Fast telescope

[Neil H]

Hi guys I know this is going to get technical, a fast telescope is F6 or lower hope that's right 

What makes it a fast telescope?

Why does it make it a fast telescope?

Why buy a fast telescope over a medium?

P!ease try and keep it easy to understand thank you 

Edited December 21, 2019 by Neil H

[teoria_del_big_bang]

Maybe this can explain better than I could:

How Telescopes Work

 

Also some good answers on this old thread:

slow-scope-fast-scopewhat-does-it-all-mean

 

Steve

[alacant]

6 hours ago, Neil H said:

a fast telescope is F6 or lower

No mention of cameras. Phew, that was close!

Hi

If a fast telescope is one which collects a lot of light, then no.

A 200mm diameter f6 telescope collects more light than an 80mm f6 telescope, so the former is faster.

AFAICT, that's it.

HTH

[Mr Spock]

1 hour ago, alacant said:

A 200mm diameter f6 telescope collects more light than an 80mm f6 telescope, so the former is faster.

No, that's wrong. A scope working at f2 is fast, a scope working at f10 is slow for example. Speed has nothing to do with aperture or light collection, it is the focal ratio.

[Neil H]

Ok so it's focal , but why it's  called a fast scope of its focal length 

[dannybgoode]

15 minutes ago, Neil H said:

Ok so it's focal , but why it's  called a fast scope of its focal length 

The focal ratio determines whether it is a fast or slow scope and is calculated by dividing the focal length but the aperture. 
 

So my refractor has a focal length of 650mm and an aperture of 105mm so it’s focal ratio is f6.2 and is a moderately fast scope. 
 

My Skywatcher Newtonian has a focal length of 750mm and an aperture of 150mm so has a focal ratio of f5 and is moving toward the realms of fast. 
 

My Meade 10” SCT has a focal length of 2500mm and an aperture of 250mm so has a focal length of 250mm so has a focal ratio of f10 so is a slower scope. 
 

Note of my three scopes the one with the longest focal length is the slowest. 
 

For visual observations the focal ratio doesn’t particularly matter. For astrophotography, a scope with a focal ratio of f5 will collect light twice as quickly as a scope of f10. So, you could take subs of say 3 minutes instead of 6. 
 

The focal length though has an impact of field of view in respect of astrophotography and magnification in respect of visual. 
 

The magnification of a scope is calculated by focal length of scope/focal length of eye piece. 
 

So to achieve a magnification of 250x on my 10” SCT I only need a 10mm eyepiece but of my  105mm I would need a 2.6mm or so. 
 

Short focal eyepiece are expensive to make, especially to achieve high image quality, 10mm ones not so much. 
 

Again for my Meade I can use my 2” 24mm and have lovely wide field 100x mag views that are pin sharp and with nice long eye relief. 

 

[andrew s]

Following the terminology used with camera lenses the smaller the f number the "faster" it is. This comes from the ability to stop down a camera lens, reducing the aperture, and with a fixed focal length increasing the f number. As less light gets in the smaller aperture you need to increase the exposure compared the fully open aperture. Hence it is slower.

Its use in telescope can be confusing and is much debated on here as the focal ratio myth.

Regards Andrew 

[Neil H]

Ok so in lay man terms a fast scope draw light in faster to the mirror then  eyepiece , so this now makes me ask this 

If you have a fast telescope the eyepiece needs to be very good as well ?

[Louis D]

