. No wonder the views look good!

Do you see now?

Here are the comparable fields of view between your 8.2m scope and 12" f4 scope, both at 7mm exit pupil, one at x1200, one at x42. It the Subaru, you are looking at a tiny portion of the nebula, with maintained brightness, so it's no surprise the colour and detail were stunning, that's what aperture gives you.

image.png

image.png

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34 minutes ago, beka said:

Hi jetstream,

I took a look at the link and the distinction is now clear.  However the conditions described with the surface brightness not changing with distance does not apply to very distant objects. Looking again at the formula for image or plate scale, we see that it has no provision for the distance of the object. Thus the size of the image of Orion Nebula  on our retina or at the image plane of the telescope will be almost the same if we are 1500 or 500 light years from it. On the other hand the amount of light gathered will be nine fold -  the image size will not scale in proportion with distance as illustrated in the link, therefor it will "appear brighter" in the same way that it appears brighter in the 12" with the same focal length as a 6".  

The image scale must scale with distance. If I move closer to an object then it appears bigger to me. If I move from 1500 ly away from something to 500 ly away, it will appear bigger. The effect is obvious with other extended objects like planets. Mars for instance varies hugely in apparent size depending upon the relative orbital positions of Earth and Mars.

The object appears bigger, so it's light is more spread out. The telescope's apparent field of view will remain unchanged so you will be looking at a relatively smaller area, the object surface brightness will not increase.

One variable is whether you are in a position where the object fills the field of view or not. Before you reach that point, I agree that it will 'appear' to get brighter because it will appear larger in the fov. After that point my I would expect it to remain the same, the area you are looking at becomes smaller, the object brightness gets larger as it gets closer but the surface brightness remains the same.

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Can I just add that this has been one of the most enjoyable and informative threads I've taken part in for quite some time. Great stuff ??

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