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What the F?


Guest Turtleboy
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Guest Turtleboy

Another newbe question this time regarding focal ratios or "F" numbers.

As I understand it the lower the number the faster the scope and vice versa.

So a slower scope would take a longer time for imaging. But is that the extent of the difference? Does it make any difference for just observing or factor in to your choice of eye pieces?

I know my scope is an F13. Does this in any way effect what I will be able to see with it, or is that purely down to aperture size?

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This is from Celestron's website.

 

The focal ratio is the ratio of the focal length of the telescope to its aperture. It’s calculated by dividing the focal length  by the aperture (both must be in the same units). For example, a telescope with a 2032mm focal length and an aperture of 8" (203.2mm) has a focal ratio of 10 (2032/203.2 = 10) or f/10.

It’s variously abbreviated as f-stop, f/stop f-ratio, f/ratio, f-number, f/number, f/no., etc.

Smaller f-numbers will give brighter photographic images and the option to use shorter exposures. An f/4 system requires only ¼ the exposure time of an f/8 system. Thus, small focal ratio lenses or scopes are called “fast†and larger f/numbers are called “slowâ€. Fast focal ratios of telescopes are f/3.5 to f/6, medium are f/7 to f/11, and slow are f/12 and longer. 

Whether a telescope is used visually or photographically, the brightness of stars (point sources) is a function only of telescope aperture - the larger the aperture, the brighter the images. Extended objects will always appear brighter at lower magnifications. The main advantage of having a fast focal ratio with a visual telescope is that it will deliver a wider field of view than slower f-numbers.

 

It's the best I could find that explains it without frying your brain, however if you need a bit more then have a look at this, I did warn you!

 

http://en.wikipedia.org/wiki/F-number

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It's basically slow scopes such as F10 are perfect for splitting stars and lpooking at planets but not very good at widefield and star clusters as the slower the scope the narrower the field of view.


 


So....


 


F10, F12, F13 etc are great for binary stars, Lunar, and planets


 


F4.5, F5, F6 etc are great for DSO, star clusters, and widefield.


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Cheers Baz.

Yup. Brain now fried. :(

Ha ha :) it gets easier, don't worry-just enjoy the views.......when get some clear skies!

Edited by catman161
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Guest Turtleboy
It's basically slow scopes such as F10 are perfect for splitting stars and lpooking at planets but not very good at widefield and star clusters as the slower the scope the narrower the field of view.

 

So....

 

F10, F12, F13 etc are great for binary stars, Lunar, and planets

 

F4.5, F5, F6 etc are great for DSO, star clusters, and widefield.

Cheers Mick.

 

I understood that. :lol:

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But that doesn't mean to say if you have a fast scope you can't look at planets and vice versa.


 


I have a F6 and Jupiter looks awesome on a clear, still night.


 


(granted it would look better in a F12 etc but I`m just making a point.)


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Faster scopes tend to be less tollerant of cheaper eyepieces. So you could find yourself spending more money to get quality optics for a decent view in something under approx f-6. That's another consideration you might like to bear in mind. :)


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Guest Turtleboy
Faster scopes tend to be less tollerant of cheaper eyepieces. So you could find yourself spending more money to get quality optics for a decent view in something under approx f-6. That's another consideration you might like to bear in mind. :)

 

So at F13 I should be able to use an old marble as an eyepiece.

 

That's good news then. ;)

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The C8 although a fairly slow scope, is good as it has a flat field of view and no chromatic abberation.


As mentioned you compensate for lack of light grasp by increasing the amount of time you keep the shutter open, so to speak.


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It's planets, moon and sun photography the sct's are good at Steve. There's loads of light, the objects are all close, and with a long focal length slow scope you can get some very sharp, contrasty, detailed images. When the light is faint and distant you need aperture to capture it - and it captures slow so you want a fast f-ratio. This is the case with dso's. An f5 scope will gather the light from a galaxy at half the time of an f-10 scope.


 


Long focal length slow refractors are also very nice to image planets with. :)


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I also find this confusing, everybody raves about the c8 sct for astrophotography but its a f10? 

 

It's good at F10 if you are photographing the moon, or planets. If you add a reducer to make it either F6.3 or F3.3 then it's great at DSO's as well.

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Guest Turtleboy
I also find this confusing, everybody raves about the c8 sct for astrophotography but its a f10?

It's good at F10 if you are photographing the moon, or planets. If you add a reducer to make it either F6.3 or F3.3 then it's great at DSO's as well.

What are these reducers of which you speak? :wacko:

Edited by Turtleboy
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It's the opposite of a barlow - a 2x barlow effectively doubles the scope focal length (or halves the eyepiece fl). A reducer does the opposite so a .633 reducer will bring an f-10 down to f-6.3 :)


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Guest Turtleboy
It's the opposite of a barlow - a 2x barlow effectively doubles the scope focal length (or halves the eyepiece fl). A reducer does the opposite so a .633 reducer will bring an f-10 down to f-6.3 :)

Hang on. My head hurts.

So of I had a 20mm eyepiece with a 0.5 reducer it would give me a faster focus time than using a straight 10mm eyepiece? :mellow:

Edit.

Hang on that's the wrong way round.

If I had a 10mm eyepiece with a 0.5 reducer it would give me a faster focus than a 20mm EP?

Confused.com

Edited by Turtleboy
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I wouldn't worry about focal reducers unless you are going to get into serious astrophotography.

There is a lot to get your head around. Don't try to figure it all out at once, your brain will come out your eyes lol

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Guest Turtleboy
I wouldn't worry about focal reducers unless you are going to get into serious astrophotography.

There is a lot to get your head around. Don't try to figure it all out at once, your brain will come out your eyes lol

Your probably right. :lol:

Its this bloody weather that's driving me mad. If it was clear I'd be out there learning as I go instead of trying to get my head around the theory.

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Your probably right. :lol:

Its this bloody weather that's driving me mad. If it was clear I'd be out there learning as I go instead of trying to get my head around the theory.

To right. All I'm doing is looking at stuff I can't afford (says SWMBO)

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