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JOC

With a little experience you start to understand things a little - or do you?

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I find I can read all the theory I like, but it's only when I start to do something that I start to develop some understanding of really what's happening.

So here are my current thought processes - how far out am I please. 

So I've been playing with different EP's - these provide the apparent changes in magnification that you see in the objects.  The OTA itself has its own focal length and provides a degree of initial magnification - that's what makes it possible for me to take a closer picture of the moon with my SLR body that the magnification my own eyes see.  A different OTA will provide different apparent magnification, but it will still be the addition of an EP that provides the rest.

The size of the mirror affects the light gathering ability of the telescope - the bigger the mirror the higher the amount of light gathered.  This quality means that a wispy nebula should appear clearer/brighter in a larger mirror because more light has been gathered.

So as far as I can see the size of the mirror doesn't have so much effect on the apparent magnification - If two telescopes are say 1200mm long and one has an 8" mirror in it and one has a 12" mirror in it the 12" one will show the wispies' brighter, but with the same EP's will be apparent magnification of the objects be significantly different?  i.e. if I wanted to see a planetary object 'bigger' then buying a telescope with a larger Mirror width isn't necessary going to do this - it will still depend on the size of the EP being used - yes, no or maybe..................???

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For the same focal length of mirror, you will get the same mag, but the bigger mirror will give more brightness, and show more small, faint objects.  And sky darkness is a limit to this improvement.

Doug.

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10 minutes ago, JOC said:

I find I can read all the theory I like, but it's only when I start to do something that I start to develop some understanding of really what's happening.

So here are my current thought processes - how far out am I please. 

So I've been playing with different EP's - these provide the apparent changes in magnification that you see in the objects.  The OTA itself has its own focal length and provides a degree of initial magnification - that's what makes it possible for me to take a closer picture of the moon with my SLR body that the magnification my own eyes see.  A different OTA will provide different apparent magnification, but it will still be the addition of an EP that provides the rest.

The size of the mirror affects the light gathering ability of the telescope - the bigger the mirror the higher the amount of light gathered.  This quality means that a wispy nebula should appear clearer/brighter in a larger mirror because more light has been gathered.

So as far as I can see the size of the mirror doesn't have so much effect on the apparent magnification - If two telescopes are say 1200mm long and one has an 8" mirror in it and one has a 12" mirror in it the 12" one will show the wispies' brighter, but with the same EP's will be apparent magnification of the objects be significantly different?  i.e. if I wanted to see a planetary object 'bigger' then buying a telescope with a larger Mirror width isn't necessary going to do this - it will still depend on the size of the EP being used - yes, no or maybe..................???

My employer is very big on the 70:20:10 'learning from experience, then peers, then formal raining'. They may have point (yet be convinced - formally :) )  

From the interweb:

Xunzi:

不闻不若闻之,闻之不若见之,见之不若知之,知之不若行之;学至于行之而止矣。

A rough translation: "Not hearing is not as good as hearing, hearing is not as good as seeing, seeing is not as good as knowing, knowing is not as good as acting; true learning continues until it is put into action."

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Spot on. Magnification is OTA focal length divided by eyepiece focal length...regardless of aperture. 

However....you need aperture to be able to get detail when magnified. Usually they say that max mag is 2x aperture in mm. So if you have 150mm aperture, max mag is 300x.

Hope this helps. 

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My brain is the same - I can read or be told how to do something...but it doesn't 'click' until I actually physically start doing it and trying to put into practice what I've been soaking up.

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2 minutes ago, davyludo said:

My brain is the same - I can read or be told how to do something...but it doesn't 'click' until I actually physically start doing it and trying to put into practice what I've been soaking up.

Dead right!  Reading and studying are important, but getting behind the eyepiece really teaches you stuff.

Doug.

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Another aspect of "brightness" is contrast. The way the eye operates good contrast makes the image appear brighter. Which I suspect is where the perpetual idea that a 6" refractor is equivalent to an 8" reflector comes from.