1. Fast relates to photography.  Smaller focal or "f" ratios (the focal length divided by the aperture in matching units) require shorter exposures to reach a given image density so images can be taken "fast"er.  That's all fast means in this context.
2. The light isn't being drawn in faster, it's just creating a brighter, per unit area, image.  This is unrelated to magnification which is strictly a function of the focal length.  Thus, a 1000mm focal length f/4 telescope will be able to achieve a denser/brighter (more photons collected per unit area) image in a given amount of time than a 1000mm focal length f/8 telescope.  However, the magnification would remain the same.  The magic here is that the objective must be twice as large in diameter to collect four times as much light for the f/4 telescope as compared to the f/8 telescope.  Thus, the f/4 scope will create the image "faster" by collecting more photons per unit of time.  The downside is increased optical tube bulk, weight, and cost.
3. For a given aperture, the smaller the f-ratio, the shorter the focal length and generally the shorter the physical tube.  Visually, fast scopes are nice because, for a given aperture, you have a shorter focal length and thus can achieve wider true fields of view.  They're also more compact because the physical tube is shorter which makes them easier to mount and transport.
4. It is possible to use a focal reducer near the focal plane and have a smaller f-ratio than the physical tube length would imply.  However, the image circle is also reduced by doing so.  You're basically squeezing the photons down to a smaller image circle.  That's why you can't use 2" eyepieces behind an SCT focal reducer.  The image circle has been compressed to much less than the initial 46mm diameter.  TANSTAAFL
5. Fast scopes are much more difficult to make to the same optical level of excellence as slower scopes.  For Newtonians, the figure of the mirror is more difficult to polish and the increased coma is more difficult to control at the eyepiece.  However, you can more easily observe with a 20" f/3 Dob than with a 20" f/5 Dob because the eyepiece is about 60" off the ground compared to 100" for the f/5.  For refractors, color dispersion is extremely difficult to control in shorter focal ratios.  In fact, as aperture goes up, the focal ratio must go up as well in order to maintain the same level of color control without changing glass types or number of objective lenses.  That's why APO telescope prices increase very non-linearly with linearly increasing aperture at a constant f-ratio.
6. Eyepieces need to have a more sophisticated design to deal with shorter focal length scopes because the light cone is converging fairly steeply near the edge.  That's why TV Ethos eyepieces cost so much.  It requires a sophisticated design along with special glass types to successfully bring together all of the various ray bundles along with their various associated wavelengths of light into their proper positions at the exit pupil across a 100 degree apparent field of view.  Older eyepiece designs generally have narrower AFOVs and were intended for slower focal ratio scopes common at the time of their design.  Older, wider eyepiece designs sacrificed good edge correction for their wider AFOVs.  Wider AFOVs were important in military applications to avoid tunnel vision.  Motion and contrast could still be perceived in the blurrier outer field, allowing the user to swing the optic around to center the object of interest as in binoculars and periscopes.
7. Why are steeply converging light cones difficult to control?  That's actually a fairly technical subject that I have only a rudimentary understanding of.  Someone like @Peter Drew could probably explain it better.

[Alan64]

Hi Neil,

In so far as Newtonians, a faster and fast telescope requires corrective and therefore more costly eyepieces.  At f/5, this 150mm Newtonian of mine is considered fast...

...but I don't have any particularly expensive eyepieces for it, and I get great views without them.  Although, when you start going up in aperture at f/5, the eyepieces required for distortion-free views become more complex and therefore expensive.  A 300mm f/5 Newtonian is another animal compared to my 150mm f/5.  I can get by with cheaper eyepieces, but the owner of the 300mm f/5 would need much more corrective, and expensive, eyepieces.  Then, wide-angle eyepieces are more costly than the standard Plossls.  The more costly and wider-angle eyepieces oft contain more lenses to effect the corrections, and the wider view.

A 200mm f/6 Newtonian-Dobson, or "Dobsonian", can get by with less-expensive eyepieces.  But a 250mm f/5 Newtonian-Dobson is going to require the more costly for distortion-free views.  It's the steeper curve of the faster primary-mirror of a Newtonian that requires correction.  A 250mm f/5 might even require a coma-corrector in addition.  The faster the Newtonian, the worse the coma.  Coma is evident when the outer edge of the view of an eyepiece shows stars that are not round, but appear as streaks, lines or "teardrops" instead.  Coma can also affect the center of the view, rendering it less sharp.

With medium-to-slow Newtonians, like our 114mm f/8 Newtonians, the need for corrections are not needed.  However, everybody wants a short-compact telescope nowadays; the shorter the better.  They're easier to handle, store, and are travel-friendly.  Faster telescopes are more costly to manufacture, their lenses and mirrors.

With refractors and Newtonians, what you see is what you get.  If the telescope has a short tube, it will be faster, and will have a shorter focal-length.  If a longer tube, it will be slower, and with a longer focal-length...

Those are the two, main staples of amateur-astronomy: refractors and Newtonians.  They are also the oldest designs of telescopes, 1608, and 1668, respectively.

The only other design you will encounter, generally, when shopping for a telescope, is a catadioptric: either a Maksutov-Cassegrain, or a Schmidt-Cassegrain.  They came about much later, in the 1940s, but are based upon the classical-Cassegrain of the 1600s.  This is my 127mm f/15 Maksutov-Cassegrain...

