Focal ratio of an eye piece?

[martinl]

In addition to the excellent explanations and clarifications above, I should point out that the terminology used in the CN quote is unfortunate. The word “critical” is typically used in physics and engineering as the point where the system moves from one state to another. There is a critical velocity for a fluid in a pipe, for example. Below the critical velocity the flow is laminar, above it will be turbulent and behave very differently. 

Eyepieces don’t have a critical focal ratio. They don’t work perfectly with an f/10 telescope but fall apart in an f/9. 

[25585]

43 minutes ago, Charic said:

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

Of sellers I use, Teleskop-Express seem to always provide everything I want to know, most importantly eye relief.

TV are great though. I appreciate the info on their optics, most recently Paracorr 2 settings....

[jetstream]

14 minutes ago, martinl said:

In addition to the excellent explanations and clarifications above, I should point out that the terminology used in the CN quote is unfortunate. The word “critical” is typically used in physics and engineering as the point where the system moves from one state to another. There is a critical velocity for a fluid in a pipe, for example. Below the critical velocity the flow is laminar, above it will be turbulent and behave very differently. 

Eyepieces don’t have a critical focal ratio. They don’t work perfectly with an f/10 telescope but fall apart in an f/9. 

VG point.

I think those theoretical spot diagrams are a useful tool to evaluate potential eyepiece performance and I wonder why many makers ie Televue and ES don't publish theirs? I can't find them anyway.

Personally I use a good barlow with my fastish newts and Abbe orthos to get a more friendly light cone for the orthos- and it seems to work well. I would really like to see the spot sizes for the Pentax XW,Delos, Ethos, Docter UWA etc...

 

 

[Ags]

17 hours ago, 25585 said:

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. 

 

 

 

You stop down an eyepiece by putting it on a slower scope. You can easily see this effect if you imagine the eyepiece as a simple 1 element design. The faster the telescope objective, the bigger the exit pupil (keeping scope focal length constant).

[martinl]

6 minutes ago, jetstream said:

VG point.

I think those theoretical spot diagrams are a useful tool to evaluate potential eyepiece performance and I wonder why many makers ie Televue and ES don't publish theirs? I can't find them anyway.

Personally I use a good barlow with my fastish newts and Abbe orthos to get a more friendly light cone for the orthos- and it seems to work well. I would really like to see the spot sizes for the Pentax XW,Delos, Ethos, Docter UWA etc...

 

 

Probably because every wannabe optics expert on the internet would over analyse them, ignoring aspects like surface accuracy, polishing, stray light control, quality control etc. (Not me, of course, but those other people.)

But I agree. It would be interesting to see. 

[25585]

1 minute ago, martinl said:

Probably because every wannabe optics expert on the internet would over analyse them, ignoring aspects like surface accuracy, polishing, stray light control, quality control etc. (Not me, of course, but those other people.)

But I agree. It would be interesting to see. 

As we all know, specs are what we have to go on generally. Hearsay, advertising & magazine reviews being careful to not to offend advertisers.

Otherwise brand loyalty, experience etc. Very little to try out physically & optically, let alone compare one make after another.

So online discussion & advice from the opticmages here on SGL are invaluable & very welcome

[jetstream]

16 minutes ago, martinl said:

Probably because every wannabe optics expert on the internet would over analyse them, ignoring aspects like surface accuracy, polishing, stray light control, quality control etc. (Not me, of course, but those other people.)

But I agree. It would be interesting to see. 

Yes, the execution of the eyepiece is a big consideration. Its nice to know the potential of each eyepiece design from the diagrams and I'm sure if they did well then they might be used for advertising...

ps the word in the coffee shop is that the Delites have nice tight little spots

 

[martinl]

7 minutes ago, jetstream said:

ps the word in the coffee shop is that the Delites have nice tight little spots

 

So I’ve heard. I’ve never seen the diagrams myself, though, but my 7mm DeLite is quite lovely...

