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What point is a 2" focuser on 6" dob?

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#1 markgf

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Posted 22 November 2012 - 02:59 AM

Hello cloudy-nighters

I think my inexperience may be showing...

The 6" dobs from Skywatcher and Saxon both have 2" (51mm) focussers, but only 34.5mm secondary mirrors.

As a 1.25" (31.7mm) eyepiece is just about big enough to capture the available light from a 34.5mm mirror, is there any any point buying a 2" eyepiece for either of these telescopes?

thanks...Mark

#2 Jon Isaacs

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Posted 22 November 2012 - 04:27 AM

Hello cloudy-nighters

I think my inexperience may be showing...

The 6" dobs from Skywatcher and Saxon both have 2" (51mm) focussers, but only 34.5mm secondary mirrors.

As a 1.25" (31.7mm) eyepiece is just about big enough to capture the available light from a 34.5mm mirror, is there any any point buying a 2" eyepiece for either of these telescopes?

thanks...Mark


Mark:

Just because the field of view out towards the edge is not fully illuminated, the added field of view can still be quite helpful, quite enjoyable. The eye is quite insensitive to the light fall off so you probably will not even notice it.

Jon

#3 markgf

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Posted 22 November 2012 - 06:04 AM

Thanks John
I don't really understand about the effect of non-illumination of the outer portion of an eyepiece 'input aperture' (should that be field stop?).

Would there be any added field of view at all if the small secondary only directs light through the middle portion of a large field stop eyepiece?

My concern was that telescope manufacturers might put in big focussers as a mere fashion statement. I feel a bit silly saying so. Getting cynical in old age perhaps.
cheers...Mark

#4 star drop

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Posted 22 November 2012 - 06:10 AM

Years ago I made a 4.25" f/4 reflector with a two inch focuser. Its diagonal mirror was ~28.5mm and I never noticed the edge of the field being dim. The telescope provided wonderful rich field views with a 9mm Nagler, a 20mm Nagler or a 32mm Televue Widefield eyepiece.

#5 Jon Isaacs

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Posted 22 November 2012 - 07:00 AM

Thanks John
I don't really understand about the effect of non-illumination of the outer portion of an eyepiece 'input aperture' (should that be field stop?).

Would there be any added field of view at all if the small secondary only directs light through the middle portion of a large field stop eyepiece?

My concern was that telescope manufacturers might put in big focussers as a mere fashion statement. I feel a bit silly saying so. Getting cynical in old age perhaps.
cheers...Mark


Mark:

You will see the entire field of view the eyepiece offers.

What happens is that you see the full field of view that the field stop provides but there is some drop-off in brightness as you near the edge of the field. But this drop off has to be quite severe to be noticeable, the eye is just not very sensitive to it.

One way to think about this is to consider that you are looking at the primary mirror through a pinhole in the focuser. If you are in the center of the field, you will be able to see the entire mirror but as you move off axis, you will come to a point where you can just barely see the entire mirror. That point is the limit to the 100% illumination diameter. If you can see the mirror, it means that light from the mirror reaches your eye.

As you move further off-axis, further away from the center, you see less and less of the primary mirror. At some point you can only see 75% of the mirror, that is the 75% illumination limit and it means that at that point, the image is 75% as bright as it would be were it in the center.

25% drop-off seems like a lot but 75% is still a lot of light, even 50% is a lot of light. At the 75% point, the scope is working like a 5.2 inch scope, still plenty bright, still very useful.

Jon

#6 howard929

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Posted 22 November 2012 - 08:41 AM

One way to think about this is to consider that you are looking at the primary mirror through a pinhole in the focuser.



Jon,

What are the mechanics behind that pinhole effect?

Howard

#7 markgf

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Posted 22 November 2012 - 10:02 PM

Thanks Jon and Ted.
I am now equipped to demonstrate a greater depth of ignorance...

How come a bigger eyepiece gives a bigger real field of view if even a pinhole will see the whole primary mirror. Surely the eyepiece diameter doesn't increase the FOV seen by the primary mirror?

