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Easy way to understand f ratio / 'speed' of a 'scope


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Does anyone have a easy way to remember which f ratio's are 'faster' or 'slower' than others and what that actually means?

If I see an advert for an eyepiece and it states 'best suited for telescopes with f ratios f6 or slower' this leaves me confused.

No matter how many times I read about f ratio's and 'speed' I still get confused.

A nice, simple & ( most importantly) easy to remember explanation would be welcomed. :(

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The f number is generally given by the focal length divided by the aperture diameter. Smaller f number is "faster".

Imagine you have two scopes of the same focal length, so would give the same field of view with a given eyepiece or sensor. If one scope has a bigger aperture than the other, the bigger aperture would collect more light and result in a lower f number.

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Short answer: the lower the f/ number, the faster.

The term "speed" only really makes sense in relation to imaging. A faster scope will get a brighter image, faster. For visual use, faster scopes basically mean you may want more expensive eyepieces and to spend more time aligning the mirrors (if it has).

It all boils down to the focal length and the aperture. Your Bresser newtonian has a 150mm aperture and a 1200mm focal length, so 1200 divided by 150 = f/8. That's SLOWER than your f/6.47 Nexstar so will be easier on eyepieces.

HTH

Andrew

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just thought of this off the cuff but Formula 1 (f1) is the fastest of all. faster means a shorter tube for the aperture and slower is the opposite.

Thanks for the quick reply and a great way to remember - the f1 analogy is brilliant, I won't forget that:D

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"Fast" and "slow" are terms that really relate more to photography and refer to (if you like) the speed at which the telescope can deliver photons to the image plane. If you need X photons to arrive in order to achieve a specific level of brightness at a given point, then a "fast" scope will reach that point more quickly than a "slow" one.

The focal ratio is the ratio of the focal length of the telescope to its aperture. A 1200mm focal length telescope with a 200mm aperture will have a focal ratio of 1200 / 200, or 6. A 1500mm focal length scope with a 125mm aperture will have a focal ratio of 1500 / 125 or 12. The lower the number, the "faster" the focal ratio. You'll sometimes see the focal ratio written as something like f/8. If f is the focal length, then what that actually tells you is that the aperture is f divided by 8.

To look at it another way, the telescope takes the light entering the aperture and concentrates it down to a size that will fit in through the pupil of your eye. If the focal length is long relative the to aperture, the optics don't have to do so much work to change the light path and the optics have an easy time of things. If however the focal length is quite short relative to the aperture then the optics have to alter the light path much more, which is much more difficult to do well and is less forgiving of average quality optics.

To look at the speed issue a different way, if you have two telescopes with a focal length of 1000mm say, and one is f/5 and the other f/10, then the first, the faster one, will have a 200mm aperture ( 1000 / 5 ) and the second will have a 100mm aperture ( 1000 / 10 ). For a fixed length exposure the amount of light each telescope can deliver to the same size image plane will be proportional to the cross-section of the scope which is the aperture diameter squared times pi divided by 4. To compare the two I don't need to work that out fully -- it's the same as the ratio of the squares of the aperture. So, in our fixed time period the first, short focal length, scope will deliver ( 200 * 200 ) / ( 100 * 100 ) or four times the amount of light to the image plane as the slower scope, which means your fixed level of exposure can be reached four times as fast, hence it's the faster scope.

James

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The focal ratio value is the focal length divided by the aperture

It's mainly useful for imaging (as a fast scope makes the image brighter and reduces exposure time), but does impact on eyepieces as a fast scope demands a better eyepiece. Focal ratio does not effect brightness for visual, only aperture does that (for the same magnification).

Low f-ratio is fast, high is slow.

Remember it like this;

low number = low image exposure time (a fast exposure)

high number = high image exposure time (a slow exposure)

so f6 or slower, would mean f6, f7, f8.....etc

F1 being fast analogy is great

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another thing to remember is that even though speed of a scope does relate mainly to imaging, there's also a real difference in my experience in terms of the ease at which planetary detail etc is visible.

I have a 16" f4 dobsonian and in theory this should provide super sharp, high resolution and very bright images of planets. on nights of excellent seeing, this is the case for sure. on averagely average nights in the UK, I find that putting in an aperture mask which effectively reduces the aperture to 170mm and creates a focal ratio of f11. The level of detail seen in the 170mm aperture is so much more defined, with more contrast and a more stable image. in the unmasked 16" aperture, the detail is certainly still there but it feels like you have to work harder for it.

I also have a 6" f11 dob and a 6" f5 dob and the views (at the same magnifications) are again like those described above.

so for me at least a slower focal ratio in the same aperture at least will provide more usable images on high contrast, bright images. on fainter images (i.e. not moon, planets and most doubles) the view will be exactly the same with the same aperture and magnification (i.e. focal length makes no perceivable difference to faint objects - to me ).

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Hi Moonshane,

Thanks for your description of different ratio scopes when visually observing planets. It's nice to learn from other peoples' experience :(

Do you think it would be worth me trying to stop down my 6" scope to provide better contrast when viewing the planets?

