Focal Ratios and eyepieces

[bomberbaz]

I have read on here a few times when discussing scopes and ep's people exclaiming that such a scope will be tough on your eyepieces.

What I am trying to figure is why from say F10 to F6 the effects of focal ratio is less dramatic than say F5.4 to F4.4. 

Anyone explain please?

Steve/Baz  

[ollypenrice]

Optics bend light. That is their role in life. When they do so they create artefacts. The more the light is bent, the more artefacts there are. Fast F ratios put big changes into the direction the light is travelling and getting this right is difficult and therefore expensive.

Olly

[ronin]

Curvature.

The image plane of an objective is simply not flat, it is curved and the faster the scope the greater the curvature for the same aperture.

Easy way to picture it is take a point at the centre of the objective and draw a circle of the focal length with the radius at that objective centre. That circle/arc is the focal plane.

All these opictures in optics books show it as flat and it is not.

So you need better eyepieces.

[bomberbaz]

Optics bend light. That is their role in life. When they do so they create artefacts. The more the light is bent, the more artefacts there are. Fast F ratios put big changes into the direction the light is travelling and getting this right is difficult and therefore expensive.

Olly

Going off this I drew a little diagram in paint and this makes sense to me.

They represent tubes at F10,6 & 4.5. The angle and length at which the light comes in from the F10 is not that much different from the sides to the centre. As you move down the F ratio that angle increases and at F4.5, this angle is going to cause aberrations.

Am I on the right track here?

[Charic]

The telescopes focal length divided by its aperture gives the result of your telescopes focal ratio. The diverging cone of light between the objective and the image plane is determined by this result. The longer and narrower the cone of light, the larger the focal ratio will be, lets say f/10. The result is fewer aberrations visible (in fact their just smaller)  and most eyepieces should work well with the telescope. The opposite is true, if you have a lower focal ratio say f/4, the eyepieces will need to be of special design to encapsulate the shorter wider light cone, aberrations become more apparent, requiring correction, but the good thing is that the surface brightness at the image plane will appear brighter at the lower f/ratio's.

We should know that the telescopes focal length divided by the eyepiece focal length gives the telescopes power or magnification. Your f/ratio will also determine the useful range of magnification that could be obtained with your telescope?

With higher f/ratio telescopes, lower powers and wider fields are difficult to achieve, yet with lower f/ratio telescopes higher powers are difficult to achieve? So depending on whether you need to get the highest magnifications or the widest views can have a bearing on the f/ratio you choose!

Its also often mentioned that  f/ratio is just the speed of your telescope,  the faster the better,  for photography, and that it doesn't matter  much for visual observations.  Well I would say it does matter for visual use, given what I explained above. 

[John]

I think F/6 is a good compromise focal ratio for a Newtonian scope for visual use. A sort of "Goldilocks" focal ratio, if you like. I'd love to see Skywatcher, Meade etc market a 10" F/6 dobsonian. You would not need coma correction, sub-premium eyepieces would deliver pretty good performance, decently wide fields would be possible with 2" eyepieces, the eyepiece would be at a good height unless you were very short or very tall, collimation tolerances would be reasonable, the secondary could be around 20% and a primary of 1/4 wave PV or better optical accuracy would be relatively easy to achieve consistently with modern manufacturing methods.

Are you listening Skywatcher / Meade ???

[Charic]

John........I'd order the Skyliner  250P  f/6.  No problem! Funny old thing, I read this earlier? http://www.rfroyce.com/10f6tel/

[timwetherell]

Any objective lens or mirror, produces what in optics is known as a real image. That is an image that you can see on a piece of paper if you place it at the focus. The job of the eyepiece is to magnify that real image floating in space in just the same way as a magnifying glass magnifies a photograph. Trouble is that  the objective doesn't create a flat field, it creates a curve. So even if the eyepiece is perfect the edges will be a bit defocused just like magnifying a bent photograph. Long focal length instruments like traditional refractors have a very slow f-ratio. So just like with a camera lens, at f22 there is a far greater depth of field that at f4. That depth of focus means that the focus zone of that real image in space is thicker than it is with a fast objective. So that any given curvature of the objective image plane or the eyepiece has a better chance of staying in the focus zone. 

