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Why is it called "fast"?


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Morning all.

My scope is an f5, and I understand what makes focal length, but why is it described as "fast"? What is fast about it?

Probably a dumb question, but if you don't know, ask! :)

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Unless you intend to use your scope for imaging, then the terms fast and slow don't mean too much. Fast telescopes (f5 or less) tend to demand more of your EP as the light comes in at a steeper angle to the EP than with a slower scope (f7 or above), so longer focul length scopes tend to show a better quality in an EP generally than faster telescopes will. However, there are some EP's that are designed to work better in faster telescopes. Stars in my f5 scope tend to deteriorate when towards approx 70% of the edge of the FOV, yet with my f11 scope the stars stay pin sharp right to the edge in the FOV.

If you want to do imaging then faster scopes are much preferred over slower scopes simply because you can gather images quicker with a faster telescope, so less problem with star trailing showing when imaging with a faster scope.

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Been wondering about this on and off myself over the past year , so thanks for this thread OP.

I've heard before that certain EPs are better for faster scopes, I'm guessing because of their particular characteristics?  If so, what should those of us with scopes of ~f5 be looking for in an EP?

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10 minutes ago, BeerMe said:

what should those of us with scopes of ~f5 be looking for in an EP?

Sharpness of stars, clarity, and uniformity from the center all the way out to the edge of the eyepiece. Cheaper eyepieces you will find start to tail off in the last 30% of the view - they become blurry and fainter than the center, and the view can contort due to aberration. So you find yourself nudging a lot more to keep the object in the center where it is at it's clearest.

More substantial eyepieces have multi lens elements which compensate for the contortions between lenses and towards the edges, as well as superior coatings to reduce internal reflections, whilst minimising light obstruction. It's not uncommon to find five or seven or more lenses in an expensive eyepiece. Hth :)

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36 minutes ago, brantuk said:

Sharpness of stars, clarity, and uniformity from the center all the way out to the edge of the eyepiece. Cheaper eyepieces you will find start to tail off in the last 30% of the view - they become blurry and fainter than the center, and the view can contort due to aberration. So you find yourself nudging a lot more to keep the object in the center where it is at it's clearest.

More substantial eyepieces have multi lens elements which compensate for the contortions between lenses and towards the edges, as well as superior coatings to reduce internal reflections, whilst minimising light obstruction. It's not uncommon to find five or seven or more lenses in an expensive eyepiece. Hth :)

Thanks for that info Brantuk, what I was more angling towards was should I be looking for EPs that have eye relief of 'x'mm, FOV of 'x'°, etc that makes an EP automatically better for a faster scope?

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10 hours ago, BeerMe said:

Thanks for that info Brantuk, what I was more angling towards was should I be looking for EPs that have eye relief of 'x'mm, FOV of 'x'°, etc that makes an EP automatically better for a faster scope?

Those sort of characteristics are not really indicative of optical quality. If the manufacturer has published the fastest focal length that an eyepiece is designed to work with you could use that with a pinch of salt as a rough guide. The best way is probably to look for reviews of eyepieces that you might be interested in to find out what other people have already found through testing. 

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Yes Rocochet is right. The point really is that, of any two eyepieces of the same size and fov, one may be cheap and the other expensive. The cheaper one will be awful in an f-5 scope whereas the expensive one will work like magic because it's made of superior glass, better coatings, better figured, and more corrective elements. The only way to tell is by directly comparing them, or reading reviews and personal reports. :)

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Actually, for telescopes up to 8"(200mm), and for visual use, the terms "fast" and slow", and "moderate" even, may most certainly be meaningful.

My 6" f/5 Newtonian is a "fast" telescope, and I know that I may expect low-power wide field views, and also that I may expect to enlist a barlow to reach the higher and highest of magnifications per its aperture.

A 150mm f/10 Schmidt-Cassegrain or a 127mm f/12 Maksutov are "slow" telescopes, and not given to low-power wide-field observations; also, a barlow is either optional or not required at all in order to reach the higher and highest of magnification as their apertures will allow.

A 200mm f/6 Newtonian on a Dobson mount is an example of a "moderate" telescope; and barlows are oft employed with same, but still optional per the user's interests.

Among refractors, my 4" f/8 apochromat is a "moderate" telescope; and I therefore know what to expect from it.

On the other hand, a 25" f/4 Newtonian on a Dobson mount is indeed a "fast" telescope, but with a relatively-extraordinary 2540mm focal-length.  A 200mm f/10 Schmidt has a focal-length of 2032mm; yet, per its aperture, it is most definitely a "slow" telescope.

Eyepieces are a constant; unchanging; static.  A 5mm orthoscopic will afford an observer who owns and operates five telescopes, of varying apertures and focal-lengths, five different magnifications or powers; but at the end of the day, that 5mm ocular is still a 5mm.

It's the oculars that make or break a telescope.

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1 hour ago, Alan64 said:

Actually, for telescopes up to 8"(200mm), and for visual use, the terms "fast" and slow", and "moderate" even, may most certainly be meaningful.

My 6" f/5 Newtonian is a "fast" telescope, and I know that I may expect low-power wide field views, and also that I may expect to enlist a barlow to reach the higher and highest of magnifications per its aperture.

