Focal ratio of an eye piece?

[25585]

I read in a CN article on Brandon eps

 All eyepieces have a critical focal ratio. If the scope is faster than the critical number, the eyepiece fails to perform .

But what do you divide an ep's FL by to calculate it's focal ratio? Is it the field lens or eye lens, or another value?

 

 

[Ruud]

It must refer to the focal ratio of the objective.

Suppose an eyepieces is  good at F/10 but not at F/5, then in terms of the quote you show, the critical focal ratio for that eyepiece is somewhere between F/10 and F/5. 

So they mean "critical focal ratio for an eyepiece" instead of critical focal ratio of an eyepiece.

Brandons aren't very good in fast scopes, so I assume that's the issue they were talking about?

[Stu]

Yes, it actually says it in the text you posted 'if the scope is faster than the critical number,.....'

Admittedly it's a poorly constructed sentence but they mean that eyepieces are designed to work down to a certain telescope focal ratio, below which they suffer poorer edge performance usually.

[vlaiv]

Yes it is property of eyepiece design.

Focal ratio of telescope implies incoming light cone spread. Take for example star dead center on optical axis. One ray will come at angle 0 to focal point, but "further out" you move, rays will be hitting focal plane at bigger and bigger angle, until you reach F/ratio angle of telescope.

Eyepiece needs to bend those rays in certain way to make them such that eye sees object as it would without telescope (only magnified). Depending on eyepiece design it will bend some rays better then others. So for some eyepieces rays coming "out" of focal plane at too big angle will not be correctly bent - thus introducing all sorts of aberrations.

This also impacts stars that are further away from optical axis - this is why you hear some eyepieces "have pinpoint stars right to the edge of AFOV" even if fast telescopes.

[Charic]

I've never looked at an eyepiece as having a focal ratio? but often recommend that exceeding the scopes focal ratio can and will  diminish the quality of the image produced by the telescope.

The purpose of the eyepiece is to magnify the image at the telescopes focal plane. Just know there are limits at either extreme, be that higher/lower magnification, also the type of eyepiece and its construction can also aid the 'comfort' afforded to the end user, by the amount of  view that is visible and the eye relief afforded, as some folk require more than others, and some EP's can even help alleviate some minor  scope aberrations as stated above.

The great thing about modern telescopes is that the end user can 'select' varying degrees of magnification, simply by changing the focal length of the eyepiece.

That said, reducing magnification (longer focal length eyepiece ) certain images can/will appear sharper, clearer, better, but all the time getting smaller!
Increasing magnification ( Shorter focal length ) the opposite is true, larger/closer image, not the best, not as clear leading to almost impossible to view hazy images?

I find for most targets, that exceeding the focal ratio of the telescope  with a  focal length eyepiece, lower in number to the  telescope's focal ratio, will degrade most images, and the further you increase the magnification, the worse the image gets, which if you try yourself, arrives very quickly. 

If eyepieces do have a focal ratio, then its just another learning day for me!

[John]

Many eyepieces have lenses of varying diameters used in them. Which lens is used to calculate the focal ratio or is it the field stop diameter ?

Edit: Ooops ! - just noticed that this Q was asked in the opening post of the thread

[jetstream]

2 hours ago, 25585 said:

I read in a CN article on Brandon eps

 All eyepieces have a critical focal ratio. If the scope is faster than the critical number, the eyepiece fails to perform .

But what do you divide an ep's FL by to calculate it's focal ratio? Is it the field lens or eye lens, or another value?

 

 

Look at spot size diagrams if you can find them, they explain this. It applies to refractors as well.

"The focal length of an eyepiece is the distance from the principal plane of the eyepiece where parallel rays of light converge to a single point. When in use, the focal length of an eyepiece, combined with the focal length of the telescope or microscope objective, to which it is attached, determines the magnification. It is usually expressed in millimetres when referring to the eyepiece alone. When interchanging a set of eyepieces on a single instrument, however, some users prefer to refer to identify each eyepiece by the magnification produced. "  From Wiki

 

[Charic]

13 minutes ago, jetstream said:

Look at spot size diagrams if you can find them, they explain this

Whats the focal ratio of the first HUYGENS on that chart? I'm none the wiser!

[jetstream]

15 minutes ago, jetstream said:

 All eyepieces have a critical focal ratio. If the scope is faster than the critical number, the eyepiece fails to perform .

It doesn't matter... all eyepieces have a "critical f ratio" where they perform best and the chart shows the performance at f 10 and f 5. If the spot is inside the airy disk alls VG. The chart shows off axis vs spot size performance as well as f ratio vs spot size performance.

I think the OP may have misinterpreted the quote from CN IMHO.

[Charic]

19 minutes ago, jetstream said:

"The focal length of an eyepiece is the distance from the principal plane of the eyepiece where parallel rays of light converge to a single point. When in use, the focal length of an eyepiece, combined with the focal length of the telescope or microscope objective, to which it is attached, determines the magnification. It is usually expressed in millimetres when referring to the eyepiece alone. When interchanging a set of eyepieces on a single instrument, however, some users prefer to refer to identify each eyepiece by the magnification produced. "  From Wiki

 

That all makes sense, but the OP is looking for the focal ratio of just an eyepiece alone, not the telescope?
 

