Author Topic: Image size based on aperture  (Read 568 times)

Danny Cruz

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Image size based on aperture
« on: December 27, 2017, 11:03:23 PM »
I have seen several posts where the poster states that image size is somehow related to aperture and I don't understand this. Let me share my understanding and you tell me where I have gone wrong.I have 2 telescopes, an 80mm/400 mm FLrefractor and an 203mm/1200 mm FLNewtonian

I am using a 10 mm Orion Plossl eyepiece with 52 degree AFOV for this discussion.1200 mm scope with 10 mm eyepiece = 1200/10 = 120X OK? 52/120 = .43 degree field of view. Good?

400 mm scope, 3X barlow - Effective FL now 1200 mm, right? 120X, .43 degree FOV. Yes? Same?So, at 120x shouldn't I expect the same size image in the eyepiece regardless of aperture? Aperture does not come into play in these calculations.

Granted an 80 mm scope and a 203 mm scope may differ in brightness but assuming that the image is acceptable, say we are looking at the full moon or Saturn or Jupiter, shouldn't the image size be the same based on magnification?I would go out and test it but we have had a week of clouds. And frankly my observation would be somewhat subjective. But I see no reason to say that one scope will have a smaller image than the other based on aperture.



longtichaten

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Re: Image size based on aperture
« Reply #1 on: December 29, 2017, 06:07:08 AM »
Same magnification, same apparent angular size. However the linear size of the "enlightened disk" (a.k.a. exit pupil) is different, almost 2mm in the case of the 20cm, 0.6mm in the case of the 8cm. Therefore the former will look "brighter" than the latter, and your brain may be mislead to judge larger even the angular size of your target (the fact that the former may look more detailed due to the larger resolving power can help the illusion too...).

mingchepspatu

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Re: Image size based on aperture
« Reply #2 on: January 01, 2018, 08:07:27 AM »
Quote
I have seen several posts where the poster states that image size is somehow related to aperture and I don't understand this. Let me share my understanding and you tell me where I have gone wrong.

I think you are right on track..

The image size is related to the magnification which depends only on the focal length of the eyepiece and the telescope. It is independent of aperture.

It could be that the author was referring to the size of the exit pupil as Hesiod suggested or it could be that the author was referring to the size of the Airy disk...

Do you have a link to a thread or website where you have seen this.

Jon

Justin Prasad

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Re: Image size based on aperture
« Reply #3 on: January 03, 2018, 01:29:35 PM »
Since aperture is the foundation of resolution perhaps the point is that image size (magnification), once that limit of resolution is reached, is useless; rather like the infamous 600x 50mm.

Nathan Roberts

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Re: Image size based on aperture
« Reply #4 on: January 08, 2018, 08:19:12 AM »
Quote
<p class="citation">Quote

I have seen several posts where the poster states that image size is somehow related to aperture and I don't understand this. Let me share my understanding and you tell me where I have gone wrong.

I think you are right on track..

The image size is related to the magnification which depends only on the focal length of the eyepiece and the telescope. It is independent of aperture.

It could be that the author was referring to the size of the exit pupil as Hesiod suggested or it could be that the author was referring to the size of the Airy disk...

Do you have a link to a thread or website where you have seen this.

Jon[/quote]

I am not looking to call out or challenge an individual, I am looking to validate my understanding. Certainly there are limits to magnification based on how much light is gathered but at the same mag I should expect the same size image.

If my understanding is correct than that is all I need. I am only 6 months into this and it is/was possible that I did not understand how things work.

Jeff Weiss

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Re: Image size based on aperture
« Reply #5 on: January 09, 2018, 05:59:30 AM »
Lets say you have eps that give you a .5 degree fov in both 80mm and 200mm scopes (a full moon).

You have a .5 degree 80mm picture and a .5 degree 200mm picture. They both are .5 or 1/2 degree of arc, one being a bigger (larger aperture) picture.

Good viewing,

Dave

lorndwatassi

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Re: Image size based on aperture
« Reply #6 on: January 10, 2018, 06:16:26 PM »
Image Size = Focal Length X Object Size/Lens to object distance.

Yes, in astronomy the object distance and object size are very large, but dividing them gives you the angular size. So that leaves the focal length to determine how large the image will be. An 80mm and 203mm will give you the same image size if they both have 1500mm focal lengths.

noerivatat

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Re: Image size based on aperture
« Reply #7 on: January 10, 2018, 07:03:18 PM »
Quote
I was not looking to call out or challenge anyone. Rather I thought that if I saw what had been written, then I could better understand what you had read, it is possible you had misunderstood what had been written.