Newbishly yours...Mark

#8 GlennLeDrew

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Posted 22 November 2012 - 11:57 PM

Mark,
Perhaps Jon chose a not quite appropriate word in "pinhole." As soon as I read his response, I knew it might cause some confusion, even though he's correct. Allow me to expand...

With the eyepiece removed, and your eye placed at the focal surface, when your eye is centered at the focuser's opening you will see the entire primary mirror reflected by the secondary. Now, as you slowly move your eye laterally, a point will be reached where the edge of the primary, as seen in reflection off the secondary, appears to just 'touch' the edge if the secondary. This distance from the optical axis that your eye has moved defines the size if the circle of full illumination.

Continue moving your eye laterally and it will be seen that a part of the edge of the primary is no longer seen, due to the secondary not being sufficiently large to do this. The farther off axis you move your eye, the greater the fraction of the primary not seen. The fraction of the primary's area which is seen is the fraction of the illumination at that distance off axis. You would have to move your eye rather farther than 1" (1/2 the 2" format) off axis in order to completely lose sight if the primary as reflected by the secondary.

You can see this by making up a scale drawing, tracing various rays which originate at the primary's center and edge, passing along opposite edges of the secondary and on to the focus. Compare the endpoints to an image circle of 2" diameter.

#9 Jon Isaacs

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Posted 23 November 2012 - 01:29 AM

Thanks Jon and Ted.
I am now equipped to demonstrate a greater depth of ignorance...

How come a bigger eyepiece gives a bigger real field of view if even a pinhole will see the whole primary mirror. Surely the eyepiece diameter doesn't increase the FOV seen by the primary mirror?

Newbishly yours...Mark


Mark:

Glenn this, hopefully that resolved your question but if not, I will give it a try:

- Without an eyepiece in the focuser, you are looking at what is essentially the focal plane of the telescope, this is where the image is formed.

- At the focal plane, looking through tiny hole, you are only looking at a very small part of that image. When that tiny hole is in the center of the focuser, you are looking at the center of the field of view. When that tiny hole moves away from the center, you are looking at something that is no long centered.

Another better way to think of this might be an eyepiece with a tiny, moveable field stop that only lets you see a very small part of the image. As you move that field stop around you see different parts of the image. Without the eyepiece in place but with the field stop in place, you see the part of the mirror that is used to form that image.

- Ideally, the entire primary mirror contributes to any point in the field of view. Generally this is only true for the central part of the field of view.

I hope this helps, it probably just confuses things more...

Jon

#10 markgf

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Posted 23 November 2012 - 03:44 AM

Glenn and Jon
Thank you for your valiant attempts at clearing the fog. I think I'm getting it about off-axis partial illumination in the eyepiece, though questions remain.

I assume that if an eyepiece field stop can 'swallow' the whole focal plane, the maximum real field of view is seen through the exit pupil. Here's the thing: if the curvature of the primary is designed so that the whole focal plane is 'swallowable' by a 1.25" eyepiece, I don't get why a 2" eyepiece should increase the total field of view.

#11 Jon Isaacs

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Posted 23 November 2012 - 11:48 AM

Glenn and Jon
Thank you for your valiant attempts at clearing the fog. I think I'm getting it about off-axis partial illumination in the eyepiece, though questions remain.

I assume that if an eyepiece field stop can 'swallow' the whole focal plane, the maximum real field of view is seen through the exit pupil. Here's the thing: if the curvature of the primary is designed so that the whole focal plane is 'swallowable' by a 1.25" eyepiece, I don't get why a 2" eyepiece should increase the total field of view.


Mark:

This stuff is not so easy to understand, you are getting the crash course...

Basically, a 1.25 inch eyepiece cannot see the entire field of view, at most, it can only see a circle that is 28mm in diameter. A 2 inch eyepiece can see a circle as large as 46mm.

Hopefully this will help:

The eyepiece has a focal plane, the telescope has a focal plane, when these two coincide, the image is in focus.