A 150mm f/10 Schmidt-Cassegrain or a 127mm f/12 Maksutov are "slow" telescopes, and not given to low-power wide-field observations; also, a barlow is either optional or not required at all in order to reach the higher and highest of magnification as their apertures will allow.

A 200mm f/6 Newtonian on a Dobson mount is an example of a "moderate" telescope; and barlows are oft employed with same, but still optional per the user's interests.

Among refractors, my 4" f/8 apochromat is a "moderate" telescope; and I therefore know what to expect from it.

On the other hand, a 25" f/4 Newtonian on a Dobson mount is indeed a "fast" telescope, but with a relatively-extraordinary 2540mm focal-length.  A 200mm f/10 Schmidt has a focal-length of 2032mm; yet, per its aperture, it is most definitely a "slow" telescope.

Eyepieces are a constant; unchanging; static.  A 5mm orthoscopic will afford an observer who owns and operates five telescopes, of varying apertures and focal-lengths, five different magnifications or powers; but at the end of the day, that 5mm ocular is still a 5mm.

It's the oculars that make or break a telescope.

Alan, great post, I agree that from around f6 to f8 or even f9 are moderate speed scopes, but I have to disagree with your last point. Eyepieces never seem to be constant, unchanging, static.... the little blighters seem to be different everytime I look in the case! ;) 

On a more serious note, picking up your point about a 5mm Ortho being a constant, that sort of illustrates the point in a way. A good Ortho should, in theory, work in any scope, fast or slow. It has a narrow afov and will not show any horrid abberations even in fast scopes.

On the contrary, a low power eyepiece with a large afov may very well perform very differently in fast or slow scopes. In a slow scope it is likely to be fine, but may show plenty of aberrations in a fast scope if it has not specifically been designed to work at that 'speed'. An Ortho can be picked up for a very reasonable cost, but the sheer complexity of making an eyepiece like an Ethos which is tested down to f4 means that they get hugely expensive. The design, number of elements, figuring, coatings, assembly and QA don't come cheap.

I have my own way of remembering fast and slow which is meaningless in reality but helps me. I think if it as how quickly the scope has to change the angle of the incoming light beam to reach the focal point, sort of like a gentle or sharp turn on a road. A slow scope 'slowly' changes the angle so everything is quite easy to control. A fast scope has to turn the light 'quickly' so it is harder to control, and is likely to end up in a messy accident if the driver (the eyepiece) does not control it properly. Utterly meaningless, but just my way of thinking about it.

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A quick diagram:

F%20RATIO%20AT%20FIXED%20FOCAL%20LENGTH;

Because the focal lengths are unchanged both scopes take the same picture, but the one on the left has 4 times more aperture, collects four times more light and is, therefore, four times faster. Although we can say that the F ratios reflect this (fast F5 against slow F10) the active agent in all this is the aperture. In camera lenses the photographer does change the aperture by stopping down or opening up the diaphragm.

Note that if we change from the scope on the right to the one on the left we catch 4x as many M33 photons. But if we put a focal reducer on the scope on the right to bring it down to F5 we do not increase the number of M33 photons we collect. We concentrate them onto fewer pixels but, as ever, the main active agent in the story is aperture.

Olly

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11 minutes ago, ollypenrice said:

A quick diagram:

F%20RATIO%20AT%20FIXED%20FOCAL%20LENGTH;

Because the focal lengths are unchanged both scopes take the same picture, but the one on the left has 4 times more aperture, collects four times more light and is, therefore, four times faster. Although we can say that the F ratios reflect this (fast F5 against slow F10) the active agent in all this is the aperture. In camera lenses the photographer does change the aperture by stopping down or opening up the diaphragm.

Note that if we change from the scope on the right to the one on the left we catch 4x as many M33 photons. But if we put a focal reducer on the scope on the right to bring it down to F5 we do not increase the number of M33 photons we collect. We concentrate them onto fewer pixels but, as ever, the main active agent in the story is aperture.

Olly

What a great diagram, that really explains this in very simple terms.  Thanks Olly.

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9 minutes ago, RayD said:

What a great diagram, that really explains this in very simple terms.  Thanks Olly.

Glad it works for you. I've tried hard to work out how to get this right on beginner courses. If you allow two variables into the story you get into a mess. The problem with F ratio is that it is a derived variable arising from both aperture and focal length, so when F ratio varies it can vary because focal length has changed or because aperture has changed. This leads directly into a quagmire!

There is a popular misconception that a fast scope has to have a short focal length. The Keck has a staggering focal length of 17.5 metres and an equally staggering F ratio of 1.75. It's both very long and very fast. I dare say you could weld with it...

:Dlly

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15 minutes ago, ollypenrice said:

 

There is a popular misconception that a fast scope has to have a short focal length. The Keck has a staggering focal length of 17.5 metres and an equally staggering F ratio of 1.75. It's both very long and very fast. I dare say you could weld with it...

:Dlly

and very wide and very heavy.........want one :icon_biggrin:

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8 hours ago, ollypenrice said:

There is a popular misconception that a fast scope has to have a short focal length. The Keck has a staggering focal length of 17.5 metres and an equally staggering F ratio of 1.75. It's both very long and very fast. I dare say you could weld with it...

:Dlly

Bet that's a nightmare to colimate!

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