Jon

bijstentetal

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Re: Image size based on aperture
« Reply #8 on: January 10, 2018, 11:17:44 PM »
This has come up in several posts. Statements related to a 6" scope would provide larger images than a 5" scope. That an 80 mm scope would be constrained by small size of image. These made no sense to me as I thought size is related to mag, not aperture.I doubted my understanding of how things work.

It is also possible that I may be taking the comments out of context, but I don't think so.There is a lot of jargon in this hobby, like any hobby.

For example, we talk about fast scopes and slow scopes. It seems to me that thatis jargon that seems to have been carried over from photography.It may have some applicability toAP but I have not seen any real meaning to these terms when it comes to visual observation. But perhaps I don't understand. How is an F10 fast and an F5 slow? What does that mean to a visual observer? An F5 and an F10 scope both gather light. Where is the speed reference applicable?

In cameras this related to how fast an exposure time I could set based on the speed of the lens. But my eye doesn't work that way and I don't think too many people are still using film for amateur astronomy. Does fast and slow have any practical meaning anymore when it comes to telescopes?So, as I struggle with learning about this huge field of astronomyI try to validate what I think I know with discussion threads like this one. Is this a jargon issue or is the poster who isincorrect in their information. I turn to all of you for help.

James Scaturro

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Re: Image size based on aperture
« Reply #9 on: January 11, 2018, 11:38:11 PM »
Fast and slow are photography terms. Fast scopes will form the same image on film faster than slow scopes. For telescopes, it means an image of the same size will be brighter in a fast scope than a short scope. If you compare an 8" F/4 scope to an 8" F/8 scope, the image size will be smaller. But the image is made from the same amount of light, therefore it will be smaller but have higher surface brightness. You will be able to tell the object from the background better in a slow scope vs a fast scope.

sandsibyno

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Re: Image size based on aperture
« Reply #10 on: January 12, 2018, 12:03:03 PM »
It may be clearer if you consider 2 scopes with the same focal length. Compare an 8" 1800mm scope to a 16" 1800mm scope. They will have the same image size, but the second one has much more light coming to the image. The 16" is thefaster scope, since it has 1/2 the f-ratio.

(Correction, I originally said the 16" was the slower scope)

olexecin

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Re: Image size based on aperture
« Reply #11 on: January 13, 2018, 03:58:52 AM »
Quote
It may be clearer if you consider 2 scopes with the same focal length. Compare an 8" 1800mm scope to a 16" 1800mm scope. They will have the same image size, but the second one has much more light coming to the image. The 16" is the slower scope, since it has 1/2 the f-ratio.


OK, now I am REALLY confused.

If the 16" scope has more light coming to the image would it not logically be the faster scope?Can you see how this would be confusing to a newbie?

Not that the slow/fast term makes any sense to mein this context. I understand F ratios but saying that the scope that has more light coming to the image is the slower scope makes no sense. In photography the one that brings in more light would be the faster lens as it would allow a shorter exposure time using the same speed film. There the trade-off was speed vs. depth of field.

No matter, I ignore these terms as they just seem to be old terms that have no practical meaning.

Today we talk about cars having automatic and manual transmissions yet some people still call the manual transmission a standard transmission because a LONG time ago that was the standard transmission that came with most cars. Today the automatic would be the standard transmission.

Hey Grandpa, I just bought a new car.
Really? Does it have a standard transmission?
Yes!
How many gears?
5
I didn't know you could drive a clutch.
What's a clutch?
You said your new car had 5 gears.
Sure. P,R,N,D, L - 5
Kids, I can't understand anything they say.
Poor Grandpa, he is staring to lose it.

Charlie Collins

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Re: Image size based on aperture
« Reply #12 on: January 13, 2018, 06:53:37 AM »
Quote
I have seen several posts where the poster states that image size is somehow related to aperture and I don't understand this. Let me share my understanding and you tell me where I have gone wrong.I have 2 telescopes, an 80mm/400 mm FLrefractor and an 203mm/1200 mm FLNewtonian

I am using a 10 mm Orion Plossl eyepiece with 52 degree AFOV for this discussion.1200 mm scope with 10 mm eyepiece = 1200/10 = 120X OK? 52/120 = .43 degree field of view. Good?