The field stop is a ring in the barrel of the eyepiece. If you look through the eyepiece, the image stops at the field stop, it is a dark, hopefully sharp ring, at the edge of the view. If you look backwards down many eyepieces, you can see the field stop.

The field stop is at the focal plane of the eyepiece, if you look through the eyepiece an try to touch the field stop with a pencil point, you will see that it is in focus.

So.. the telescope produces an image at it's focal plane. When the eyepiece is placed in the focuser and the image is in focus, the eyepiece magnifies the image that it sees. But it only sees the part of the focal plane that is inside the field stop, the rest it stops, thus the term field stop.

The greater the diameter of the field stop, the larger the field of view. In the 1.25 inch barrel, there is only room for a 28mm field stop, in a 2 inch barrel it can be much larger.

Jon

#12 markgf

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Posted 26 November 2012 - 01:28 AM

Thanks again Jon.
Your patience is legendary. I'm learning heaps but I may still be missing something.

I assume its the telescope's tube geometry that actually limits the real field of view available to an eyepiece. For a 6" F8 scope, the point of focus (at the focal plane?) is 1200mm from the primary mirror, so the mirror appears about 7 degrees wide (I think). However the geometry of a 2" eyepiece limits its real field of view to ~2.2 degrees (for some reason) so there's plenty of field of view available to it.

This raises another question (sorry). If the entire light cone from the primary, when it reaches the eyepiece, is smaller than the eyepiece field stop, why isn't the real field of view of the eyepiece the whole 7 degrees of the scope?

I feel I have drifted far off topic by now, so please don't feel obliged to respond if there's no straightforward answer. Maybe I should look up an optics paper or something if I really want to drill into this. :grin:

Cheers...mark

#13 FlorinAndrei

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Posted 26 November 2012 - 03:49 AM

This raises another question (sorry). If the entire light cone from the primary, when it reaches the eyepiece, is smaller than the eyepiece field stop, why isn't the real field of view of the eyepiece the whole 7 degrees of the scope?


The TFoV (true field of view) of a telescope is fully determined by the focal length of the primary mirror, and the size of the field stop of the eyepiece.

Think of the ray of light that goes through the center (apex) of the primary lens or mirror. It's the thicker (darker) ray in the diagram below, the ray in the center.

http://www.novac.com/nl/91/Fov.pdf

That ray is never deviated, it always goes straight from object to eyepiece (it is reflected in a reflector, but its angle to the axis remains the same). That's the only ray, in any optical system, that is never bent or deviated, it always goes ramrod-straight from object to eyepiece. It's easier to think of a refractor here, but it's the same for reflectors (plus an additional reflection that doesn't change any angle).

Now imagine that ray sweeping the whole field stop in the eyepiece, side to side. That's the maximum angle visible in that scope. When the central ray is hitting the bottom side of the field stop, it's coming from the very top of the image. When it's hitting the top side of the field stop, you're looking at the very bottom of the image.

You can't have an image bigger than that, because then the central ray would have to bend (which contradicts the laws of optics).

The maximum angle that the central ray can sweep is the true field of view of the telescope, and the formula you could deduce from the diagram is:

TFoV = arctan (d / F)

where d = field stop, and F = primary focal length

A 6" f/8 dob with a 2" focuser (46 mm max field stop, 1200 mm primary focal length) would then provide over 2 degrees true field.

http://www.wolframal...arctan(46/1200)

I've built a scope exactly like that and it works great for wide field stuff (if you don't mind the small aperture).

If you want an even wider field of view, you just need a bigger field stop. That means you'd probably have to use one of those newfangled 3" eyepieces that Scott Roberts is playing with these days. :)

why isn't the real field of view of the eyepiece the whole 7 degrees of the scope?


It doesn't work like that. There are no 7 degrees anywhere, forget that number.

You're not looking through the primary mirror like you would look through a hole in the wall. Telescopes don't work like that.

The way a scope works is the primary mirror actually projects a real image in its focal plane, and you're looking at that image with a handy little microscope called "eyepiece". See how it works? There's a big image up in the sky (stars and stuff), there's a tiny image in the focal plane which is exactly like the big image in the sky but smaller and much closer, and the eyepiece is looking at that tiny image created by the mirror. Who cares what's the angular size of the primary (your 7 degrees), you're not looking directly through it, ever.

