Slow scope, fast scope...what does it all mean?

[i Drew]

Hail SGLers

Ok for the past few weeks now I've been on the forum I've been reading fast scope and slow scope being mentioned. Someone mention to one of my post that my 6se which is f/10 is a slow scope?... anyway honestly I am a bit confuse so please explain What makes a fast scope and what makes a slow scope? Why does one need to know if your scope is slow or fast? Is it all about how long the light travels on your telescope? Thanks!

[MarkMayf]

Good question! All I know is, the lower the f number the faster the scope. But I don't know why!

[Ganymede12]

That's a question I'd liked answered too!

When will the intelligent ones get here!

[cantab]

The terms "fast" and "slow" in this sense are photographer's terms. A photographer is most interested in the image scale - with no eyepieces involved, a long focal length lens gives a "zoomed in" telephoto view, while a short focal length lens gives a wide angle view. With two lenses with the same focal length, the "fast" one will require a shorter shutter speed to get the same image brightness as the "slow" one. The faster lens will have a bigger aperture.

For a visual observer it's less important, but fast optics tend to show more aberrations than slower ones. Some eyepieces don't work well in fast scopes, for example, and the ones that do tend to be more expensive. For Newts and refractors, fast optics mean a shorter tube. With a slow scope it can be hard to get wide-angle views since eyepieces have an upper limit to their own focal length imposed by their barrel diameter, too long and the field of view is restricted giving no benefit. (The exact maximum depends on eyepiece apparent field as well as barrel diameter, but it's 32mm for a 1.25" format Plossl for example.)

The f number is simply the focal length divided by the aperture. f/10 is slow compared to Dobsonians in particular, but it's similar to a lot of refractors and faster than some Schmidt/Mak-Cassegrains.

Edited August 2, 2012 by cantab

[barkis]

The terms fast and slow are basically related to astrophotography, much as in terrestrial photography.

The fast instruments are of short focal length, and the slow, are longer focal lengths.

Short focal scopes will produce wider fields of view of the sky, and long focal lengths narrower fields of view.

For a given eyepiece FL, a fast scope will give a lower magnification, and a slow scope will give a higher magnification.

The magnification math, is simple the telescopes focal length, divided by the eyepiece focal length.

That is a rough outline of how it works, but I'm sure a more robust explanation will be forthcoming.

Ron.

[Capricorn]

Fast or Slow is a photographic term.

There is a number that is defined as:

FN = (Scope Focal Length) / (Scope objective diameter)

Actually think it is strictly the other way but easier like this.

If the number you get is around the 5-6 mark then the scope is termed Fast

If the number is 8 and above then it is termed Slow.

Between 6 and 8 call it what you feel like.

Below 5 it is Very Fast,

The number as said is a carry over from photography, effectively the Faster the scope the less exposure time you need.

The shutter can move faster and get a picture.

To us in astromony it can be more a measure of how "easy" the scope works.

As a fast scope will have greater curvature on a lens this will cause more problems, Chromatic Aberration and Spherical Aberation being the main.

On a Reflector ther is more Coma and as the secondary can move the alignment (Collimation) is more critical.

Basically a Fast scope is more difficult to produce and can show more errors.

An F/5 achromat will show a fair degree of CA, an achromat of f/10 will show very little.

Fast scope tend to need better eyepieces the slow scopes.

[WaveSoarer]

I puzzled about this too. A quick google found this page:

http://science.howstuffworks.com/telescope18.htm

The explanation is:

"Focal ratio or f/number relates to the brightness of the image and the width of the field of view. The focal ratio is the focal length of the objective lens or primary mirror divided by the aperture. The focal ratio concept comes from the camera world, where a small focal ratio means a short exposure time for the film, and was said to be "fast." Although the same is true for a telescope, if a "fast" and a "slow" telescope are compared at the same magnification for visual rather than photographic viewing, then both telescopes will have the same quality image."

There are some other interesting pieces of information there too. I expect that the boundary between Fast and Slow is rather fuzzy and for telescope use is perhaps a little unhelpful.

[WaveSoarer]

Wow. Some people are incredibly fast in posting.

[i Drew]

Wow. Some people are incredibly fast in posting.

haha that's why I liked this forum from the beginning, quick response and intellectual. Anyway, now I understand thank you for the response.. again the reason why i asked because I was asking is it was mentioned on my post where I was asking about eyepiece. Thanks guys!