If your eyesight is less than perfect you can see the same effect by making a small hole in a piece of paper. Looking through that, your vision will be dramatically better than without it because again, that long f-ratio increases the depth of field and "forgives" imperfections in your eye.

A absolutely perfect f4 scope is no better or worse than an absolutely perfect f22 scope. But the latter is far, far easier to make

[kev]

John. Could i dare to suggest that meade, celestron etc dont do f6 as standard precisely because you wouldnt need premium eyepieces etc? The smaller f also means the mount can be less robust.

Personnaly f6 to me makes sense. Mind you an f8 8 inch would be nice for planetary.

Sent from my iPhone using Tapatalk

[TiTanecd]

If higher f/ratios can deal with cheaper eye pieces and lower f/ratios require better eye pieces designed for the larger 'light cone' coming into the telescope, what happens when you use really good quality eyepieces in a really long f/ratio telescope. I assume that your better quality eyepieces will cater for the medium to low end f/ratio range of telescope, or am I wrong and better quality eyepieces work well across the range?

Tx

[Stu]

All these explanations are very good, but what they don't address is Steve's specific question which is why the sudden 'change' at below f5 say?

f10 to f6 seems to make comparatively little difference, but going below f5 seems to change things more rapidly.

Is there some form of exponential curve to the calculation which causes this sudden difference?

Stu

Btw, I have no idea what the answer is!!

[bomberbaz]

I think my diagram above goes some where to explaining it Stu. I work better with pictures. The angle of the light reflected back as shown by the red lines increases quicker as you head past F6 and below, this will I assume increase the object curvature faster and so the effect is multipled and this in term affects what you see with aberrations. 

Erm at least I think thats what it is 

Oh and agree with John about F6 scopes and tbh, an F5.5 12" scope wouldn't be out of the limitations of most peoples viewing height and I also think Kev might have something regards premium eyepieces demand might decrease if everyone had more forgiving OTA's! 

[John]

.....Is there some form of exponential curve to the calculation which causes this sudden difference?

Perhaps it's more to do with thresholds for perception of the optical issues that we human beings have, which might vary person to person, coupled with the exponential increase in the severity of the aberration as the optics of the scope get faster ?

Maybe each person has a sort of "trip point" at which something moves from being a non-issue into something that they are much more aware of ?

Also, if you are not aware of an issue I think you just get on and use the equipment. When you become aware of it you start to look more for it and "see" what perhaps was always there.

Trying other equipment at star parties can cause the above !

My belief is that the current specifications of imported scopes is driven as much by shipping considerations and manufacturing efficiencies as it is by anything else although you have to admire the fact that Synta (the manufacturer) can mass produce 250mm F/4.7 primary mirrors to a decent quality. A couple of decades ago such a mirror would be considered challenging and expensive to produce.

[Stu]

This may go some way to explaining it. If you plot focal ratio against the angle of incidence of the light then you get the attached graph which is hopefully correct!!

The highlighted point is f4, and the incidence angle really starts to ramp up at that point which confirms what Steve was saying with his diagrams.

I guess it shows that there is a valid reason why things start to get much harder very quickly at around this point because much more correction is needed.

From f16 to f10 is a 60% increase in angle

From f10 to f6 is a 66% increase

From f6 to f4 is a 50% increase

From f6 to f3 is a 100% increase

Stu

[Laurie61]

Perhaps it's more to do with thresholds for perception of the optical issues that we human beings have, which might vary person to person, coupled with the exponential increase in the severity of the aberration as the optics of the scope get faster ?

Maybe each person has a sort of "trip point" at which something moves from being a non-issue into something that they are much more aware of ?

This was the same conclusion I came to, I think there are several factors at work here. As we know aberrations build toward the field edge but illumination drops away at the same time, I think this tends to mask what's going on initially. As a scope gets faster and aberrations worse they reach a point where you can't miss them. I think light pollution can also hide the affects as the non focused light is hidden within background light ?  

[kev]

Mmm. Interesting point.

Sent from my iPhone using Tapatalk