In theory, the size of that real image in the focal plane is unlimited. Objects further and further away from the axis simply project an image further and further away from the center of the focal plane. If you had an eyepiece with a field stop the size of the wheels on a cement truck, then you'd get ultra-wide views.

But in practice, the performance of any optical system decreases quickly away from center. That ultra-wide view would be a total blur at the edge.

#14 Jon Isaacs

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Posted 26 November 2012 - 06:50 AM

Mark:

These are difficult concepts to grasp, I am not quite sure how to proceed, hopefully Florin has answered you questions but I will take a stab at it myself.

The 7 degrees you refer to is the included angle of the light cone for an F/8 telescope, one eighth of a radian.

But that is not the field of view. To understand the field of view, one has to think of an off-axis ray that enters the telescope and it's position in the focal plane.

Let's imagine a tree that is 100 feet tall that is 4000 feet away. This represents an angle of 0.025 radians or 1.43 degrees. If you are looking at the middle of the tree, rays from the top and bottom are about 0.71 degrees off-axis, that is from the center.

How tall is the image of the tree that is formed at the focal plane???

This depends on the focal length of the telescope, the longer the focal length, the larger the image. Since both the center and top/bottom of the tree are in focus and the top and bottom are separated by 1.43 degrees, the size of the image of the tree will be:

0.025 radians x 1200mm = 30mm. (Angle x focal length of the scope)

So, the image of the tree at the focal plane of the telescope is 30mm tall. If the field stop of the eyepiece is smaller than 30mm, you will not be able to see the entire tree.

I hope this example helps, understanding the size of the image formed at the focal plane is the key.

Jon

#15 markgf

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Posted 26 November 2012 - 07:03 AM

Florin Andrei
Thank you. I've got it. Its simple once the penny has dropped. :bow:

My brain is full. All the wrinkles on its outer surface have now been squashed out flat against the inside of my skull.

I think I was looking down the wrong end of the telescope and thinking about it as a microscope with an AFoV = 7 degrees. Fail. I have it now. :waytogo:

This brings me back to the title of this thread. I suppose that if the minor axis of the secondary mirror (34.5mm) of a 6" F8 dob is only 0.75 x the max field stop diameter of a 2" eyepiece(46mm), and the secondary is close to the field stop, the max real FoV may be constricted a bit by the secondary, but who maxes out the FoV anyway? Might have to make do with a 35mm Pan instead of a 41mm. :lol:

So...a 2" focuser in a 6" dob does make sense.

Thanks All...Mark :cool:

#16 Jon Isaacs

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Posted 26 November 2012 - 07:35 AM

This brings me back to the title of this thread. I suppose that if the minor axis of the secondary mirror (34.5mm) of a 6" F8 dob is only 0.75 x the max field stop diameter of a 2" eyepiece(46mm), and the secondary is close to the field stop, the max real FoV may be constricted a bit by the secondary, but who maxes out the FoV anyway?



Mark:

Glad it finally all came together. Just one last tidbit:

The size of the secondary generally does not limit the size the field of view, rather it governs the brightness of the off-axis portions of the field of view, that is, the edge will be there to see, it just won't be as bright.

Jon

#17 markgf

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Posted 26 November 2012 - 07:49 PM

Yep. Thats the off-axis illumination ratio thingy from the start of this thread...
Mark

#18 lamplight

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Posted 26 November 2012 - 11:11 PM

is this still the beginners forum?

#19 KWB

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Posted 26 November 2012 - 11:36 PM

is this still the beginners forum?

Absolutely,it still is and IMHO,the best forum here at CN for this excellent primer on optical theory 101. :ubetcha:

#20 galexand

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Posted 26 November 2012 - 11:48 PM

Wow! Quite a few interesting posts here. I don't know that anyone answered your practical question though. :)

If you're like me and you don't use a finder scope, then there is a point to buying a 2" EP for any scope that will take it: the wider field of view will help you in a "finder eyepiece". Poorly-illuminated, ridiculous giant exit pupil, no matter. Having a 2 or 3 degree true field of view to look through makes star hopping *much* easier. To say nothing of the aesthetic delight of such a rich field.

