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Daft question - are narrow telescopes more difficult to...errr.....aim?!


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I see loads of pictures of refractors on SGL - with some exceptions most seem to fall into category of long narrow telescopes, whilst the Dobs like my one on the whole are wider and squatter taking both both measurements into account.  Yet I see many frac owners extolling the virtues of their scopes for planetary and lunar views and often more besides.  I have also seen postings from beginners like me who might have inherited/obtained 'decent' Fracs.  as first scopes or bought the standard supermarket offerings which often seem to be this type.  I keep looking at the initial difficulties that I had pointing an 8" circle at the sky and finding things in it, without a finder things are even more awkward (thinking of finding the sun the other week).  Many fracs are far narrower, I hesitate to ask what might be a daft question, but given the praise that many fracs get, are they more difficult to find things in the sky with due to their narrow size?  Instinct says they ought to be more tricky, but is their a trick in their optics that lets them 'see' as much of the sky in one go as my 8" Dob does.

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This all relates to focal length, which dictates the magnification the scope gives with each eyepiece. This in turn combines with the apparent field of view of the eyepiece to give you a true field of view.

So put simply, a skinny refractor of 900mm focal length will give the same field of view as an apparently short stubby 200mm f4.5 Newtonian.

Refractors can range from small, fast, short focal lengths which give lovely widefield views, for instance the TV60 or Tak FS60C which have focal lengths of 360mm, up to some of the crazy f12 and f15 scopes which could have 1500mm focal lengths and which would have narrower fields of view. With a 24mm 68 degree eyepiece as an example, the TV60 would give around a 4.5 degree field of view, whilst the latter f15 scope would only show 1 degree of sky with the same eyepiece.

Things can be deception though too. I used to have an OMC200 which looked nice and compact, almost stubby, but infact was f20 i.e. 4000mm focal length. The maximum field of view in that even with a 2" 41mm Panoptic was under 0.7 degrees.

So, don't judge the book by its cover, check the focal length and that will tell you more.

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No, the diameter of the tube has nothing whatever to do with the diameter of the piece of sky it reveals.

What governs the field of view, in a given eyepiece, is the focal length and only the focal length.

In a given eyepiece the longer the focal length the higher the magnification and the smaller the field of view so as field of view becomes smaller, so the telescope becomes harder to aim. It also needs to be better mounted to give a stable image.

Aperture (the size of the objective and, by implication, the size of the tube) has an effect on resolution (the ability to present fine detail) and on the amount of light captured.

Olly

Stu beat me to it. :icon_biggrin:

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My instinct says that they both have the same FOV and I haven't looked them up.  I was thinking more of those telescopes that have this overall shape:

831_2.jpg

 

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From a purely mechanical position, a long skinny tube can be pointed more accurately by sighting along it than a short stubby tube. Ultimately you are normally using a skinny finder that has a wide field of view and a crosshair to help or some form of red dot projection.    :icon_biggrin:

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it was more in terms of the amount of sky it can see - if I hold a 8" circle up to the sky I can see more sky through it than if I hold a 2" circle up to sky, and thinking about how the light can enter a tube an angle surely that angle has to be narrower for the light to reach the far end of a long skinny tube than a wider tube, ergo it seems counter intuitive that a long skinny tube can see as much sky.

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3 minutes ago, JOC said:

it was more in terms of the amount of sky it can see - if I hold a 8" circle up to the sky I can see more sky through it than if I hold a 2" circle up to sky, and thinking about how the light can enter a tube an angle surely that angle has to be narrower for the light to reach the far end of a long skinny tube than a wider tube, ergo it seems counter intuitive that they can see more sky.

no, it's all about focal length.
Your eyes are only 5mm big 'circles' and yet you see like 120°, hows that possible? :icon_biggrin:

The angle I highlighted here in red governs the field of view. If you think about it and take this example for the eye - your retina is pretty close to the lens of your eye, so the angle is wide. In a long and narrow refractor the angle is narrow. The same applies to newtonian reflectors. Slow mirror - at say f/10 - means, that the light converges at a sharper-narrower angle, which means that the FOV is smaller.

