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gorann

Traveling close to M42

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48 minutes ago, gorann said:

all the pretty nebula that we image from here would slowly fade away and we would just see stars when we get there....

That is exactly the point I was trying (and failing) to make :wink:

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Yes Derek,

Thank you all for a good discussion that helped me think about it and cleared this up, even if the conclusion is a bit sad: Nebulosity looks best at a distance and will fade away when you get close. I will try not to think about it when I see Sci-Fi movies.....

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11 hours ago, gorann said:

This is getting interesting,

Stu, I think you nailed it. If you get 100 times closer, i.e. if you get to 13 light years from M42 (which is now at 1300 light years), the nebula would be 100 times wider, and the nebula area would be 100 x 100 = 10000 times larger, which is the same as the increase in light intensity, so the surface would be as bright as seen from here, which is very faint with the naked eye (=we cannot see it). On the other hand, the trapezium stars would also be 10000 times brighter, and since we can see them well from here, they would be extremely bright (better bring your sunnies on this trip!). So, I think I was right when I suggested that the trapezium would be so dominating that all faint stuff would be invisible. Conclusion: we can see all the wavelengths of light in the image I posted, but we would be blinded by the stars and not see any nebulosity. This suggest that we are better off at a distance. So sadly, all the pretty nebula that we image from here would slowly fade away and we would just see stars when we get there....

That reasoning is not quite accurate. Even if it is not a point source like a star each point on the surface radiates photons in all directions. As you get closer you will capture more of these per unit area, from each point. The nebula will thus appear brighter as you  get closer. You are thinking like an image from a telescope in which the same number of photons are spread over a larger area when the magnification increases. Regarding it's visibility from inside,  it would depend on a lot of factors - density of the parts, your exact location, 3D structure etc. Without a detailed analysis it would be hard to say either way. Although this is an interesting debate,  I am not sure this is the place for it :-)

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9 minutes ago, beka said:

That reasoning is not quite accurate. Even if it is not a point source like a star each point on the surface radiates photons in all directions. As you get closer you will capture more of these per unit area, from each point. The nebula will thus appear brighter as you  get closer. You are thinking like an image from a telescope in which the same number of photons are spread over a larger area when the magnification increases. Regarding it's visibility from inside,  it would depend on a lot of factors - density of the parts, your exact location, 3D structure etc. Without a detailed analysis it would be hard to say either way. Although this is an interesting debate,  I am not sure this is the place for it :-)

It is definitely a complex subject, but the basic premise that the nebula does not get brighter as you get closer is correct I'm afraid.

Some interesting info here.

http://www.universetoday.com/99989/in-reality-nebulae-offer-no-place-for-spaceships-to-hide/

http://blogs.discovermagazine.com/badastronomy/2008/04/26/what-does-a-nebula-look-like-up-close/#.VrcY_EXfWJI

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I'm not sure that is the complete picture. The links talk about being up close; inside the nebula. I would agree they would not be brighter. However, if you go far enough away they would also be dim simply because photons would not arriving at the observer very often.

Somewhere in between there must be a sweet spot. So I'm guessing as you get closer more receptors are receiving light and it would appear brighter. At some point it would be spread over a larger area and not get brighter. Of course I might be talking tosh.

Nurse, nurse, I've been left alone again!

cheers

gaj

 

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1 hour ago, Stu said:

It is definitely a complex subject, but the basic premise that the nebula does not get brighter as you get closer is correct I'm afraid.

Some interesting info here.

http://www.universetoday.com/99989/in-reality-nebulae-offer-no-place-for-spaceships-to-hide/

http://blogs.discovermagazine.com/badastronomy/2008/04/26/what-does-a-nebula-look-like-up-close/#.VrcY_EXfWJI

Well Stu, despite the links I remain unconvinced, and would agree with gajjer. A larger scope makes the image brighter because it gathers light over a larger area corresponding to more flux. Closer the same telescope would gather more flux or more of the emitting objects luminosity. Inside the nebula would be a different story but even then we might see something in the same manner that we see the milky way from inside the galaxy.

... anyway I still like the image without the stars :-)

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5 minutes ago, beka said:

Well Stu, despite the links I remain unconvinced, and would agree with gajjer. A larger scope makes the image brighter because it gathers light over a larger area corresponding to more flux. Closer the same telescope would gather more flux or more of the emitting objects luminosity. Inside the nebula would be a different story but even then we might see something in the same manner that we see the milky way from inside the galaxy.

... anyway I still like the image without the stars :-)

Not true again ?. A telescope does not make objects brighter, it simply allows them to be magnified whilst maintaining the same brightness. A larger scope gives you more image scale whilst maintaining the brightness, making the object easier for you eye to detect. It may seem counter intuitive but it is correct.