[MarkMayf]

Excellent answers. Knowledge broadened again. Thank you!

[brantuk]

"I expect that the boundary between Fast and Slow is rather fuzzy and for telescope use is perhaps a little unhelpful."

Generally f-7 or below are considered fast scopes - anything above is considered slow - this is the "arbitrary" boundary.

The way it helps is quite straightforward. Imaging dso's with a telescope requires long exposures - the lower the focal ratio, the less time you will need to take an exposure. You can easilly double the number of subs taken in the time available during a session with a fast scope (or halve the time taken for a set number of exposures) - than when using a slow scope.

Another way it helps is with observing - if you have a fast dobsonian (eg f-5 SW 300P fl=1500mm) cheaper eyepieces like the supplied ones will offer poorer views than with a high quality well corrected eyepiece. This can be as significant as the difference between a £20 eyepiece and a £100 eyepiece. So it can save you money too picking the right f-ratio scope for you.

Also - longer focal length scopes (with a higher focal ratio) are sharper and crisper on planets/moon/sun and give very pleasing views with mid range eyepieces that don't cost an arm and a leg. So you'll save on eyepiece costs and amputation medical fees lol

[Ant]

There are a few threads that explain this...

http://stargazerslounge.com/topic/7921-primer-focal-lengths-and-ratios/

But it does get very very complicated... which is touched upon in this thread - http://stargazerslounge.com/topic/20542-questioncomments-on-primer-focal-lengths-and-ratios/

Enjoy

Ant

[knobby]

Imagine the telescope is a funnel and you are using it to fill a bottle with water ... (please, please don't try this in reality) if your scope was short and wide the water would flow 'fast' ... if it was thin and long then the water would flow 'slow'

Substitute light for water and the bottle for your eye, bobs your uncle.

Very crude / basic analogy but it works on my kids

[JasonHohman]

Does the f/ratio have anything to do with how fast you have to "track" the image in the scope's view?  For example do you have to adjust the position of the scope faster to keep the image in view?  I am a newbie and have a F/10 3" chepo scope, an F/4.4 4.5" chepo, and a nicer Class X 10" scope that I'm thinking is a F/5.5ish maybe?  I haven't worked out my camera hookups yet but that is my goal.  Will be attempting looking at the eclipse tomorrow.  Thanks for any input!!

[brantuk]

10 hours ago, JasonHohman said:

Does the f/ratio have anything to do with how fast you have to "track" the image in the scope's view?

Only in terms of field of view. With a slow scope you have a narrower field of view and an object will travel across the fov in less time than a fast scope with a wide fov. However, if you're using an alt/az mount with tracking motors, the motors will be working to track in two planes. An  equatorial mount which is accurately polar aligned will only be tracking in a single plane cos RA is handled by the spin of the Earth and mechanically it's much easier - especially for astro photography. The moon is very close compared to dso's so a slow scope will give more magnification and brighter views. If it's a manual mount though you'll need to be turning the slow mo controls quite frequently to cope with the narrow fov. Hth

[JOC]

Errr.......Let me try..........

A slow scope is a 16" OTA that takes 45 minutes to lug out of the shed with a willing neighbour to where you want it and by the time you get it there the clouds have rolled over and you need to take it inside again.

A fast scope is one that you can race in, grab and dump on the table/ground outside to take advantage of that 10 minute break in the clouds.    

Yes??

[ollypenrice]

On 8/4/2012 at 16:44, knobby said:

Imagine the telescope is a funnel and you are using it to fill a bottle with water ... (please, please don't try this in reality) if your scope was short and wide the water would flow 'fast' ... if it was thin and long then the water would flow 'slow'

Substitute light for water and the bottle for your eye, bobs your uncle.

Very crude / basic analogy but it works on my kids

...but if it was wide and long the water would flow just as fast as it would in an equally wide short scope. And what does that bring us to? Oh no, it brings us with perfect precision to the F ratio myth!

Olly

[Skinnypuppy71]

In the photography world..f stop is related to the amount of light a lens let's into the camera sensor...f1.8 very wide aperture (fast lens)and enabling faster shutter speeds f22 very narrow aperture and slower shutter speeds and as focal length increases so does the length of time the shutter needs to gather the required amount of light....it pretty much translates over to telescopes in much the same way...but instead of sensors...we use our eyes.