It has a short tube, as you can see, but it has a very long focal-length inside: 1900mm(!).  In the cases of Maksutov- and Schmidt-Cassegrains, what you see is NOT what you get.  Both are slow telescopes, the Maksutov being very slow.  Therefore, they can use the most inexpensive eyepieces on the planet, particularly the Maksutov, and for sharp, pleasing views.  I suspect that some are drawn to them for their short tubes(easy to manage, store, and travel with), but upon arrival quite a few new purchasers encounter more than they had bargained for, or less rather, and in the form of narrow, drinking-straw like views of the sky, even at the lowest power.  The interior of a Maksutov, its folded focal-length, in action...

But such telescopes are excellent for views at high magnifications; for the Moon, the planets, and the double-stars.

Edited December 22, 2019 by Alan64

[joe aguiar]

F10 would be medium

F12 to f20 is slow

F3 to f5 fast

F7 to f10 med

Joejaguar 

Edited December 22, 2019 by joe aguiar

[Stu]

8 hours ago, Louis D said:

1. Fast relates to photography.  Smaller focal or "f" ratios (the focal length divided by the aperture in matching units) require shorter exposures to reach a given image density so images can be taken "fast"er.  That's all fast means in this context.
2. The light isn't being drawn in faster, it's just creating a brighter, per unit area, image.  This is unrelated to magnification which is strictly a function of the focal length.  Thus, a 1000mm focal length f/4 telescope will be able to achieve a denser/brighter (more photons collected per unit area) image in a given amount of time than a 1000mm focal length f/8 telescope.  However, the magnification would remain the same.  The magic here is that the objective must be twice as large in diameter to collect four times as much light for the f/4 telescope as compared to the f/8 telescope.  Thus, the f/4 scope will create the image "faster" by collecting more photons per unit of time.  The downside is increased optical tube bulk, weight, and cost.
3. For a given aperture, the smaller the f-ratio, the shorter the focal length and generally the shorter the physical tube.  Visually, fast scopes are nice because, for a given aperture, you have a shorter focal length and thus can achieve wider true fields of view.  They're also more compact because the physical tube is shorter which makes them easier to mount and transport.
4. It is possible to use a focal reducer near the focal plane and have a smaller f-ratio than the physical tube length would imply.  However, the image circle is also reduced by doing so.  You're basically squeezing the photons down to a smaller image circle.  That's why you can't use 2" eyepieces behind an SCT focal reducer.  The image circle has been compressed to much less than the initial 46mm diameter.  TANSTAAFL
5. Fast scopes are much more difficult to make to the same optical level of excellence as slower scopes.  For Newtonians, the figure of the mirror is more difficult to polish and the increased coma is more difficult to control at the eyepiece.  However, you can more easily observe with a 20" f/3 Dob than with a 20" f/5 Dob because the eyepiece is about 60" off the ground compared to 100" for the f/5.  For refractors, color dispersion is extremely difficult to control in shorter focal ratios.  In fact, as aperture goes up, the focal ratio must go up as well in order to maintain the same level of color control without changing glass types or number of objective lenses.  That's why APO telescope prices increase very non-linearly with linearly increasing aperture at a constant f-ratio.
6. Eyepieces need to have a more sophisticated design to deal with shorter focal length scopes because the light cone is converging fairly steeply near the edge.  That's why TV Ethos eyepieces cost so much.  It requires a sophisticated design along with special glass types to successfully bring together all of the various ray bundles along with their various associated wavelengths of light into their proper positions at the exit pupil across a 100 degree apparent field of view.  Older eyepiece designs generally have narrower AFOVs and were intended for slower focal ratio scopes common at the time of their design.  Older, wider eyepiece designs sacrificed good edge correction for their wider AFOVs.  Wider AFOVs were important in military applications to avoid tunnel vision.  Motion and contrast could still be perceived in the blurrier outer field, allowing the user to swing the optic around to center the object of interest as in binoculars and periscopes.
7. Why are steeply converging light cones difficult to control?  That's actually a fairly technical subject that I have only a rudimentary understanding of.  Someone like @Peter Drew could probably explain it better.

Great post Louis, I reckon that covers it! 👍👍

[Stu]

Love this scope Alan, which model is it?

