Silly Question About Telescope Magnification and Distance

[guillespiers]

Hi everyone!

What is the "apparent travel in KM" that a person does by looking through a telescope?

For example, If I were watching Jupiter with a 1500 mm focal length telescope, and a 25 mm eyepiece; how close am I getting to it in a KM scale?

Consider Jupiter to be at an average distance of 630,000,000 km from Earth.

Also, what will be the answer if I added a 3x barlow lens, but only use a camera (no eyepiece involved). How "close" am i getting to Jupiter to take that shot?

Those are a silly questions, but I´m very curious!

Thanks for your time,

Guillermo

[laudropb]

With tat fl and eyepiece you have a magnification of 60x

[guillespiers]

Hi lauropb! You are right! Buy my question is related to distance,

If 1500 FL / 25 mm = 60x magnification; this means that:

630,000,000 km / 60x = 10,500,000 km

In other words, am I 10,500,000 km closer to Jupiter?

[acey]

Hi lauropb! You are right! Buy my question is related to distance,

If 1500 FL / 25 mm = 60x magnification; this means that:

630,000,000 km / 60x = 10,500,000 km

In other words, am I 10,500,000 km closer to Jupiter?

Magnification by M makes an object look M times closer (or 1/M times actual distance, same thing).

Magnification by 60 makes it look 60 times closer.

So it looks as if you are 10,500,000km away from Jupiter (not 10,500,000km closer).

Not a silly question at all.

[guillespiers]

Magnification by M makes an object look M times closer (or 1/M times actual distance, same thing).

Magnification by 60 makes it look 60 times closer.

So it looks as if you are 10,500,000km away from Jupiter (not 10,500,000km closer).

Not a silly question at all.

Thanks a lot acey! That makes more sense

And what about using only a telescope, a barlow lens, and a camera? (no eyepiece). Asuming I use a 3x barlow, I have 4500 mm of FL. How do I get the magnification number?

[calli]

You cant tell the magnification with a camera. Because it will depend how big you print or look at it.

Carsten

[miguel87]

I have often wondered what magnification i get with my dslr on my 200p with barlow. Is there a way of figuring this out? It seems to be around 50x based on eyepiece viewing of the moon compared to the image on my dslr live view.

[acey]

Magnification means the factor by which the angular size of an object is increased. If an object subtends an angle of one arcminute when seen by eye, and 100 when seen through a telescope, then the magnification is 100. If you project the Sun through a telescope onto a piece of paper, and move the paper nearer and further to the eyepiece, the cone of light issuing from the telescope is unchanged, and so is the magnification, but the image grows and shrinks on the paper. So, as has been said, it is meaningless to speak of the magnification of a real image (e.g. photograph). The nearest thing, given a real image of a certain size, is to calculate the distance at which it should be viewed, such that its angular subtense at the eye equals what it would be when viewed directly through a telescope.

You look at a photo of Jupiter and wonder what the magnification is. Put it another way: you look at a photo of the Eiffel Tower and wonder what the magnification is. The question is then clearly meaningless.

[miguel87]

I disagree entirely.

The analogy with the sun projected onto a piece of paper; the magnification is still increasing when the paper moves away, just because the resolution is not increasing does not mean it is not magnification.

It is the same as digital zoom as opposed to optical zoom, both still increase magnification. Or using a very short focal length eyepiece beyond the resolving power of a telescope.

Digital cameras have measureable zoom lenses. Even tho the picture can be changed in size depending on how it is viewed/ printed.

With the dslr attached to the telescope, the image is still being bought to focus at a focal plane; the sensor chip of the dslr. And the telescope still has a focal length so there must be a meaningful magnification.

I understand what you mean about the magnification being 'meaningless' because the image can be enlarged or shrunk. But there is still an image formed at tge dslr chip, the same as at the focal plane of an eyepiece. These two images could be compared, therefore measured.

Mike

[miguel87]

Also if i remove my barlow from the dslr, the image halves in magnification.

This alone means it must have a measureable magnification.

[Stu]

I disagree entirely.

The analogy with the sun projected onto a piece of paper; the magnification is still increasing when the paper moves away, just because the resolution is not increasing does not mean it is not magnification.

It is the same as digital zoom as opposed to optical zoom, both still increase magnification. Or using a very short focal length eyepiece beyond the resolving power of a telescope.

Digital cameras have measureable zoom lenses. Even tho the picture can be changed in size depending on how it is viewed/ printed.

With the dslr attached to the telescope, the image is still being bought to focus at a focal plane; the sensor chip of the dslr. And the telescope still has a focal length so there must be a meaningful magnification.

