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

[CraigT82]

Field of view is purely down to focal length and nothing else.

'Skinnyness' of the tube is down to the focal ratio (if we ignore compound scopes here)

Image below shows how a skinny tube can see more sky that a squat one ... purely because it has a shorter focal length! 

 

 

[ollypenrice]

Let's be careful with 'focal ratio.' Of itself it tells you absolutely nothing about field of view.

If the focal length is the same in an F5 and an F10 scope they have exactly the same FOV.

If you take an F10 scope and fit a focal reducer to make it F5 then it will have a wider FOV (barring internal obstructions) but only because it has a reduced  focal length. 

Coming back to the original question about tubes (and so, in effect, apertures) and assuming that we can't really make decent visual optics at less than about F3.5, what we find is that the bigger the instrument the longer the focal length has to become and so the smaller the FOV becomes. This is, in fact, the oppositie of the OP's opening supposition. 

Olly

[SilverAstro]

What an interesting topic ! An 8" lens of 1m focal length will see the same area of sky (for a given eyepiece) as an 8" mirror of 1m focal length, will it not ? The only difference is that the 8" mirror is in the pantomime mode - it's behind you !

The prob that JOC has is that most refractors are long and thin and pointy cos they are generally not 8" aperture for similar focal length.

An 8" mirror is just a convenient surrogate for a (costly) 8" lens.

[ollypenrice]

5 hours ago, SilverAstro said:

What an interesting topic ! An 8" lens of 1m focal length will see the same area of sky (for a given eyepiece) as an 8" mirror of 1m focal length, will it not ? The only difference is that the 8" mirror is in the pantomime mode - it's behind you !

The prob that JOC has is that most refractors are long and thin and pointy cos they are generally not 8" aperture for similar focal length.

An 8" mirror is just a convenient surrogate for a (costly) 8" lens.

Yes, it's cheaper to make reflectors than refractors because there are fewer optical surfaces to prepare. A triplet has six surfaces, a primary mirror only one.

Olly

[Peter Drew]

Try this easy one. Take three tubes of the same diameter but 6" long, 12" long and 24" long. Look at the sky through them with the naked eye and see the difference in the field you can see.  

[ollypenrice]

4 hours ago, Peter Drew said:

Try this easy one. Take three tubes of the same diameter but 6" long, 12" long and 24" long. Look at the sky through them with the naked eye and see the difference in the field you can see.  

I get more magnification out of the longer tube! (Small objects fill the field of view.)

lly

[Peter Drew]

17 minutes ago, ollypenrice said:

I get more magnification out of the longer tube! (Small objects fill the field of view.)

lly

Proves the other point, the longer the focal length the higher the magnification. 

[Louis D]

On 5/27/2017 at 01:17, ollypenrice said:

Yes, it's cheaper to make reflectors than refractors because there are fewer optical surfaces to prepare. A triplet has six surfaces, a primary mirror only one.

Olly

Oh, it goes way beyond that.  Large glass blanks are difficult to pour strain free, bubble free, etc.  It is difficult to mount them strain free as well.  They tend to deform under their own weight.  Unlike a mirror, you can't support them at multiple points on the backside as with a mirror in its cell.  To top it all off, the larger the diameter of a refractor, the larger the chromatic error for a given focal length, optical design, and glass type.  They do not scale like mirrors.  Thus, to keep chromatic aberrations to a minimum, large diameter refractors must also have very large focal ratios as well as use exotic, super expensive low dispersion glass.  Taken together, it's no wonder the Lick and Yerkes scopes were the last of their kind.

[Louis D]

Technically, with long focal length eyepieces of a very large size (think 5" diameter, 100mm+ eyepieces) in a massive focuser, it is possible for a refractor to see a much larger chunk of the sky than a reflector of equivalent focal length and focal ratio because it isn't limited by secondary mirror size or obstruction as in a Newtonian design or by the baffle size as in a catadioptric design.  This is a common approach taken by both the Lick and Yerkes refractors to achieve lower powers and thus larger true fields of view while maintaining a reasonable apparent field of view.  They have custom made "observatory" class eyepieces that would not interest most amateur astronomers except as collector's items.

[rockystar]

On ‎27‎/‎05‎/‎2017 at 09:16, Peter Drew said:

Try this easy one. Take three tubes of the same diameter but 6" long, 12" long and 24" long. Look at the sky through them with the naked eye and see the difference in the field you can see.  

I get this, this makes sense to me, in fact I've used the analogy myself.

I even accept the fact that it is all down to focal length, but I think I'm missing something.

On ‎26‎/‎05‎/‎2017 at 11:25, CraigT82 said:

Field of view is purely down to focal length and nothing else.

'Skinnyness' of the tube is down to the focal ratio (if we ignore compound scopes here)

Image below shows how a skinny tube can see more sky that a squat one ... purely because it has a shorter focal length! 

 

 

Let's take the 100/500mm refractor (for easy, and forget about how difficult it is to make), if I make it 200/500mm, why does it not increase the angle at the focal point and thus show more sky?

[Peter Drew]

Because the focal length is still the same  so would give the same magnification with the same eyepiece in either telescope. This is where the simplistic empty tube analogy falls down. 

[Stu]

7 minutes ago, rockystar said:

I get this, this makes sense to me, in fact I've used the analogy myself.

I even accept the fact that it is all down to focal length, but I think I'm missing something.

Let's take the 100/500mm refractor (for easy, and forget about how difficult it is to make), if I make it 200/500mm, why does it not increase the angle at the focal point and thus show more sky?

If you look at the diagrams, the fields of view are not related to the aperture. The 60mm f12 has a larger fov than the 100mm f9 despite an apparently steeper light cone

[Louis D]

5 hours ago, rockystar said:

Let's take the 100/500mm refractor (for easy, and forget about how difficult it is to make), if I make it 200/500mm, why does it not increase the angle at the focal point and thus show more sky?

You won't see any more sky (TFOV), but you will see everything as being brighter at a given magnification due to doubling your exit pupil.  You'll also see way more detail due to the doubling of your aperture increasing your angular resolution (assuming equivalent scope quality).