As another example, M31 is approaching us (albeit slowly, well quickly but it has along way to go!) As it gets closer, it will appear physically larger and larger in the sky, but the overall brightness remains constant so the surface brightness will also remain the same.

If acey is around, I'm sure he could explain this all far better than me!

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Stu. I can't agree with you there. The light in a solid angle from the distant object will remain the same, so by increasing the diameter of a telescope you are increasing the solid angle by the square of the diameter. The light in each unit solid angle is not increased but the solid angle viewed by the telescope is bigger. That's why people go for big Dobs isn't it?

 

cheers

gaj

 

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16 minutes ago, gajjer said:

Stu. I can't agree with you there. The light in a solid angle from the distant object will remain the same, so by increasing the diameter of a telescope you are increasing the solid angle by the square of the diameter. The light in each unit solid angle is not increased but the solid angle viewed by the telescope is bigger. That's why people go for big Dobs isn't it?

 

cheers

gaj

 

No, it's not. They go for big dobs to allow them to get increased image scale whilst maintaining brightness. A telescope does not increase the brightness seen. I will try to find some info to support this.

BTW, can you clarify your term 'solid angle' so we make sure we are talking in the same terms?

Note that this all applies to extended objects, not point sources, they behave differently.

Cheers

EDIT

Try these:

http://www.rocketmime.com/astronomy/Telescope/SurfaceBrightness.html

http://www.skyandtelescope.com/astronomy-resources/astronomy-questions-answers/telescope-never-increases-an-objects-surface-brightness/

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Hi Stu

here is a definition taken from Wiki       " In geometry, a solid angle (symbol: Ω) is the two-dimensional angle in three-dimensional space that an object subtends at a point."

What I am saying is that you are gathering a larger solid angle with a big scope. You are gathering more light. That is without doubt. Each element of that solid angle has the same magnitude as it did before. Those elements are not brighter.

I believe that p-resenting more light to the eye would make it appear brighter.

 

cheers

gaj

 

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30 minutes ago, gajjer said:

Hi Stu

here is a definition taken from Wiki       " In geometry, a solid angle (symbol: Ω) is the two-dimensional angle in three-dimensional space that an object subtends at a point."

What I am saying is that you are gathering a larger solid angle with a big scope. You are gathering more light. That is without doubt. Each element of that solid angle has the same magnitude as it did before. Those elements are not brighter.

I believe that p-resenting more light to the eye would make it appear brighter.

 

cheers

gaj

 

Thanks for that. Did you check my links above?

We may be saying the same thing. The surface brightness cannot increase, but the object appears larger so is easier for your eye to perceive. So long as you are agreeing that surface brightness cannot be increased by the scope then we are probably basically along the same lines.

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Yes. I think that's right. As you get closer more receptors get included so there is am apparent increase in brightness. Problem comes when it's a surface of light and as you get closer you will not be including more receptors but you will be seeing less of the source. So the perceived brightness will go down. Somewhere in between being inside it and an infinite distance from it will be a maximum. I suspect we a quite a way from being at the optimum point for M42.

Interesting thinking about it though! My brain needs a rest now.

 

cheers

gaj

 

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2 hours ago, Stu said:

Not true again ?. A telescope does not make objects brighter, it simply allows them to be magnified whilst maintaining the same brightness. A larger scope gives you more image scale whilst maintaining the brightness, making the object easier for you eye to detect. It may seem counter intuitive but it is correct.

As another example, M31 is approaching us (albeit slowly, well quickly but it has along way to go!) As it gets closer, it will appear physically larger and larger in the sky, but the overall brightness remains constant so the surface brightness will also remain the same.

If acey is around, I'm sure he could explain this all far better than me!

Then Stu, 12" f4 and 6" f8 focal lengths are both 48" so same magnification. Would the image brightness be the same? If the Same telescope got closer by half the distance you will gather four times more light. The image scale depends only on the focal length of the telescope (206265 / focal length(mm) giving arcsec/mm) at light year object distances (effectively infinity) and will not increase proportionally to the amount of light gathered - hence we would get a brighter image.Getting closer by half the distance is equivalent to doubling the aperture (logically from similar triangles). 

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Are we forgetting that the heart of the nebula, at least, is easily viewable with the naked eye?

If the dust clouds around the trapezium are visible from earth, so they could only become more impressive as you get closer, even if the fainter parts would remain invisible.

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20 hours ago, DRT said:

So your eyes are capable of taking 46 x 3 minute exposures (equivalent to a 2hr 18min unblinking stare), stacking them one on top of the other and then allowing your brain to process them for hours before deciding what you just seen?