I understand what you mean about the magnification being 'meaningless' because the image can be enlarged or shrunk. But there is still an image formed at tge dslr chip, the same as at the focal plane of an eyepiece. These two images could be compared, therefore measured.

Mike

Mike, I think if you ask any of the imaging guys they will agree with acey. It's ok to talk about image scale, but magnification is meaningless in imaging terms.

[ollypenrice]

Also if i remove my barlow from the dslr, the image halves in magnification.

This alone means it must have a measureable magnification.

Fact: My new car is 27% larger than my old car. Question: How large is my new car?

Clearly you have no idea. I would need to tell you the size of my old car first, so you could work it out. But wouldn't it be more sensible for me just to tell you the size of my new car?

Imagine a world in which car makers gave the size of all their cars as a percentage of the size of Olly's old car, without telling you how big that old car was. This is the problem with using 'magnification' as a term in photography.

To magnify means to make larger. But larger than what? Larger than the image you first started with. But then we'd all need to know how large that was. Yes, you can make your projected image four times larger by area by using a 2x Barlow lens but I know no more about the projected size of your old image than you know about the size of my old car.

That's why the term is effectively meaningless.

The equivalent of my sensibly telling you how big my cars are in mm would be your sensibly telling me how many mm on your chip are are covered by an arcsecond of sky. Now we are talking in absolute rather than in relative terms. We are talking about image or plate scale.

In visual observing there is no problem. A given optical train makes an object appear 100x bigger. Bigger than what? Bigger than it appeared to the naked eye. The point is that there is no photographic equivalent of that definitive initial value. There is no naked eye in a camera.

Olly

[miguel87]

Yoir central question was that to magnify makes things larger, but larger than what?

You then answered your own question at the end saying larger than with the naked eye. This is always what magnification is compared to.

The car analogy seems entirely irrelevant. Im not asking how big the moon is in km, or how big the old moon is?? Just how much bigger it appears through my scope+dslr from the same distance away compared with the human eye.

I can work out a reasonably accurate answer to my own question i suppose:

Photo the moon with my dslr on tripod only. Then photo the moon with dslr attached to telescope and barlow.

I could then measure the width in pixels of both moons, and figurr out the magnification quite precisely?

Feel free to tell me why this wouldnt work as i am by no means an astrophotographer.

[Scott]

Yoir central question was that to magnify makes things larger, but larger than what?

You then answered your own question at the end saying larger than with the naked eye. This is always what magnification is compared to.

The car analogy seems entirely irrelevant. Im not asking how big the moon is in km, or how big the old moon is?? Just how much bigger it appears through my scope+dslr from the same distance away compared with the human eye.

I can work out a reasonably accurate answer to my own question i suppose:

Photo the moon with my dslr on tripod only. Then photo the moon with dslr attached to telescope and barlow.

I could then measure the width in pixels of both moons, and figurr out the magnification quite precisely?

Feel free to tell me why this wouldnt work as i am by no means an astrophotographer.

Which lens on your dslr is naked eye?

[ollypenrice]

Yoir central question was that to magnify makes things larger, but larger than what?

You then answered your own question at the end saying larger than with the naked eye. This is always what magnification is compared to.

The car analogy seems entirely irrelevant. Im not asking how big the moon is in km, or how big the old moon is?? Just how much bigger it appears through my scope+dslr from the same distance away compared with the human eye.

I can work out a reasonably accurate answer to my own question i suppose:

Photo the moon with my dslr on tripod only. Then photo the moon with dslr attached to telescope and barlow.

I could then measure the width in pixels of both moons, and figurr out the magnification quite precisely?

Feel free to tell me why this wouldnt work as i am by no means an astrophotographer.

You can certainly do that, just as I can tell you that my new car is 27% larger than my old. (My car analogy is far from irrelevant.)

Let's say that you do the test and find that the scope and barlow magnify the lens image by 122x. What does this allow you to say? It allows you to say this (inventing a number for agrument's sake):

The image projected onto my chip by my scope and barlow is  132x larger than that projected by my 55mm camera lens.

There is nothing wrong with this, just as there is nothing wrong with me saying my new car is 27% larger than my old one. But if we are going to communicate then I need to know your initial value and you need to know mine. It is easier if we both use absolute rather than relative terms.

However, you also say before this, larger than with the naked eye. This is always what magnification is compared to.   What 'naked eye' image are you going to measure, in mm, to compare with the projected image on your chip? What exactly are you going to measure??

Olly

[acey]

Miguel87, you confuse magnification and enlargement. Magnification means the factor by which angular subtense is increased. It is only meaningful in relation to angular subtense. Enlargement means the factor by which physical dimension is increased. It is meaningful in relation to real objects.