[25585]

43 minutes ago, martinl said:

So I’ve heard. I’ve never seen the diagrams myself, though, but my 7mm DeLite is quite lovely...

Outside the TV universe, what are the Delites' main rivals in respect of specs?

[martinl]

2 minutes ago, 25585 said:

Outside the TV universe, what are the Delites' main rivals in respect of specs?

I’m not sure there are any. The Pentax XF and ES 62 degrees, perhaps, but neither have the reputation for optical excellence that the DeLites enjoy. You also have the older TV Radians (yes, still TV) that are supposed to be similar, except the very early examples that are known to show excessive lateral CA. I haven’t used either of them, though. 

[25585]

23 minutes ago, martinl said:

I’m not sure there are any. The Pentax XF and ES 62 degrees, perhaps, but neither have the reputation for optical excellence that the DeLites enjoy. You also have the older TV Radians (yes, still TV) that are supposed to be similar, except the very early examples that are known to show excessive lateral CA. I haven’t used either of them, though. 

Can't think of a gap in my ep fl roster that could be filled with a Delite. A 15mm just possibly squeezed in.

http://www.skyatnightmagazine.com/review/eyepieces/tele-vue-delite-eyepiece-range

 

[Paz]

On 1/27/2018 at 13:25, 25585 said:

But what do you divide an ep's FL by to calculate it's focal ratio? 

The literal answer is the exit pupil of the image.

The working focal ratio of the eyepiece always equals the focal ratio of the scope it is being used with. This is because the more "work" the scope is doing (the steeper the light cone) the more work the eyepiece has to do at its end. This all works out as it does because the exit pupil for a given eyepiece is determined by the focal ratio of the scope it is being used with. I've had a go at the maths below.

Focal ratios are the same for eyepieces as for telescope objective lenses so for a telescope objective:

Focal Ratio = Focal Length of Scope / Aperture or say R=F/A

For an eyepiece:

Working Focal Ratio = Focal Length of Eyepiece / Exit Pupil Diameter or say r=f/e

The working focal ratio of the eyepiece happens to equal the focal ratio of the telescope it is being used in because...

exit pupil diameter = Scope Aperture / Magnification or say e=A/M and...

Magnification = Focal length of scope / Focal length of eyepiece or say M=F/f

If you describe exit pupil size in terms of A and M you get:

r=f/(A/M) = fM/A

if you then describe M in terms of the focal lengths of the scope and eyepiece you get:

r=(f/A)(F/f) = fF/Af

which simplifies to:

r=F/A

you will also recall 

R=F/A

so after all that.....

R=r

i.e. the focal ratio of the scope equals the working focal ratio of the eyepiece or putting it another way the amount of work the scope is doing defines how much work the eyepiece has got to do. A hard working scope (a fast focal ratio) required the eyepiece to work hard.

Older eyepiece designs like Keplers were designed to work at f30 or even longer(!), modern eyepieces are designed to work down to (or even below) f5. As has been said if you push an eyepiece beyond its design intentions it will struggle.

[Louis D]

On 1/28/2018 at 12:40, 25585 said:

Outside the TV universe, what are the Delites' main rivals in respect of specs?

I would feel quite comfortable putting the older Pentax XLs up against them.  My 5.2mm has amazing polish, stray light control, and is sharp from edge to edge.  They're only 3 degrees wider than the Delites, so comparable in that respect.  Perhaps they need updated coatings, but that's about it.

[Sporadic Dobstronomer]

On 27/01/2018 at 23:24, Charic said:

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

I can't resist the temptation to put an explanation in my words...

The difference between the two ends of a telescope is that light from, say, a star passes through the whole surface of the objective but passes through only a small part of the eyepiece -a different part for each star visible.  That means the eyepiece inherits its f/number from the objective (as long as the exit pupil is no bigger than your eye pupil).

[Charic]

On 30/01/2018 at 23:47, Sporadic Dobstronomer said:

I can't resist the temptation to put an explanation in my words...