#21 FlorinAndrei

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Posted 27 November 2012 - 03:53 AM

there is a point to buying a 2" EP for any scope that will take it: the wider field of view will help you in a "finder eyepiece".


6" f/8 newtonian tube with a 2" focuser and a 1.83" secondary (30% the size of primary). It gives me over 2 deg TFoV in a 30 mm ES82 eyepiece, pretty decent illumination at the edge, actually. The Pleiades fill the whole field and are visible pretty much entirely at once.

It's nice that I can use good 2" eyepieces that would otherwise be unusable in a classic 6" reflector with a small focuser.

I can also do prime focus photography with a Micro Four Thirds camera, using a low profile T-minus adapter, which wastes much less back focus than a classic T-adapter/T-ring and allows the back focus to stay small (also helped by the small flange-focus distance of a MFT camera). The sensor gets decent illumination all the way to the edge - not 100% perfect, but good enough for Moon and solar shots.

Incidentally, the entire lunar or solar discs fit on the sensor entirely in one shot, with a little extra room on the edge. That would not be doable in a small focuser at this focal length. The results are fairly pleasing:

http://imgur.com/a/TGqlG#0

The price paid for all that is the large 30% secondary. But in 9 cases out of 10, atmospheric turbulence blurs details much more than the effects of the large secondary. When seeing is good, at high magnification it resolves the galilean moons as tiny discs, no problem (but without any surface details, of course). So the large secondary ain't a big issue, if the rest is in good shape.

This is the scope I use to do demos for my kids, their friends, my friends, neighbors, random passersby on the street, etc. Being small it's perfect for ad-hoc sessions.

So yeah, it's totally worth it having a 2" focuser on a 6" reflector for a general purpose, flexible instrument. This is not a single-purpose "apo killer" with a tiny secondary.

#22 Jon Isaacs

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Posted 27 November 2012 - 04:26 AM

Wow! Quite a few interesting posts here. I don't know that anyone answered your practical question though. :)

If you're like me and you don't use a finder scope, then there is a point to buying a 2" EP for any scope that will take it: the wider field of view will help you in a "finder eyepiece". Poorly-illuminated, ridiculous giant exit pupil, no matter. Having a 2 or 3 degree true field of view to look through makes star hopping *much* easier. To say nothing of the aesthetic delight of such a rich field.


:waytogo:

I think the original question was whether a 6 inch F/8 Dob with a 1.33 inch secondary could take advantage of the larger field of view offered by a 2 inch eyepiece and the short answer is yes...

Jon

#23 markgf

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Posted 27 November 2012 - 08:34 AM

OK then! Guess I'd better start saving for a 2" wide field eyepiece and a 2" barlow/telextender.

A pity the Orion 6" dob only has a 1.25" focuser, but the Saxon/Skywatcher 6" dobs don't have intelliscope. Oh well.

Thanks All...Mark

#24 howard929

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Posted 27 November 2012 - 10:22 AM

Depending on how much time and money you'd want to invest into that telescope, this focuser might be a worth while upgrade if you can see yourself opening the hole in the OTA a bit larger.

#25 Jon Isaacs

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Posted 27 November 2012 - 12:09 PM

OK then! Guess I'd better start saving for a 2" wide field eyepiece and a 2" barlow/telextender.

A pity the Orion 6" dob only has a 1.25" focuser, but the Saxon/Skywatcher 6" dobs don't have intelliscope. Oh well.

Thanks All...Mark


That's the catch 22. And if you spend the money to add the 2 inch focuser to the 6 inch Orion, you might as well just buy the 8 inch Orion.

There was a short time when Optics Planet was selling the 6 inch Synta Dob under the Bushnell name with the 2 inch focuser for $220 shipped to your door.

That was a deal.

Jon






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