The Moon has a diameter of approximately 3500km. I take a photograph of it and find that it has diameter 3.5cm. Then I have reduced the Moon by a factor of 100,000.

The Moon has angular diameter roughly half a degree. I place my 3.5cm photograph 1 metre away and look at it. It subtends an angle at my eye of 2 degrees. I am seeing something equivalent to the real Moon at magnification 4.

I move my photograph to a distance of 4m. It now subtends the same angle as the real naked-eye Moon, i.e. magnification 1.

I enlarge my photograph by a factor of 2, so that it has diameter 7cm. I have now reduced the Moon by a factor of 50,000. Viewed at a distance of 1 metre it looks like the Moon at magnification 2.

I could count pixels instead of cm but the idea is the same.

(Apologies if I happen to have miscalculated any numbers, I'm doing this quickly - but you get the idea I hope).

[miguel87]

It doesnt matter which lense. None of my current lenses have a magnification if 1x. But take the fical length of any camera lense and divide it by 50 to give the magnification value.

So 50mm lense would be naked eye if youre asking.

[ollypenrice]

It doesnt matter which lense. None of my current lenses have a magnification if 1x. But take the fical length of any camera lense and divide it by 50 to give the magnification value.

So 50mm lense would be naked eye if youre asking.

There may be a convention in daytime photography to regard a 50mm lens as having a magnification of 1x. If I'm not mistaken it is to do with the relative sizes of near and distant objects in the photographs they create. In such a lens near and distant objects have about the same relative size as they do with the naked eye. However, this is merely a convention which arises from one property of photographs taken with these lenses. It is, in all other ways, entirely baseless to describe a 50mm lens as having a magnification of 1x. I can only repeat, one times what?

Olly

[Scott]

is the 50mm lens in a full chip or crop chip dslr?

I'm not trying to be awkward, just pointing out there are too many variables

[miguel87]

Thanks acey, i do undertand the difference in something taking up a wider field of vision in degrees (angular something or other) and enlargement which is based on physical dimensions.

However, none of the objecs in the night sky are going to change their physical dimensions, so we are only talking about angular size.

I get that you can move a photo towards and away from your eyes but for the sake of normal conversation lets pretend we view all pictures at the same distance, or like i say, refer to pixels and eliminate the problem.

The reason i feel it is urrelevant is that there isnt a lens or eyepiece in the world that will ENLARGE objects. They all just make things appear bigger, i.e take up more of our field of view. So the conversation is only about magnification.

The barlow and telescope essentially become a crazy camera lense. And if any other camera lense can have a magnifucation calculated (according to the cannon website by dividing focal length by 50) then so can my 'lense' i just thought the maths might be different.

[Yearofthegoat]

The key bit for me is I can tell my children that my eyepieces are like spaceships.

All aboard.

[themos]

I think the confusion arises from cheap cameras being sold as having a "4x zoom lens". All that means is that the image scale varies by a factor of 4 between smallest and largest.

[themos]

They all just make things appear bigger

They do precisely the opposite. The telescope, or a lens, creates an image of an object the size of the Moon (aproximately the size of Europe). The image itself is a few millimetres wide. 

In astronomy, telescopes create much smaller images of the object under study. But, and this is crucial, they create this image very near us so we can use a magnifying glass on it.

[ollypenrice]

Thanks acey, i do undertand the difference in something taking up a wider field of vision in degrees (angular something or other) and enlargement which is based on physical dimensions.

 And if any other camera lense can have a magnifucation calculated (according to the cannon website by dividing focal length by 50) then so can my 'lense' i just thought the maths might be different.

I may not have found exactly the site you're referring to but a quick run round the internet led me to a Canon website in which I found this.

To get the binocular-like viewfinder magnification value of a lens, divide the focal length by 50.

It is important to note that they are talking about an SLR viewfinder and lens combination here and not about either a projected-onto-chip image or final image. The division by 50 simply makes the veiwfinder behave like a binocular of the same magnification. So if you use a DSLR viewfinder and a 500mm lens it will give you the same magnification as a pair of 10x binoculars, which means the apparent size of an object will be 10x larger than the naked eye view. The key thing is that this about the viewfinder image and not the final image. 

Olly

PS I don't know why the quotation copied oddly but it was a single grab from screen.

[miguel87]

So what i gather from you guys is that the 5mm lense = 1x magnification (naked eye) is not an accurate description?

Is it even roughly correct? Enough to estimate the vue through my dslr? Or is it mostly a useless figure?