RefractorDiagram.jpg.3b565eeb97c530ec8eb3e602b1e03977.jpg

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I can see what your saying but you haven't taken into account that the circles your looking through are fixed, imagine that not looking straight through but moving your eye left and right to change the field of view

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10 minutes ago, JOC said:

it was more in terms of the amount of sky it can see - if I hold a 8" circle up to the sky I can see more sky through it than if I hold a 2" circle up to sky, and thinking about how the light can enter a tube an angle surely that angle has to be narrower for the light to reach the far end of a long skinny tube than a wider tube, ergo it seems counter intuitive that a long skinny tube can see as much sky.

As said in my post, it's all in the focal length, nothing to do with the aperture (or skinniness of the tube.

The larger Newt in the picture is an f6 300mm with a focal length of 1800mm, the little scope is the OMC200 f20 scope with a focal length of 4000mm so the field of view is over double in the longer so.

I know this is not so easy to relate to the skinny scope you showed, but the thing to get your head around is that the focal length is what matters, not the aperture.

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1 minute ago, JOC said:

it was more in terms of the amount of sky it can see - if I hold a 8" circle up to the sky I can see more sky through it than if I hold a 2" circle up to sky, and thinking about how the light can enter a tube an angle surely that angle has to be narrower for the light to reach the far end of a long skinny tube than a wider tube, ergo it seems counter intuitive that they can see more sky.

It would seem that the distance from the opening at the front to the focal point determines the angle of light.

Or something.

Anyway, it's good to see written out that it's the focal length that determines max. field of view. And not, as was confusing to me at first, the focal ratio.

So anyone relating their narrow field of view to their f/15 scope is implicitly assuming that I know roughly what aperture they could possibly have. Which, as a beginner, I certainly didn't. And perhaps still don't.

Whatever.

Focal length it is, then.

:happy11:

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2 minutes ago, darknight said:

imagine that not looking straight through but moving your eye left and right to change the field of view

but surely the amount you can move your head is more limited in a long thin tube?

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hehe, didt explain that well, how about your eye is 3ft away from the 8 inch circle, but only 3inch from the small one, you have a wider field of view due to lenses, I do see where your coming from, light gathering in general is more from bigger scopes, but fiels of view is governd by lenses present

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and btw, the refractor on the image you posted here certainly is not a wide field instrument, it seems to be pretty long with small aperture, so the FOV will be quite narrow, narrower than your dob.

On the other hand, this guy is going to have extra wide field of view:
small frac
 

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1 minute ago, iPeace said:

It would seem that the distance from the opening at the front to the focal point determines the angle of light.

Or something.

Anyway, it's good to see written out that it's the focal length that determines max. field of view. And not, as was confusing to me at first, the focal ratio.

So anyone relating their narrow field of view to their f/15 scope is implicitly assuming that I know roughly what aperture they could possibly have. Which, as a beginner, I certainly didn't. And perhaps still don't.

Whatever.

Focal length it is, then.

:happy11:

Your first point is roughly correct for a refractor, but not correct for folded optic scopes like maks and SCTs.

Last point, exactly right! You need to know the aperture in order to work out the focal length from the focal ratio.

If you took a 100mm f15 scope, a classically long skinny tube, you might be fooled into thinking it had a narrower field of view than a 450mm f4 scope due to the difference in width and the focal ratio.

But, the skinny refractor has a 1500mm focal length, the dob has an 1800mm focal length, so its field of view will be a little narrower.

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1 minute ago, JOC said:

but surely the amount you can move your head is more limited in a long thin tube?

It's exactly how long the tube is what matters, not how thin.

:grin:

I agree it's somewhat counterintuitive. Just try it out, whenever possible. Same eyepiece in shorter, skinnier tube. Less magnification and more field.

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1 minute ago, kilix said:

and btw, the refractor on the image you posted here certainly is not a wide field instrument, it seems to be pretty long with small aperture, so the FOV will be quite narrow, narrower than your dob.

On the other hand, this guy is going to have extra wide field of view:
small frac
 

No! Not necessarily.

It looks like it might be a 60mm f15 scope, that will have a 900mm focal length. An 8" f6 dob has a 1200mm focal length, so has a narrower fov.

Even if it were an 80mm f15 it would still only have the same focal length.