It is not equivalent but the contrast created by the different optics give the "brighter" interpretation to the eye/brain combination. This is why a good 8" mirror will be said to give a brighter image then a not so good 8" mirror.

When talking of viewing it is common that the eye and it's operation is overlooked, although it seems slightly relevant.

I find that you need the information in order to understand what it is you are experiencing. Even if for some time you cannot fit the information in, it is likely at some time that something will come along to make it become applicable. Night vision is a good one for this, it is chemical as well as just a bigger pupil.

Edited by ronin
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2 hours ago, davyludo said:

My brain is the same - I can read or be told how to do something...but it doesn't 'click' until I actually physically start doing it and trying to put into practice what I've been soaking up

Yup, that's just it.  I am quite certain that I've been told what I wrote above several times and probably read it on other people's threads, but the practical application of looking and changing EP's has shown me what I've been told in action and that has had the effect of cementing the information for me.  For the person in the street buying a telescope they probably don't realise this - most people would counter-intuitively think that the 'bigger' the telescope the 'bigger the image', but it suddenly struck me when looking at Jupiter two nights ago, that the image I was seeing probably wouldn't be significantly 'bigger' within the EP even if I'd purchased the 10" or 12" version of my own telescope - the only difference possible would be any increased apparent magnification from the length of the OTA itself - I'd still be putting the same EP in it (which OK would effectively magnify that increased OTA focal length - perhaps exponentially? -  but it would still be the same EP!).  That seemed quite an game-changer in my thought processes for me.

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I think EPs which perform well with shorter/faster local ratios can be significantly more expensive, with the longer focal ratios favouring cheaper EPs.

I have two different scopes - an 8.5" f/7.6 reflector and a 102mm f/5 refractor - and one set of EPs. None of which were particularly expensive, but all suggested to perform well (enough?) at my focal ratios. I guess therefore you may see distortions or aberrations in a faster scope that don't present in a slower scope?

Is another rule of thumb the wider the field, the lower the contrast?

 

Here's a quicky on the physical calculations.

epcalcs.png

[Edit: I imagine the internal diameter of the field stop of the EP affects exit pupil, too]

 

I've not spent enough time at the eyepiece to learn the differences in feel from my set, but looking forward to it.

Edited by furrysocks2
field stop
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I've used wide, ultra wide and hyper wide eyepieces extensively and compared them with eyepieces with a more regular field of view. I have not noticed that the wider angle eyepieces have reduced contrast on either deep sky objects or the moon and planets over their narrower field equivalents.

Sometimes using an eyepiece with a wider field of view can improve contrast because you can get the same true field at a higher magnification which helps to darken the background sky making dso's stand out a little more.

I do try and keep the largest exit pupil created to 6mm or less which I think suits my 50+ year old eye and moderately light polluted back garden better.

I don't pretend to understand the maths and physics behind all this though - I've just looked to try as many combinations of eyepieces and scopes on as wide variety of objects as possible over the years and have learned which combinations produce results that are satisfying :icon_biggrin:

Maybe there are little imps inside the eyepieces creating the images and more imps in widefield eyepieces ? :grin:

Edited by John
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50 minutes ago, John said:

....I don't pretend to understand the maths and physics behind all this though ....

Just to clarify, what I mean by this is that I don't understand all of it. I wasn't implying that anyone else is pretending. I'm sure that some folks do have a very thorough understanding of what lies behind what we see at the eyepiece :smiley:

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You were, however, IMPlying something else.

:icon_biggrin:

P.S. Too subtle? "IMP", get it?

Edited by iPeace
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I found the fov calculator on the following website helped me:

http://astronomy.tools

I've been thinking about a new scope and have been using it to compare views with my current eyepieces on various telescopes. I was then really sad and made a spreadsheet with the magnification and field of view so that I could compare different scopes...I'm just that cool! 