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I have two scopes with 114mm mirrors.

A 500mm tube and the other is a 900mm tube.

The 500mm (F4.3) lets me see millions of stars and nebula but the 900mm (F7.8) lets me see the planets with incredible detail. Both scopes are completely different even though they are the same width (focal length).

Short/fast tubes are great for galaxy hunting and longer tubes are better for planet hunting.

Thats a bit of a simple description, but it is what I find. If I could go back in time, I would not have bought the faster scope. Slower is incredible for me.

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I don't know if this is correct. I hope someone says soon! But there was one thread where someone explained fast and slow in a way I could understand. I've added a bit of my own.

Imagine you have a projector projecting the image onto a screen. If the projector is close to the screen (short focal length) the image is smaller but brighter. As the image is bright, if you were to then take a photo of that image, the exposure need only be quite short ('fast'). Fast scopes are therefore good for faint objects like deep sky objects (DSOs) as you get a lot of light in fast scopes.

If you moved the projector away from the screen (longer focal length) the image spreads out but so does the light so it's a darker image as the light is less concentrated. A photo exposure would need to be longer ('slow') to get a decent, bright image. So, I've been told, these scopes are better for planets which are very bright. But that's a simplified generalisation!

HTH

Alexxx

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another thing to remember is that even though speed of a scope does relate mainly to imaging, there's also a real difference in my experience in terms of the ease at which planetary detail etc is visible.

Hi Shane,

I would think that a large part of your experience is down to the differences in central obstruction of the systems, rather than the focal ratio. Your off-axis masked 16" will be free of secondary diffraction so will provide a cleaner image. Also, your slower 6" scope will have a smaller CO, again providing a cleaner image.

Another aspect is that larger apertures show up the seeing more, so seeing will appear worse at a given magnification in a bigger scope. This is because you are viewing a larger column of air.

I wouldn't think the differences you have experienced are directly affected by focal ratio, but rather by the optical design "side-effects" entailed.

I don't know if this is correct. I hope someone says soon! But there was one thread where someone explained fast and slow in a way I could understand. I've added a bit of my own.

That's quite a good way of seeing it, but remember this applies to imaging only. For visual, focal ratio has NO effect on image brightness of image. Aperture and magnification affect brightness.

Andrew

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Hi Shane,

That's quite a good way of seeing it, but remember this applies to imaging only. For visual, focal ratio has NO effect on image brightness of image. Aperture and magnification affect brightness.

Andrew

Thanks Andrew. But I, now, am confused. I assumed a smaller image would be brighter than a bigger image of the same object, with the same aperture. But I don't want to hijack the thread!

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Yes, that's right, but in visual that's a feature of magnification rather than focal ratio. Magnification is affected by the eypiece used, and in these comparisons you must always keep all other factors equal.

In imaging, a change in focal length, and hence focal ratio, will have the same effect on the size and brightness of the image on the chip, as magnification in visual.

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Hi Shane,

I would think that a large part of your experience is down to the differences in central obstruction of the systems, rather than the focal ratio. Your off-axis masked 16" will be free of secondary diffraction so will provide a cleaner image. Also, your slower 6" scope will have a smaller CO, again providing a cleaner image.

Another aspect is that larger apertures show up the seeing more, so seeing will appear worse at a given magnification in a bigger scope. This is because you are viewing a larger column of air.

I wouldn't think the differences you have experienced are directly affected by focal ratio, but rather by the optical design "side-effects" entailed.

Andrew

you may be right Andrew but I think it's the combination of all aspects coming together to give better visual images; smaller aperture = less brightness on the image so more apparent contrast, and as you say handles the seeing better. plus the central obstruction in my unmasked 16" is less than 20% by diameter so according to theory should not have any effect upon image quality (although I am not 100% convinced about this) so I do actually think focal ratio matters.

after all planetary specialist scopes (Maks and SCTs), many of which have very large secondary obstructions, have a slow focal ratio and still produce good results.

unfortunately I don't know enough about optics to be totally clear.

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  • 6 years later...

Fast or Slow,  for me, the focal ratio of a Newtonian scope can also offer some guidance for the end user when choosing  eyepieces?

Choosing an eyepiece that matches the telescopes focal ratio will provide you with your high power eyepiece.
Choosing an eyepiece of double the focal ratio often provides the best from your scope, a  sort of 'sweet spot' as it were?
Finaly, multiplying the focal ratio by your eye/s  entry pupil will provide a suitable focal length for your  wide angle, low powered eyepiece.

The type or eyepiece branding is of your own personal choice and taste, but this method will provide a set of three eyepieces that should be optimal for the scope in question.


 

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Great Zombie thread Charic :)

I think your logic is sound for larger newts, but for smallish apo or long focal length refractors, venturing into sub 1mm exit pupils does give you a benefit in image scale and perceived detail I think. A decent 100mm frac will easily exceed x200 under good conditions, requiring 0.5mm exit pupil or an eyepiece of half the focal ratio. Floaters certainly come into play at and below thi a sort of level, and I now find my aself Binoviewing at these levels to reduce the effects.

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