Its easy to get confused by this!

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16 minutes ago, Stu said:

Your first point is roughly correct for a refractor, but not correct for folded optic scopes like maks and SCTs.

Really? Wow. So when I wrote:

21 minutes ago, iPeace said:

It would seem that the distance from the opening at the front to the focal point determines the angle of light.

Was I misunderstanding the term "focal point"? It's true I didn't qualify the "distance" as "folded or otherwise".

:icon_biggrin:

I am sure it's more subtle than this. I feel a StuPlenation coming on...

:grin:

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5 minutes ago, iPeace said:

Really? Wow. So when I wrote:

Was I misunderstanding the term "focal point"? It's true I didn't qualify the "distance" as "folded or otherwise".

:icon_biggrin:

I took your comment as basically 'the length of the scope from aperture opening to focuser'.

If what you actually meant was 'unfold the light path and measure from opening to focal point' then that's more like it!

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2 minutes ago, Stu said:

I took your comment as basically 'the length of the scope from aperture opening to focuser'.

If what you actually meant was 'unfold the light path and measure from opening to focal point' then that's more like it!

Yeah, indeed it's not just a shorter tube. It's the distance the light travels within the tube. Bounce the light off mirrors in the tube and the distance increases.

:happy11:

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1 hour ago, JOC said:

I see loads of pictures of refractors on SGL - with some exceptions most seem to fall into category of long narrow telescopes, whilst the Dobs like my one on the whole are wider and squatter taking both both measurements into account.  Yet I see many frac owners extolling the virtues of their scopes for planetary and lunar views and often more besides.  I have also seen postings from beginners like me who might have inherited/obtained 'decent' Fracs.  as first scopes or bought the standard supermarket offerings which often seem to be this type.  I keep looking at the initial difficulties that I had pointing an 8" circle at the sky and finding things in it, without a finder things are even more awkward (thinking of finding the sun the other week).  Many fracs are far narrower, I hesitate to ask what might be a daft question, but given the praise that many fracs get, are they more difficult to find things in the sky with due to their narrow size?  Instinct says they ought to be more tricky, but is their a trick in their optics that lets them 'see' as much of the sky in one go as my 8" Dob does.

Once the 'scope is lined up with the crosshairs or red dot, aiming is easy with any instrument, although any small errors will be heightened if you do this at high mag, so start with low.  And the mag of course is proportional to the 'scope's FL (for a given eyepiece).

Doug.

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Let me chip in here.  The short answer to the original question is probably 'no' or 'it depends'. 

What makes a telescope difficult to aim? Asides from the practical mechanics of the mount, finderscopes, etc, I am assuming we are referring to the apparent field of view of the telescope. 

The apparent field of view in a modern eyepiece is typically 50 degrees - less in an old-fashioned eyepiece, more in a fancy high-priced one. If you put it in a telescope such that the resultant magnification is x100, the actual field of view is 50 divided by 100, or half a degree (the diameter of the full moon).  In a practical example, if the eyepiece focal length is 10mm and the objective focal length is 1000mm, that's what you get - x100 and an apparent field of half a degree. 

You will note that I have not mentioned the aperture (diameter) of the telescope, because it's irrelevant. So, if you take an eyepiece and an objective focal length, that's what determines the difficulty of aiming it at an object in the sky.  

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2 minutes ago, Cosmic Geoff said:

I am assuming we are referring to the apparent field of view of the telescope. 

Yes, I guess that is what I was  getting at - in a huge area that we'll call 'the whole sky' was the amount of that sky 'seen' by the telescope limited by the apparent circular area of telescope that you were looking through.  i.e. did you see an apparently bigger circle of the sky (EP's not withstanding) because you happened to be looking through a squatter wider tube.  I was just looking all the relatively thin narrow tubes that I've seen pictures and wondering how they could possibly 'see' as physically big an area of sky as something much wider.  I've seen people raving about their Taks and then looking at pictures of long narrow tubes and thinking well they might give lovely sharp images, but surely of a more limited portion of sky (per EP) than a wider scope.  It just seemed so counter-intuitive esp. as I've never looked down a 'long thin tube' style telescope, and well they say there is no such thing as a 'daft question' so I asked. 

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