How does diffraction affect telescope magnification?

[Emilyjoint]

Homework Statement

Vague class discussion. Stars are points and a telescope does not magnify them.
How then can more stars be seen with a telescope?

Homework Equations

I know magnification for a telescope is fo/fe but that is not much help

The Attempt at a Solution

Is it because diffraction makes bigger images?

 

[turbo]

Here's a hint. How does a telescope differ significantly from your naked eye?

 

[Emilyjoint]

The main thing about a telescope is that it magnifies.
That is what I think fo/ fe means

 

[turbo]

You are missing an important quality of telescopes.

 

[Emilyjoint]

I know that they cause diffraction which makes point sources larger. Is that what you mean.
Ps... I have looked on wiki for telescope magnification and could not find an explanation

 

[turbo]

I know that they cause diffraction which makes point sources larger. Is that what you mean.

Nope. You're over-thinking this. Go to the "telescope" thread and see if you can figure it out. I can't give you the answer (forum roles), but I can show you the way to good resources.

https://www.physicsforums.com/showthread.php?t=391086

 

[Emilyjoint]

I have just spent half an hour or so looking through that reference. It gives lots of advice about which telescope to buy...I already own a telescope (Meade)...but there is nothing to help me explain how I can see more stars !
"I am missing an important quality of telescopes"..."I am over-thinking this"... I don't get it... is there a straight forward answer ?
Do you have any other references that might help.
Is my diffraction idea wrong?

 

[turbo]

What are the critical physical qualities of the telescopes in that thread?

 

[Emilyjoint]

?
Thank you for your help

 

[turbo]

Maybe this would help.

http://www.telescope.com/Telescope-Buyers-Guide/p/99897.uts

 

[Emilyjoint]

I have a telescope and am not looking for advice about which telescope to buy.
I am no wiser about how it is that more stars can be seen through a telescope.
My first and only idea was that diffraction make bigger images and I will stick with that in our class discussion.

 

[turbo]

Don't do that emily, because it is wrong. Which telescopes will show you more stars than others and why?

 

[Emilyjoint]

I think I am correct when I say that diffraction makes stars look bigger. We have done calculations to show what is meant by 'resolution' which is to do with separating images.
I also know the equation for calculating angular width to find the size of a point source.
I do not know how it is wrong that this explains how stars can be seen !
If they look bigger because of diffraction it sounds right to me.

 

[turbo]

Again, what qualities would you want in a telescope to show you more and more (fainter and fainter) stars?

 

[Emilyjoint]

they should have bigger diffraction patterns... I don't know... that is why I came here... for help.

 

[turbo]

they should have bigger diffraction patterns... I don't know... that is why I came here... for help.

That's not the answer. I'm trying to throw resources at you, but I can't simply give you the answer. Forum rules, sorry.

 

[Emilyjoint]

But am I correct in saying that the diffraction pattern will be larger ?
If so I can use that in my discussion, something will come of it
I do not understand your 'rules'

 

[turbo]

Do you think huge professional telescopes have large diffraction patterns, and that's why they can show more stars?

 

[Emilyjoint]

you tell me ! It sounds reasonable to me !...if it works

 

[Jimmy Snyder]

Emily, you already know that the telescope doesn't make the star larger. What else does a telescope do. Something that's different from what your eyes do.

 

[Evo]

you tell me ! It sounds reasonable to me !...if it works

We don't give out answers here. We will guide you, but you have to do the work.

 

[Emilyjoint]

I am lost ! I think that the telescope does make the image of the star larger...isn't that what the diffraction pattern is.
That is the only thing that I can think of that makes it possible to see more stars.
I know that I have to do the work...I realize that no one here is going to give me an answer... I do not need to be told that ! is this a physics forum ?

 

[Emilyjoint]

no one has been able to give a better explanation than mine... am I wrong?

 

[micromass]

Why is it better to have large telescopes??

 

[Jimmy Snyder]

Emily, is there a book to go with your class? If so, does it have a discussion on how a telescope works? It does not make stars appear larger. At least let that soak in.

 

[Emilyjoint]

The textbooks I refer to are by Muncaster(my favourite) Nelkon (a bible) and Duncan.
All explain optics and telescopes in great detail.
I have no difficulty with that theory. I am beginning to feel a little stupid...am I missing something so obvious.
How is it that more stars can be seen through a telescope?

 

[micromass]

Again: why might I prefer a larger telescope to a smaller one?

 

[Jimmy Snyder]

It might help to consider that a telescope and an eye work in ways that are similar. What are some of the differences though? What happens to your eyes when you go from a bright room to a dark one?

 

[Emilyjoint]

My original question was "how/why can more stars be seen with a telescope", not why should I have a larger or smaller telescope !
I am still in the position that I can see more stars because diffraction makes the images larger, this is undoubtedly correct and I will use this argument in my presentation and see what reaction I get.
Thankyou foryour help

 

[micromass]

My original question was "how/why can more stars be seen with a telescope", not why should I have a larger or smaller telescope !