I know there's a lot more to it than mag and fov, but it helped me get a feel for it. I kind of struggle with degrees, arc minutes and seconds and can't always work out what mag I want to view something. I guess experience and a fair amount of trial and error helps this. 

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20 minutes ago, davyludo said:

 

I was then really sad and made a spreadsheet with the magnification and field of view so that I could compare different scopes...I'm just that cool! 

 

Not sad at all!  I have made tables showing EP focal length, mag, and TFOV for each 'scope.  They are I reckon vital "in the field" so you know exactly what you're doing and seeing.

(I've even added further details of the EPs - construction, AFOV, cost.  Very handy for general reference.)

Doug.

 

 

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8 hours ago, JOC said:

 the 10" or 12" version of my own telescope - the only difference possible would be any increased apparent magnification from the length of the OTA itself - I'd still be putting the same EP in it (which OK would effectively magnify that increased OTA focal length - perhaps exponentially? -  but it would still be the same EP!).  That seemed quite an game-changer in my thought processes for me.

M = (mirror focal length) / (EP FL), so for constant (EP FL), M is just proportional to (mirror FL) rather than an exponential relationship.  But if the bigger 'scope has the same (mirror FL), the mag will thus still be the same, for the same EP.

Doug.

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one thing, that helped me realize what is really happening was using a Nikon camera in telescope's prime focus. All that theory that I had read suddenly clicked into understanding.
Using the camera in prime focus, I realized that the telescope produces some kind of image, which is static basically. All changes to this image happen inside the eyepiece. With that eyepiece removed, you get to see what the telescope actually produces (but you cannot do that with your own eyes).
Then I realized, that all the magnification is just the eyepiece "looking" at a smaller part of this static image produced by a telescope. The bigger the mag, the smaller part of the original image is viewed. I imagine it somewhat like this:

The telescope collects light according to the surface area of the lens/mirror. The bigger the mirror/lens, the more photons are falling on it's surface because of bigger surface area. Image from mirror/lens gets projected somewhere to be actually used by humans - this is the focal point, point where the eyepiece rests. Now, the eyepiece decides if it will project the entire image to your eye (low mag EP), or if it will use and project only a small part of the image (high mag).

My brain understands it something like that. Very similar situation to understand would be this: telescope projects an image on a piece of paper, this image cannot be magnified or changed, it is still the same, only other telescope will provide other image. Using the eyepiece, you view only a small part of the paper and magnify it.

With bigger aperture scope, you collect more photons, so the produced image is brighter. And you can go to bigger magnifications without loosing too much contrast because of more light collected (pi * r  squared).

That's why it eventually starts loosing brightness and - accordingly - contrast and detail with big enough magnification.

What I still don't practically understand is how those widefield EPs work compared to normal EPs.

Edited by kilix

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The reality is that going bigger in terms of aperture almost inevitable means going longer in terms of focal length and thus magnification. This is for a number of reasons including difficulty of manufacture, physical restraints, physics etc. One benefit of this though is that it keeps exit pupils sizes acceptable.

Imagine a hypothetical 16" (400mm) dob with a 600mm focal length. First up it would be f1.5, crazy fast, very hard to make and extremely (impossibly?) hard on eyepieces.

Physically it would be hard (actually impossible) to make as the secondary would be huge and the focal point would be just on the edge of the tube, focussing would be impossible without eating into the aperture I imagine.

The other issue is exit pupil. Say you are using a 20mm eyepiece in this hypothetical stubby monster, you would get x30 magnification, giving you an exit pupil of 13.33, wasting light, giving a washed out image which would likely show a secondary shadow.