[jetstream]

I believe the question stems from the misinterpretation of the CN quote.

[Charic]

1 minute ago, jetstream said:

I think the OP may have misinterpreted the quote from CN IMHO.

Thats quite possible?
Not trying to catch you out here jetstream, if there is a focal ratio for an eyepiece, then its a new thing for me!

[vlaiv]

2 minutes ago, Charic said:

That all makes sense, but the OP is looking for the focal ratio of just an eyepiece alone, not the telescope?
 

No such thing ...

[Charic]

18 minutes ago, vlaiv said:

No such thing ...

Im sure your right. Ive never heard or seen anything in writing that refers to the eyepiece focal ratio, and a better search today has found  absolutely nothing!

I think its good that 25585  asked the question in the first place, especially if you don't know,  and if you don't know, then you'll never know unless you ask, that's where forum help can help sometimes.

[TheLookingGlass]

There is no "focal ratio" of an eyepiece, only the focal ratio of a telescope.

[Merlin66]

Yeah but.....

The focused beam from the objective forms a cone of light, the "steepness" of the cone depends on the focal ratio. After forming the virtual image at the focus, the cone of light starts to expand - at the same focal ratio that it entered. The eyepiece field lens must be large enough to embrace this expanding light cone. So you could say, that an eyepiece working successfully in a f10 light cone was functioning as an f10 eyepiece.

We have similar considerations when designing a spectrograph - the collimating lens (like the eyepiece in a telescope) must be sized to accept the incoming beam at the telescope focal ratio and present a parallel (exit beam) beam to the diffraction grating.

Hope this helps.

 

 

[Charic]

By that method ALL and every one of your eyepieces would function at  f/10, and with so many focal lengths involved I cant see how this would work?
For now, I'm on the side that says there's no focal ratio involved with an eyepiece alone.

 

[Merlin66]

But would they function without vignetting in an f8 or f5 beam?

Again, in spectrographs the size of the collimator (= field lens in eyepiece) needs not only to match the focal ratio but must suit the size of the diffraction grating. ie a 50mm grating in an f5 system would need a 50 x 5 = 250mm focal length collimator, shorter focal lengths would not fill the grating, but longer focal lengths (>250mm) would.

So an eyepiece with a  field lens OK for f10 may not work well with a f4 or f5 input beam.

 

 

[Ruud]

59 minutes ago, Merlin66 said:

The eyepiece field lens must be large enough to embrace this expanding light cone. So you could say, that an eyepiece working successfully in a f10 light cone was functioning as an f10 eyepiece.

I don't think you could say that. Rather, if the field lens is not large enough the image will vignette, and in an extreme case the edge of the field lens becomes a very much out-of-focus field stop, with the original field stop invisible.

[Charic]

2 minutes ago, Merlin66 said:

But would they function without vignetting in an f8 or f5 beam?

I've no idea? Im just referring to the f/10  you mentioned below!

57 minutes ago, Merlin66 said:

..............at the same focal ratio that it entered. The eyepiece field lens must be large enough to embrace this expanding light cone. So you could say, that an eyepiece working successfully in a f10 light cone was functioning as an f10 eyepiece.

Therefore it was my assumption from your statement that if any eyepiece works successfully in an f/10 scope, and I'd suppose all and every eyepiece one chooses for their f/10  scope  would work successfully, then I can only conclude  from your text that they'll all be operating at f/10? or did I miss something?

I'm still on the no focal ratio side for now, lol

[25585]

Would stopping down the aperture of an eye piece make any difference, apart from darkening what is seen (possibly improving contrast)? Might the optical quality at the edges be improved, as for camera lenses? So by changing focal ratio, better suitability to telescopes of varying fl & fr is achieved?

The CN article BTW is linked to in the description for a 32mm Brandon on ebay. 

 

 

 

[Ruud]

Making the field stop smaller would make the view narrower.

Edit: And stopping down the field lens will cause the image will vignette. In an extreme case the edge of the field lens itself becomes a very much out-of-focus field stop, with the original field stop invisible. But I already said that.

Another edit: The purpose of the field stop is to limit the visible field to the circle that the eyepiece can show properly. If you remove a field stop you get a wider view with very poor edge correction.

[Merlin66]

The field stop is located at the image plane to define the largest image diameter visible (outside this diameter everything is blocked from view). This in itself doesn't reaaly have anything to do with the focal beam which would emerge from the image (ie from the field stop) to the field lens in the eyepiece.

The size of the field lens therefore must accommodate the "fastest" f ratio it's designed for - f10 or f5 or what....

Somewhere along the line the telescope f ratio comes into play with eyepiece design.........

[vlaiv]

I think that most eyepieces are designed to handle, in terms of light cone, quite ridiculous focal ratios.

Take for example Tele Vue specs on eyepieces here:

http://www.televue.com/engine/TV3b_page.asp?id=214

Even 8mm plossl has 6.5mm field stop diameter - that would make it accept F/1.23 beam, so I'm not sure that we should really be considering this in real life

[Charic]

.....its a pity other suppliers don't provide as detailed as product specification list, well laid out and informative.