Scope speed??

[craigfoot]

Hi all

Just wondering if anyone can help me with "speed"? If I'm on the right track. .

My skywatcher 130 is rated at f/6.92

But I'm not sure what this means or how it effects my viewing?

Cheers

Craig

[ScottS]

It's your focal length divided by aperture. A fast scope being a lower number.

[craigfoot]

And what exactly does "fast" mean? And what's the benefit?

[swamp thing]

Basically it won't effect your viewing at all.

Its only at the extremes of scope length will it effect viewing. With very long focal length (slow) scopes FOV can become an issue, and with very short focal length (fast) scopes, eyepieces must be well corrected else they will give poor performance. 

[ScottS]

Have a search for the thread "Telescope speed" there are some useful posts there.

[Uranium235]

As mentioned above, for viewing the speed doesnt really matter that much. But when it comes to imaging, photographic speed becomes the priority - so you will benefit from a lower f number (f5 or f4). However, faster speed comes with its problems (must be well corrected to be of any use).

[triton1]

Slower scopes are more forgiving on eyepieces whereas f5 and lower more money on ep,s and coma corrector aswell

[craigfoot]

So are you suggesting at almost f7 I can get away with poorer quality eye pieces?

[John]

So are you suggesting at almost f7 I can get away with poorer quality eye pieces?

F/7 is reasonably forgiving on eyepieces. The Baader Hyperions, for example, show most (not all) of the field of view as sharp stars in an F/7 scope but in an F/4.7 the outer parts of the field will show quite a lot of distortion.

Faster scopes tend to bring their own abberations too such as coma in newtonians and chromatic abberration in achromat refractors. On the flip side faster scopes can show wider fields of view so for some uses are very popular. Also an F/7 12" newtonian tube will be 7 feet in length !

[ronin]

At about f/7 your scope would to many be medium tending to slow.

Anything to do with the f number really relate to photography, so don't get too concerned.

10 is slow, 5 is fast, so as said 7 sits in the middle.

A "fast" scope sort of 5 or 6 will create a "small" image, this in turn creates a brighter image (all the light gets squeezed into it) thus in photographic terms you do not need as long an exposure.

Fast scope will tend to be short relative tpo the aperture, so there is curvature of the image plane. It is aspects like this that mean you need batter eyepieces when using a fast scope.

On a reflector the collimation is more critical on a fast scope, so if you get a fast scope get used to the idea of checking and adjusting collimation fairly often.

As said the f number is the focal length divided by the aperture.

So at 130 and 6.92 you should have an 899.6mm focal length scope, lets call it 900mm.

[craigfoot]

Yeah, its 900mm ☺ cheers

[Charic]

..........'Speed' is your telescope's focal length, divided by its aperture,  giving you your f/number, and is of little importance for visual observations. If you have a FAST/ER  telescope it simply implies that you have a shorter  focal length and a larger field.  Some image issues can be observed in faster telescopes, due to the tightness of the light cone, so selective eyepieces my be the order of the day to get a flatter view across the field. My Skyliner is an f/6 (its really f/5.91)  but any number lower than this is deemed to be FASTER, and an f/8 is said to be slower!  Is there a plate or label on your telescope showing the f/number, NO! Why? Not important!