Fast Newtonian scopes will have a lot of coma, needing coma correction to keep the stars round off axis. You will see that as mirrors get larger, scopes tend to get faster to make them practical and to avoid having to use a long ladder to get to the eyepiece. An f6 22" scope would be around 10.5' or 3m tall, and f2.5 22" would be around 4' or 1.2m tall. They still have much longer focal lengths than smaller scopes though, your f2.5 22" would have a focal length of around 1400mm despite its speed. A 17mm Ethos would probably be your lowest power giving x82 with a 6.8mm exit pupil and a 1.2 degree fov.

So, why bigger aperture then? The answer is almost always to get bigger image scale on DSOs. A larger aperture can never make an object have a higher surface brightness than with the naked eye.  That may sound counter intuitive but it is correct. What the aperture CAN do is maintain surface brightness as the image gets bigger. Many of the guys with big dobs take their scopes to very dark sites and hunt faint, small galaxies. The aperture allows them to be magnified whilst maintaining the surface brightness, allowing them to be seen. Your eye perceives the contrast more easily on larger objects. So the benefit is being able to get more detailed, up close views of smaller objects, rather than being able to take in the whole view of larger objects such as M31. Probable targets are more likely to be spotting globular clusters around M31 or detail in the spiral arms.

Of course, resolution also comes with aperture, so in theory with a good sky, you should get large and detailed planetary images from these scopes. I think that actually is the case in high altitude desert locations, I'm sure the planetary views at x500 or more are quite amazing. Not something you could do very often unless you live outside the U.K.!

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What eyepieces of decreasing focal lengths do, is to allow you to focus on the image produced by the telescope from decreasing eyepiece distances. If you hold your hand out at arms length a finger looks "finger size" but if you hold your finger 6mm away from your eye and you were able to focus the finger looks huge. This is the principle difference between using a 40mm eyepiece and one of 6mm.  :icon_biggrin:

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1 hour ago, Peter Drew said:

What eyepieces of decreasing focal lengths do, is to allow you to focus on the image produced by the telescope from decreasing eyepiece distances. If you hold your hand out at arms length a finger looks "finger size" but if you hold your finger 6mm away from your eye and you were able to focus the finger looks huge. This is the principle difference between using a 40mm eyepiece and one of 6mm.  :icon_biggrin:

And after reading that i am now holding my finger out and moving it backwards and forwards to my eyes. I'm glad i'm on my own at the minute!! :)

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On 27/02/2017 at 07:23, kilix said:

one thing, that helped me realize what is really happening was using a Nikon camera in telescope's prime focus. All that theory that I had read suddenly clicked into understanding.
Using the camera in prime focus, I realized that the telescope produces some kind of image, which is static basically. All changes to this image happen inside the eyepiece. With that eyepiece removed, you get to see what the telescope actually produces (but you cannot do that with your own eyes).
Then I realized, that all the magnification is just the eyepiece "looking" at a smaller part of this static image produced by a telescope. The bigger the mag, the smaller part of the original image is viewed. I imagine it somewhat like this:

The telescope collects light according to the surface area of the lens/mirror. The bigger the mirror/lens, the more photons are falling on it's surface because of bigger surface area. Image from mirror/lens gets projected somewhere to be actually used by humans - this is the focal point, point where the eyepiece rests. Now, the eyepiece decides if it will project the entire image to your eye (low mag EP), or if it will use and project only a small part of the image (high mag).

My brain understands it something like that. Very similar situation to understand would be this: telescope projects an image on a piece of paper, this image cannot be magnified or changed, it is still the same, only other telescope will provide other image. Using the eyepiece, you view only a small part of the paper and magnify it.

With bigger aperture scope, you collect more photons, so the produced image is brighter. And you can go to bigger magnifications without loosing too much contrast because of more light collected (pi * r  squared).

That's why it eventually starts loosing brightness and - accordingly - contrast and detail with big enough magnification.

What I still don't practically understand is how those widefield EPs work compared to normal EPs.

Wider field eyepieces.

This was a puzzler for me to start with because of my understanding of optics from a photographic background. Some said a photographic background is a hindrance but it isn't once you understand the somewhat different terminology used in astronomy and the various mechanisms at work.