That's impressive :wink:

I think I will just retire back to the observation section and look at grey fuzzy things with my inferior eyes :lol:

Even if we stack 46 and average them they don't get any brighter, the result will only be as bright as one image.

Suppose this is a futile argument as no is going close to this nebula, or any other in the near future.

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20 minutes ago, beka said:

Then Stu, 12" f4 and 6" f8 focal lengths are both 48" so same magnification. Would the image brightness be the same? If the Same telescope got closer by half the distance you will gather four times more light. The image scale depends only on the focal length of the telescope (206265 / focal length(mm) giving arcsec/mm) at light year object distances (effectively infinity) and will not increase proportionally to the amount of light gathered - hence we would get a brighter image.Getting closer by half the distance is equivalent to doubling the aperture (logically from similar triangles). 

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 ??

 

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25 minutes ago, Stub Mandrel said:

Are we forgetting that the heart of the nebula, at least, is easily viewable with the naked eye?

If the dust clouds around the trapezium are visible from earth, so they could only become more impressive as you get closer, even if the fainter parts would remain invisible.

True, and that's not in debate. The view would become more impressive in that the object would appear much larger, but as said (probably too many times now! ;) ), the surface brightness will not increase because the area increases as you get closer.

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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

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36 minutes ago, beka said:

Then Stu, 12" f4 and 6" f8 focal lengths are both 48" so same magnification. Would the image brightness be the same? If the Same telescope got closer by half the distance you will gather four times more light. The image scale depends only on the focal length of the telescope (206265 / focal length(mm) giving arcsec/mm) at light year object distances (effectively infinity) and will not increase proportionally to the amount of light gathered - hence we would get a brighter image.Getting closer by half the distance is equivalent to doubling the aperture (logically from similar triangles). 

Another thought. If the scope gets closer by half, you do not gather 4 times as much light because. Assuming the object filled the eyepiece in both instances, and the fov of the scope remains the same, the light from the object is now spread out far more so the overall brightness would be the same, not four times higher.

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13 minutes ago, Stu said:

Another thought. If the scope gets closer by half, you do not gather 4 times as much light because. Assuming the object filled the eyepiece in both instances, and the fov of the scope remains the same, the light from the object is now spread out far more so the overall brightness would be the same, not four times higher.

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

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9 minutes ago, Stu said:

Another thought. If the scope gets closer by half, you do not gather 4 times as much light because. Assuming the object filled the eyepiece in both instances, and the fov of the scope remains the same, the light from the object is now spread out far more so the overall brightness would be the same, not four times higher.

My argument is that the image will not be proportionally spread out as the relationship between object distance and image scale in not linear. I am not really sure what you mean by surface brightness but a brighter image to me means more light per unit area of image as in the larger diameter telescope with the same focal length as one with a smaller diameter.

Its been an interesting discussion but I think I am starting to lose my weekend stargazers lounge high... tomorrow is Monday - how is that for a downer? Sky is clear so I will try to gather some energy to lug the CPC 1100 out, but I will be sure to stay away from M42 :-)

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1 hour ago, beka said:

Then Stu, 12" f4 and 6" f8 focal lengths are both 48" so same magnification. Would the image brightness be the same? If the Same telescope got closer by half the distance you will gather four times more light. The image scale depends only on the focal length of the telescope (206265 / focal length(mm) giving arcsec/mm) at light year object distances (effectively infinity) and will not increase proportionally to the amount of light gathered - hence we would get a brighter image.Getting closer by half the distance is equivalent to doubling the aperture (logically from similar triangles). 

Extended objects surface brightness stays the same, but what are you comparing it to? Our sky? What we see is the difference between our sky brightness and the surface brightness of the extended object.

Given the fact that this surface brightness stays the same, the only way to increase it's perceived contrast is to make it bigger in the eyepiece, assuming its not large enough already. I think you may be forgetting the role that the eye/brain plays in how we see things and how are brain interprets different exit pupils in terms of contrast and acuity.

There is much information out there if your interested. Blackwell is a good place to start.

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20 minutes ago, gajjer said:

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

Here is something else to keep you going during the day: What does being "close" mean in relation to something the size of a human and an amorphous mass of gas which is 24 light years from one side to the other? :eek:

Edited by DRT

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15 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

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.

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Just now, DRT said:

Here is something else to keep you going during the day: What does being "close" mean in relation to amorphous mass of gas which is 24 light years from one side to the other? :eek:

I think that's what the links were about. If you get really close they don't get brighter. Don't have a problem with that. I can see that a diffuse gas would not get brighter. A bit like getting closer to the TV.

But there would be an optimum distance.

cheers

gaj

 

 

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