[Neil H]

Thanks guys for your time I will have to read it a few times for it to sink in but it has answered it 

[Alan64]

9 hours ago, joe aguiar said:

F10 would be medium

F12 to f20 is slow

F3 to f5 fast

F7 to f10 med

Joejaguar 

Newtonians: f/3 to f/5(fast), f/6 to f/7(medium), and f/8+(slow)

Refractors: f/4 to f/6(fast), f/7 to f/11(medium), and f/12(medium-slow) to f/20(slow)

Maksutovs/Schmidts: f/10 to f/15(slow)

Edited December 22, 2019 by Alan64

[Alan64]

2 hours ago, Stu said:

Love this scope Alan, which model is it?

Stu, that's a Celestron "AstroMaster" 70/900.  It has a dreadful, proprietary focusser, and the mount that came with it, a CG-2(EQ-1), is too small for it.  I have had it up to 225x, with little image-breakdown, on Polaris. 

[joe aguiar]

7 hours ago, Alan64 said:

Newtonians: f/3 to f/5(fast), f/6 to f/7(medium), and f/8+(slow)

Refractors: f/4 to f/6(fast), f/7 to f/11(medium), and f/12(medium-slow) to f/20(slow)

Maksutovs/Schmidts: f/10 to f/15(slow)

That's what I said

[Alan64]

3 hours ago, joe aguiar said:

That's what I said

Well, I simply narrowed it down a bit.  Each and every telescope should be evaluated individually.  How would you classify this 32"(812mm) f2.8; f/2.8, yet with a focal-length of 2276mm...

...keeping in mind that eyepieces range, in general, from 4mm to 40mm.  I would also think that, if on a motorised equatorial platform, that a camera's CMOS sensor would take its sweet time collecting those precious photons of light.

[joe aguiar]

It's based on focal ratio not focal length so f2.8 is fast

[mattjanes100]

Off topic but just to say this thread confused me for a minute when a user with Mr Spock as their avatar was in conversation with somebody with the username Mr Spock!

Some very useful information posted here, thanks all.

[Alan64]

3 hours ago, joe aguiar said:

It's based on focal ratio not focal length so f2.8 is fast

"Slow" and "fast" are photographic terms.  They do not apply to telescopes, per se; only when a camera is combined or contemplated with a telescope.  If you were to attach a camera to that 32" f/2.8 Newtonian, would an image be collected fast, or slowly, given the telescope's 2276mm focal-length?

[Louis D]

18 minutes ago, Alan64 said:

"Slow" and "fast" are photographic terms.  They do not apply to telescopes, per se; only when a camera is combined or contemplated with a telescope.  If you were to attach a camera to that 32" f/2.8 Newtonian, would an image be collected fast, or slowly, given the telescope's 2276mm focal-length?

For a given ISO, at exactly the same rate as any other f/2.8 camera lens or telescope.  The only difference would be the magnification based on the focal length.

[Alan64]

11 minutes ago, Louis D said:

For a given ISO, at exactly the same rate as any other f/2.8 camera lens or telescope.  The only difference would be the magnification based on the focal length.

If I understand correctly, then the photographic speed has nothing to do with focal-length, but rather the focal-ratio?

In any event, when dealing with a telescope only, no photography, we should instead refer to the focal-lengths only as "short" and "long"; likewise to the focal-ratios, instead of "fast" and "slow".  Although, we are in the bad habit of describing telescopes solely for visual-use as being "fast" or "slow", up to a certain aperture, say, less than that of 300mm.

Edited December 23, 2019 by Alan64

[Stu]

5 hours ago, Alan64 said:

Although, we are in the bad habit of describing telescopes solely for visual-use as being "fast" or "slow", up to a certain aperture, say, less than that of 300mm.

The descriptions fast and slow do have some relevance for visual, namely the amount of coma in a newt and impacts on eyepiece astigmatism etc particularly for wide afovs. So long as you don't misunderstand and believe that you would get a brighter image at the same magnification with a faster scope then they are useful terms still I think.

[johninderby]

Lots of threads already on this subject. 

https://stargazerslounge.com/topic/132599-fast-telescope-slow-telescope/

https://stargazerslounge.com/topic/157758-slow-scope-fast-scopewhat-does-it-all-mean/

https://stargazerslounge.com/topic/76973-telescope-speed/