A f5 telescope is also related to the amount of light it gathers.....a 750mm focal length scope with a 150mm(6inch)aperture...750mm divided by 150mm =5 the f number of the scope..so for this scopes focal length /aperture,it is a fast scope it allows more light gathering ability than say a 1500 mm scope with the same aperture....1500 divided by 150=10....f10...the focal length of this scope related to it's aperture let's less light through and therefore the image will be less brighter than the f5 scope . you'd have to double the aperture of the 1500mm scope for the same light gathering ability of the 750/150mm scope...but in turn it'd be able to retrieve the light from much dimmer objects with it's longer focal length this would then become a very expensive scope I'd imagine....especially in refractor from.lol.I hope I have done a decent job of explaining as I'm pretty new to telescopes myself but more in tune with camera's and lenses.lol.

Edited January 31, 2018 by Skinnypuppy71

[Juan from Madrid]

4 hours ago, Skinnypuppy71 said:

...
it allows more light gathering ability than say a 1500 mm scope with the same aperture....
...

From what I've been learning recently (mainly in this great forum, I must say), the amount of light depends on the aperture, period. Same aperture, same light gathering ability.

The issue here is related to AP when trying to obtain the same picture with different telescopes.

We should obtain the same field of view with telescopes that have the same focal length, let's say for example, 1500 mm.

These very same 1500 mm could be obtained with a 300 mm aperture telescope, an f/5, or with a 150  mm aperture one, an f/10.

The first one gathers 4 times the light of the second one (4 times the area), so for the very same picture the f/5 telescope requires 1/4 the exposition time of the f/10 one, hence one is named "fast" and the other one "slow".

Edited January 31, 2018 by Juan from Madrid

[laowhoo]

On 8/2/2012 at 16:34, Capricorn said:

 

Between 6 and 8 call it what you feel like.

 

That's called the sweet spot where everything works well and looks good 

[CodySydney]

On 03/08/2012 at 06:34, barkis said:

Short focal scopes will produce wider fields of view of the sky, and long focal lengths narrower fields of view.
 

I know this is a ridiculously old post but this was a real dawn-of-realisation moment for me.  I've been trying to figure out why focal length has anything to do with lens speed... doesn't aperture = more light?  And your post finally got through to me.. of course! focal length -> narrow field of view -> less light = how did I not get this until now.  So I hope you're still living the dream and thanks!

[Louis D]

4 hours ago, CodySydney said:

I know this is a ridiculously old post but this was a real dawn-of-realisation moment for me.  I've been trying to figure out why focal length has anything to do with lens speed... doesn't aperture = more light?  And your post finally got through to me.. of course! focal length -> narrow field of view -> less light = how did I not get this until now.  So I hope you're still living the dream and thanks!

Assuming a fixed aperture.  If aperture is allowed to grow in proportion to focal length (constant f-ratio), you get the same amount of light but at a higher magnification.  That's the draw of big, fast Dobs.  You get increased image scale at the same image brightness.  That, and increased resolution, so it's a win-win proposition all around.

[AstroMuni]

On 31/01/2018 at 01:27, JasonHohman said:

Does the f/ratio have anything to do with how fast you have to "track" the image in the scope's view?  For example do you have to adjust the position of the scope faster to keep the image in view? 

Simple answer is that tracking is following objects as they move across the sky and that speed is constant for stars and different for the moon. So your F ratio has no bearing on the speed. If you had the same eyepiece on fast vs slow scope all that would change is the FOV as someone has pointed out already. 

Edited August 20, 2021 by AstroMuni

[Louis D]

4 hours ago, AstroMuni said:

Simple answer is that tracking is following objects as they move across the sky and that speed is constant for stars and different for the moon. So your F ratio has no bearing on the speed. If you had the same eyepiece on fast vs slow scope all that would change is the FOV as someone has pointed out already. 

You responded to a "member" who posted exactly that one post in 2018 and never visited again.

[AstroMuni]

17 minutes ago, Louis D said:

You responded to a "member" who posted exactly that one post in 2018 and never visited again.

I should read the timing of the posts too 🙂