The thing is that your eyes ARE basically a very small telescope. A usual telescope is usually much larger than the human eye. Why is that you think??

 

[Emilyjoint]

"my eye is a small telecope"...thank you for your help.

 

[micromass]

OK, a final attempt, expanding on Jimmy's hint:

If I stand in a bright room and I turn off the light, then I can't see anything. However, a little while after, I can see quite well in the dark. What happened??

 

[micromass]

Let's say I'm in a dry african country and I want water. Suddenly it starts raining. What is the best strategy to catch a lot of water??

1) Standing outside, with your mouth open and pointed up to the sky.
2) Putting a bucket outside.
3) Building a large pool to catch water.

 

[Emilyjoint]

How does that help me in this problem/ discussion.
33 responses and I am no better off...not your fault...if you cannot help/give an answer just accept it and say so.
It is only a discussion question...at least I have some things to talk about.

 

[Jimmy Snyder]

Q: Stars are points and a telescope does not magnify them. How then can more stars be seen with a telescope?
A: It magnifies them.

This doesn't pass the giggle test. Are you absolutely certain that's what you want to go into the classroom with? I recommend you reread micromass's post #32 and tell us what you think about it.

 

[Jimmy Snyder]

if you cannot help/give an answer just accept it and say so.

It has already been said that we cannot give an answer and it's starting to appear that in your case, that means we cannot help.

 

[Emilyjoint]

I have read post 32 and am no wiser.
Where else can help be found for physics problems?
How Is it that more stars can be seen though a telescope?
I am afraid that the contributions here have been of little help.
It must be my fault

 

[turbo]

Emily, this is my last post here. You can find the relevant qualities that allow telescopes to see more stars with perhaps a 30-second Google search. Maybe a minute or so if your search criteria are looser. We have helped, hinted, and tried to steer you as much as possible. I have been an amateur astronomer for ~50 years now and have more books on astronomy than your local library does, probably. We cannot give out answers. You have to dig and show your work. When you have made progress, then we can help more, but not before.

 

[Fredrik]

Where else can help be found for physics problems?

There's no better place than here.

How Is it that more stars can be seen though a telescope?
I am afraid that the contributions here have been of little help.
It must be my fault

It is your fault actually. People have been giving you hints that are very close to a complete answer, and you just get irritated and ignore them every time. Things would have been different if you had tried to answer their questions instead of just suggesting that they shouldn't have asked them.

The reason why more stars can be seen through a telescope than through your eyes, is the same as the reason why more stars can be seen through a "good" telescope than through a "bad" telescope. This is why a web page that helps people choose what telescope to buy can tell you something that answers your question.

Suppose that you and a friend are looking at stars through your own two telescopes, and you discover that there's a star that can be seen clearly through your friend's telescope, but not yours. What do you think is the difference between the light that reaches your location from that star, and the light that reaches your location from a star that can be seen clearly in both telescopes? If you could change one property of your telescope (i.e. if you could replace it with one that's very similar, but different in one important way), then what property would you choose to change if you really want to be able to see that star without having to borrow your friend's telescope?

By the way, I don't know anything about telescopes. I just read this thread and saw what people are trying to tell you. It has nothing to do with diffraction. It's much simpler than that.

Edit: Just to clarify, I'm not saying that telescopes have nothing to do with diffraction. I just meant that that the specific detail that people have been trying to tell you about in this thread has nothing to do with diffraction.

 

[vela]

I am lost ! I think that the telescope does make the image of the star larger...isn't that what the diffraction pattern is.
That is the only thing that I can think of that makes it possible to see more stars.

Diffraction does make the image larger but not in a useful way. If the telescope could focus perfectly, the image of a point source would be a point; however, because of diffraction, actual telescopes smear the light from a point source into a smudge. In other words, because of diffraction, the image from a telescope will always be slightly out of focus. You've probably seen a movie or a picture that was out of focus. The image of an object is larger in an out-of-focus picture than when the picture is in focus, but the larger size doesn't help you see the object more clearly. In fact, an object is often harder to see because its image overlaps with the image of other things in the picture.

Another reason diffraction doesn't help is that it takes the light from a source entering the telescope and spreads it out, so the smudge will be fainter than the (unattainable) single point. So again, diffraction works to make an object harder to see.

What you want to find out is what's so important about a telescope's aperture. Try reading up about that.

 

[Emilyjoint]

Thanks Vela...I got it now.
I was confused about size of lens (aperture) and diffraction and brightness...thank you again

 

[davenn]

and in her last post she answers the question without realising it !

Emily, if you read this thread again, look at what you wrote and take out the one word -- diffraction
what are the other 2 words left ? and how does one affect the other ?

cheers

Dave