The difference between the two ends of a telescope is that light from, say, a star passes through the whole surface of the objective but passes through only a small part of the eyepiece -a different part for each star visible.  That means the eyepiece inherits its f/number from the objective (as long as the exit pupil is no bigger than your eye pupil).

Not sure what your saying here? plus, any light  can only reflect from my mirror, not having two ends!

The only article I find on the web written by Robert Morein https://www.astromart.com/reviews/article.asp?article_id=764 suggests that ALL eyepieces have a critical focal ratio, and specifically for Abbes and Brandon's it's  f/7?  I believe that this number is a random?  preventing those EPs from performing their best on the faster telescopes! 

It's still my understanding , that if you venture way past the telescopes focal ratio by using an eyepiece with a smaller focal length than the focal ratio, most EPs will provide less than perfect images?
 
I'm off the fence now! the eyepieces themselves  not having some official focal ratio or an equation for finding a focal ratio?

I still often mention that  when buying new EP's, if you  stick with the focal ratio of the scope itself, and match that number closely to the focal length an eyepiece, you won't go far wrong with an eyepiece, providing high power, that matches the performance and ability of the scope in question, doubling the ratio for a medium power eyepiece and trip....No! multiplying the focal ratio by the size of your dilated pupil, providing you with a  low powered wide angle eyepiece.

There you go, three practical and suitable eyepieces, especially if its your first scope, and your needing to upgrade!
Now you just need to find eyepieces that are both affordable and comfortable in use having the correct amount of eye relief and a field of view suitable for your needs. Other EP's are  optional, but do offer the end user more choices.

[Louis D]

I wonder if critical focal ratio for eyepieces refers to the ratio at which the axial spot exceeds the e-line airy disc as seen in the spot diagrams in figure 213 on this webpage.  You can see that for Abbes, the spot is well contained at f/10, but spilling at f/5.  The same thing happens for a Plossl which is very closely related to the Brandon.  It's quite possible it is exactly contained at F/7 for both, making that the critical focal length.

[Sporadic Dobstronomer]

12 hours ago, Charic said:

Not sure what your saying here? plus, any light  can only reflect from my mirror, not having two ends!
...
It's still my understanding , that if you venture way past the telescopes focal ratio by using an eyepiece with a smaller focal length than the focal ratio, most EPs will provide less than perfect images?
 ...
 

1. Think refractors because they are easy to visualise.  The type of telescope makes no difference in this matter.

2. If the eyepiece has a focal length equal to the objective's f/ratio you will get a magnification of about 25x per inch in any telescope.  If you have typical eyes, this is about the maximum physically possible before over-magnification starts to make the image visibly blurred.

But this is a different matter from blurring caused by using an eyepiece in a too-fast telescope.

Suppose you consider two telescopes with equal focal lengths but one has an objective lens twice the size and therefore has half the f/ratio.  In that faster telescope the beam of light from any one star will be twice as wide as it passes through the eyepiece.  The width of that beam dictates the effective f/ratio of the eyepiece which turns out to be equal to the f/ratio of the objective.

The beam being wider means any optical aberrations in the eyepiece will do more damage (I read somewhere that it goes with the cube meaning the aberrations will be 8x as bad but I don't know for certain).

Because this worsening of aberrations seen in an eyepiece increases quite sharply with decreasing f/ratio there will be, almost suddenly, an f/ratio below which the image gets noticeably blurred.  This must be what they mean by "critical" f/number.  The precise value of minimum f/number that is acceptable will be a matter of personal taste.

Modern eyepieces such as the newest TeleVue designs have less aberration and therefore can be used in very fast telescopes before all this starts to matter.

[Louis D]

7 minutes ago, Sporadic Dobstronomer said:

Suppose you consider two telescopes with equal focal lengths but one has an objective lens twice the size and therefore has half the f/ratio.  In that faster telescope the beam of light from any one star will be twice as wide as it passes through the eyepiece.  The width of that beam dictates the effective f/ratio of the eyepiece which turns out to be equal to the f/ratio of the objective.

