What is....

[RedNyte]

I'm trying to understand what the F/? is? I have been looking around for a answer that I can understand. I have 2 scopes one is a refractor with a f/10 and a reflector that is a f/8. What does this mean. I know it has been discussed before just not finding it.

[Kaptain K]

F/ratio is the ratio of the focal length of the objective (lens or mirror) to its diameter. So, your refractor has a lens with a focal length that is 10 times the diameter and your reflector's mirror has a focal length 8 times its diameter.

[Tog_]

What Kaptain K said.

Also, a lower F number will mean lower magnification for the same Eyepeice. If your both scopes have a 6 inch lens/mirror, the f/8 will have a focal length of 48 inches, or about 1200mm. The f/10 will have a FL of 60 inche, or about 1524mm.

A 10mm EP will give you either 120x or 152x depending on which scope you use.

The lower the f number, the wider field of view you will have. This is becsue of the lower magnification. Lower f numbers will also tend to make brighter images. This is, in part, due to the lower magnification again.

A higher f number will usually be better for seeing detail in planet and splitting close binary stars. This is becasue the light isn't bent or reflected quite as much as with a faster (lower f number) system, which reduces the anomalies.

Note, that while the f number from a scope is similar to the f number in a camera lens, the two are just different enough that you really need to understand one before making any comparisons.

[Dave Mitsky]

Just so that there is no misunderstanding, a fast telescope does not produce brighter images simply by virtue of having a lower f/ratio. All telescopes of the same aperture are capable of producing the same image brightness as long as they are working at the same magnification.

http://www.televue.com/engine/page.asp?ID=141 Myth #3

A lower f/ratio means greater coma and more critical and more frequent collimation in the case of reflecting telescopes and increased chromatic aberration in the case of refractors.

Dave Mitsky

[JohnW]

Just so that there is no misunderstanding, a fast telescope does not produce brighter images simply by virtue of having a lower f/ratio. All telescopes of the same aperture are capable of producing the same image brightness as long as they are working at the same magnification.

http://www.televue.com/engine/page.asp?ID=141 Myth #3

A lower f/ratio means greater coma and more critical and more frequent collimation in the case of reflecting telescopes and increased chromatic aberration in the case of refractors.

Dave Mitsky
I'm sure Dave meant to say "... increased chromatic aberration in the case of refractors."

[redshifter]

C'mon Dave, you're slipping... :)

[RedNyte]

Just too make sure I'm understanding, The refractor would be better for close-ups, and the reflector would be better for the whole picture.

[redshifter]

Just too make sure I'm understanding, The refractor would be better for close-ups, and the reflector would be better for the whole picture.


Not necessarily, again it depends on the F ratio. A more accurate statement might be: a short F ratio would be better for the whole picture and a long F ratio would be better for close ups. Now, in practice reflectors tend to have shorter F ratios simply because it's much easier to get larger aperatures with a mirror (reflector) than a lens (refractor), though there are examples of short F ratio refractors on the market. Also, SCT scopes have both larger aperatures and long F ratios and 'fold' the light path such that the tube can be more compact.

Edit: Binoculars are great examples of short focal ratio refractors.

[Dave Mitsky]

C'mon Dave, you're slipping... :)

To err is human, to forgive divine.

Anyone who has to keep the schedule that I do is entitled to a few mistakes.

Dave Mitsky

[Dave Mitsky]

I'm sure Dave meant to say "... increased chromatic aberration in the case of refractors."

I did and it's been corrected.

Dave Mitsky

[Thanatos]

Agreed, but low f ratio means wider field of view and a shorter tube. f ratio becomes a serious issue with large [>10 inch aperature] scopes. Almost everyone loves light buckets, but dread lugging around and setting up anything with a tube more than about 4 feet long.

[hhEb09'1]

Agreed, but low f ratio means wider field of view and a shorter tube. f ratio becomes a serious issue with large [>10 inch aperature] scopes. Almost everyone loves light buckets, but dread lugging around and setting up anything with a tube more than about 4 feet long.Just saw Meade's LightBridge truss dobs over at the Sky and Telescope website (http://skytonight.com/equipment/home/3956786.html), heaviest part of the 12 inch is 35 pounds.