Think of it this way, a telescope tube without an eyepiece is exactly like a camera lens.  Light enters the front and is focused at the focal plane. At the focal plane, the image that is formed is not a point but a circle.  The shorter the focal length of the objective, the wider the field of view it can see and the lower the magnification.  In 35mm camera terms, a 20mm lens is a wide angle lens with a big field of view and everything looks pretty small. A 1000mm lens has a narrow field of view and everything looks big. A telescope is just the same.  A 300mm telescope is considered to best suited to wide field, low magnification work. A 1500mm telescope is best suited for moon and planets where a wide field isn't needed. So, without an eyepiece, the field of view and the magnification seen at the focal plane is determined only by the focal length of the objective. This is the "first stage" of the telescope.

The "second stage" is the eyepiece. Cameras don't have them!

Eyepieces are really just kind of like magnifying glasses that look at the optical circle produced by the objective and and additionally magnify parts of it.  Short focal length eyepieces magnify parts of the optical circle more than longer ones but see less of the circle as a result. Longer ones magnify less but can see more of the optical circle so give wider fields of view.

What has this got to do with the so called super/extra/ultra wide eyepieces like Naglers and the like?

I'll use an imaginary telescope and made up number to illustrate as I don't have real figures to hand:

Let's say I have a simple refractor with a 1000mm focal length.  Let's say that without an eyepiece it creates an image circle at the focal plane that is 40mm in diameter.   If you stick a camera at the prime focus, that is what it would record, a circle 40mm in diameter (let's assume we have a big sensor!).

When you use a "standard" eyepiece, you zoom in and see a magnified piece of that 40mm circle.

The amount you will see depends on the focal length of the eyepiece. A short FL ep will "zoom" in an magnify the central region and exclude the edges of the circle from view. A longer ep will see more of the circle but everything will be less enlarged.

However, short or long focal length, the eps will still see only a part of the circle, say the inner 10mm section for a high magnification ep and the inner 20mm for a low magnification ep.  There will still be some parts of the out ring of the circle that is missed by the eyepiece. It is this missing part of the circle that explains where widefield eyepieces do their stuff.

So what do exotics like Naglers do?  What they do, is at any given focal length they see more of the objective's optical circle than the standard ep.  A 10mm Nagler will magnify the same amount as a standard 10mm (eg an ortho or a plossl) but they can see more of the optical circle. Using these exotics you can have increased  magnification AND a wider field of view because they can make use of some of the objective's optical circle that is wasted by lesser optics.  It's similar to the using the same full frame 35mm lens with a 35mm sensor, a APS-c sensor and a 4/3 sensor. The lens produces the same optical circle with all three camera types but only the full frame sensor can see the full circle.

There are limitations, of course:

No matter how wide field the eyepiece, it obviously can only go as far the edge of the circle produced by the objective.  If you use a exotic eyepiece of low magnification, eventually it will see all of the circle and the limit of eyepiece tricks will have been reached.

Typically, other factors come into play, such as vignetting by the eyepiece barrel or the focusing tube itself.  The reason 2" eyepieces permit wider fields of view in simply because the bigger eyepiece barrel and focusing tube remove a source of vignetting and thus you can get to see more of the circle produced by the objective.

Sorry the above is a bit of a baby talk explanation but I think it covers the essence of how exotic eyepieces do their stuff. 

Edited by digital_davem
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Great explanation @digital_davem! Now I understand it and your explanation fits my mental image of the function of a telescope very well.

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killix - yup, I totally agree, brilliant explanation of how things work esp. how the EP work with the size of image available :hello2:thanks digital_davem

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Latest thing I might know: Our entire Universe is actually located inside the Black Hole at the center of a galaxy located within another Universe - which is located in the Black Hole at the center of a galaxy located within another Universe - which is.....

ad infinitum,

Dave

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