Are you sure?  The larger diameter telescope will collect four times as many photons per area on the sky and so the star will appear brighter, but it will still be a point source.  Perhaps due to sensor bloat it might appear bigger because individual detectors could become saturated and nearby detectors might pick up on the image as well from spill over.  Bright stars appear wider than dimmer stars to both sensors and the human eye, but they are still point sources.  The atmosphere also plays a role in scattering bright stars' light making they appear larger.

If you were talking about extended object like a planet, then you would have been absolutely correct about the size of the object.

[vlaiv]

 

8 minutes ago, Louis D said:

Are you sure?  The larger diameter telescope will collect four times as many photons per area on the sky and so the star will appear brighter, but it will still be a point source.  Perhaps due to sensor bloat it might appear bigger because individual detectors could become saturated and nearby detectors might pick up on the image as well from spill over.  Bright stars appear wider than dimmer stars to both sensors and the human eye, but they are still point sources.  The atmosphere also plays a role in scattering bright stars' light making they appear larger.

If you were talking about extended object like a planet, then you would have been absolutely correct about the size of the object.

I don't think that Sporadic Dobstronomer is talking about "width" at focus point, he is rather talking about converging beam.

But, I don't think that is important. Eyepiece is not "concerned" with width of beam but angles. Bigger angles are harder to control and that is why some eyepieces work better with slow scopes - they are unable to handle big angles that fast scopes are producing. Same thing happens with stars close to field edge opposed to stars on optical axis. Eyepieces tend to have poorer correction in outer part of field compared to center. But if you think about it, same "width" of light cone is hitting center and outer field, it is the angles that differ.

[Sporadic Dobstronomer]

18 minutes ago, Louis D said:

Are you sure?

Yes.  The light from a star is only a small almost-point at the focal plane.  It converges from the objective to that point and diverges again as it continues on its way into the eyepiece before being focused to a point again by your eye

[Sporadic Dobstronomer]

5 minutes ago, vlaiv said:

...But, I don't think that is important. Eyepiece is not "concerned" with width of beam but angles. ..

That is one way of looking at it.  I wrote in terms of the width of the beam within the eyepiece because I think it is easier to visualise.

[Charic]

58 minutes ago, Sporadic Dobstronomer said:

 .......dictates the effective f/ratio of the eyepiece which turns out to be equal to the f/ratio of the objective.

Going back the the original post....

On 27/01/2018 at 13:25, 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?

 

..........I still cant accept or understand how you put it! that an eyepiece can 'adopt' the scopes focal ratio?  when a focal ratio is just the result of dividing the focal length by the aperture,  be that of a telescope or camera lens.

I do accept how an eyepiece could be 'dictated' by the scopes focal ratio, just exceed it?  but still not sure of your explanation.
There's no formulae used or referenced that calculates solely, the focal ratio for an eyepiece alone, as far as I'm aware, which should answer 25585's original post.

[Sporadic Dobstronomer]

22 minutes ago, Charic said:

 

...I still cant accept or understand how you put it! that an eyepiece can 'adopt' the scopes focal ratio?...

There's no formulae used or referenced that calculates solely, the focal ratio for an eyepiece alone, as far as I'm aware, which should answer 25585's original post.

Focal ratio of telescope = focal length / width of beam entering the objective
Focal ratio of eyepiece = focal length / width of beam through the eyepiece from any one star

It so happens that these are measurements of the two sides of the same cone of light.

The "width" of the beam here would only be simple to measure if you have a telescope with one thin lens for its eyepiece but the principle is valid for any eyepiece.  In that simple case, the "width" is the diameter of the spot of light hitting the eye lens from any one star.  It is also equal to the exit pupil.

The beam from any one star does not pass through the whole width of the eye lens so for any given focal length the width of the beam through the eyepiece is determined by the f/number of the objective and not by the eyepiece itself.

The beams from different stars pass through different parts of the eye lens so that lens needs to be much bigger than is implied by its effective f/number.

Hopefully I have now found words that work...