400 mm scope, 3X barlow - Effective FL now 1200 mm, right? 120X, .43 degree FOV. Yes? Same?So, at 120x shouldn't I expect the same size image in the eyepiece regardless of aperture? Aperture does not come into play in these calculations.

Granted an 80 mm scope and a 203 mm scope may differ in brightness but assuming that the image is acceptable, say we are looking at the full moon or Saturn or Jupiter, shouldn't the image size be the same based on magnification?I would go out and test it but we have had a week of clouds. And frankly my observation would be somewhat subjective. But I see no reason to say that one scope will have a smaller image than the other based on aperture.

Without reading all the other posts yet, a good "guide" for magnification without getting useless magnification is to use 30x per inch of a scope. Of course the quality of the optics and seeing come into play. Some great APO's can go to 40+x per inch.

So for me, I have an 80mm ED f/600mm that I can go a little higher, say 120x and still have a good image on a good night. My 8" f/2000 I use the 30x/in. rule that tops me out at 240x....but seeing in my area usually keeps me under 200x 25x/in.

David Johnson

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Re: Image size based on aperture
« Reply #13 on: January 13, 2018, 09:36:45 PM »
Quote
Quote

It may be clearer if you consider 2 scopes with the same focal length. Compare an 8" 1800mm scope to a 16" 1800mm scope. They will have the same image size, but the second one has much more light coming to the image. The 16" is the slower scope, since it has 1/2 the f-ratio.


OK, now I am REALLY confused.

If the 16" scope has more light coming to the image would it not logically be the faster scope?Can you see how this would be confusing to a newbie?

Not that the slow/fast term makes any sense to mein this context. I understand F ratios but saying that the scope that has more light coming to the image is the slower scope makes no sense. In photography the one that brings in more light would be the faster lens as it would allow a shorter exposure time using the same speed film. There the trade-off was speed vs. depth of field.

No matter, I ignore these terms as they just seem to be old terms that have no practical meaning.

Today we talk about cars having automatic and manual transmissions yet some people still call the manual transmission a standard transmission because a LONG time ago that was the standard transmission that came with most cars. Today the automatic would be the standard transmission.

He Grandpa, I just bought a new car.
Really? Does it have a standard transmission?
Yes!
How many gears?
Drive, Low, Reverse, Neutral and Park - 5
Kids, I can't understand anything they say.
Yes, the 16" is the faster scope. Mixed myself up typing.

Mario Evans

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Re: Image size based on aperture
« Reply #14 on: January 15, 2018, 02:12:05 PM »
Quote
Quote

Quote

It may be clearer if you consider 2 scopes with the same focal length. Compare an 8" 1800mm scope to a 16" 1800mm scope. They will have the same image size, but the second one has much more light coming to the image. The 16" is the slower scope, since it has 1/2 the f-ratio.


OK, now I am REALLY confused.

If the 16" scope has more light coming to the image would it not logically be the faster scope?Can you see how this would be confusing to a newbie?

Not that the slow/fast term makes any sense to mein this context. I understand F ratios but saying that the scope that has more light coming to the image is the slower scope makes no sense. In photography the one that brings in more light would be the faster lens as it would allow a shorter exposure time using the same speed film. There the trade-off was speed vs. depth of field.

No matter, I ignore these terms as they just seem to be old terms that have no practical meaning.

Today we talk about cars having automatic and manual transmissions yet some people still call the manual transmission a standard transmission because a LONG time ago that was the standard transmission that came with most cars. Today the automatic would be the standard transmission.

He Grandpa, I just bought a new car.
Really? Does it have a standard transmission?
Yes!
How many gears?
Drive, Low, Reverse, Neutral and Park - 5
Kids, I can't understand anything they say.
Yes, the 16" is the faster scope. Mixed myself up typing.

We have answered the original question so we can stop here. Now we are debating jargon.Looking at your statement,we say it is faster based on the FR,not the amount of light that is coming in. Does that make sense, speed wise?

If you had a second 16" scope with half the FL, it would bring in the same amount of light based on the 16" mirror, right? But the FR would be half.

So, why would one be faster than the other?They are both delivering the same amount of light to the eyepiece based on the size of the mirror.

How does fast and slow even apply here?No need to debate it. Just something to think about. I can be a real PIA sometimes.