A question about F/Ratings

Shano592 (Shane) · #1 11-02-2009, 09:15 PM

I am still gathering information on the scope(s) I will eventually get ...

Having looked, and looked, and read whatever I can find, I am not 100% sure on what the F/rating has to do with anything scope-related.

I understand that a 200mm diameter, 800mm focal length scope has an F/4 rating, but what does this mean for me, the very average user?

Is an f/4 telescope necessarily better or worse than an f/10 one? I am assuming that the f/number is listed for a reason on your average telescope.

Experts, the floor is yours, with thanks ...

Terry B · #2 11-02-2009, 09:25 PM

As you state the F number is just the ratio of the focal length and the diameter.
For visual use the f number is not very important. The overall light collecting ability (Diameter) determines the brightness of the image. Stars are point sources of light. The magnification you see of extended sources is determined by what eyepiece you use.
For photography the F number determines the field of view. For a constant diameter scope the one with the lower f number will have a larger field of view. This will concentrate extended sources (ie nebulas and galaxies) into a smaller area making them appear brighter on the image. The trade off is that they are smaller but for big objects this is good. For little galaxies or nebulas it isn't as good.

Omaroo (Chris Malikoff) · #3 11-02-2009, 09:26 PM

Hi Shane

The term is f/ratio, and is not a rating, as such, but the numeric raio of focal length divided by the diameter of the lens or iris opening. So, your example is correct in that a 200mm diameter, 800mm focal length scope is going to deliver an f/ratio of f/4.

Here's a paragraph off Wiki for reference:

http://en.wikipedia.org/wiki/F-numbe..._in_telescopes

Quote:

Focal ratio in telescopes

Diagram of the focal ratio of a simple optical system where f is the focal length and D is the diameter of the objective

In astronomy, the f-number is commonly referred to as the focal ratio (or f-ratio). It is still defined as the focal length f of an objective divided by its diameter D or by the diameter of an aperture stop in the system.

Even though the principles of focal ratio are always the same, the application to which the principle is put can differ. In photography the focal ratio varies the focal-plane illuminance (or optical power per unit area in the image) and is used to control variables such as depth of field. When using an optical telescope in astronomy, there is no depth of field issue, and the brightness of stellar point sources in terms of total optical power (not divided by area) is a function of absolute aperture area only, independent of focal length. The focal length controls the field of view of the instrument and the scale of the image that is presented at the focal plane to an eyepiece, film plate, or CCD.

For example, the SOAR 4m telescope has a small field of view (~f/16) which is useful for stellar studies, whereas the LSST 8.4m telescope, which will cover the entire sky every 3 days has a very large field of view (f/1.2), due to a special optical design.

Cheers
Chris

Shano592 (Shane) · #4 11-02-2009, 09:32 PM

Thanks to Terry and Chris.

Both answers have cleared things for me.

Looks like I may need a couple of scopes ....

Don't tell the wife.

BerrieK · #5 11-02-2009, 09:48 PM

Shane why dont you GET a scope for the wife?

Shano592 (Shane) · #6 11-02-2009, 10:12 PM

Because, what's mine is hers, and what's hers is hers .... I thought you knew this?

toryglen-boy (Duncan) · #7 12-02-2009, 08:03 AM

Quote:

Originally Posted by Terry B

For visual use the f number is not very important.

ya think?!?!?

i would say that for visual astronomy the F ratio is still important, maybe not as important as photography, but still rather important anyway

The F "size" will help to govern such things as eyepiece magnification, field of view, the actually physical size of the OTA, on a reflector it will have a big input on how easy it is to collminate your scope, and how much coma it suffers from, and on a refractor it has alot to say about the amount of CA.

i would say get as short as possible without compromising on quality, for a reflector i wouldnt get anything shorter than F6, anything less and colmination has to be exacting, and coma might start being a hassle, although the trade off is a wider field, and a more portable scope.

its up to you !!

rider · #8 12-02-2009, 08:21 AM

There is a related spin off to F ratio, and it has to do with go-to mounts and field size for eyepiece mm.

With a high f value, , it is much less likely that the requested go-to star will be in the field of view (Unless you have access to a VERY low magnification wide-angle eye piece).

for example, I have an f14 Mak. my wife has a f4.7 Newt. and both my EQ5pro go-to and her Argo Navis Push-to are much nicer to use with the F 4.7 because you don't have to chase a go-to object that is just out of view.

...and this is even when I use a 40mm 72degree EP with the mak.

Rider2

Terry B · #9 12-02-2009, 08:43 AM

Quote:

Originally Posted by toryglen-boy

ya think?!?!?

i would say that for visual astronomy the F ratio is still important, maybe not as important as photography, but still rather important anyway

The F "size" will help to govern such things as eyepiece magnification, field of view, the actually physical size of the OTA, on a reflector it will have a big input on how easy it is to collminate your scope, and how much coma it suffers from, and on a refractor it has alot to say about the amount of CA.

i would say get as short as possible without compromising on quality, for a reflector i wouldnt get anything shorter than F6, anything less and colmination has to be exacting, and coma might start being a hassle, although the trade off is a wider field, and a more portable scope.

its up to you !!

I agree absolutely but I was trying to be relatively simplistic and only considering the actual view through the scope,

toryglen-boy (Duncan) · #10 12-02-2009, 08:44 AM

Quote:

Originally Posted by Terry B

I agree absolutely but I was trying to be relatively simplistic and only considering the actual view through the scope,

fair enough then !!

ausastronomer (John Bambury) · #11 14-02-2009, 09:31 PM

Quote:

Originally Posted by toryglen-boy

ya think?!?!?

The F "size" will help to govern such things as eyepiece magnification, field of view

Hi,

This is not correct. It is a very common mistake. These two things are solely dependent on the focal "length" of the telescope, not it's F-ratio. The F-ratio is a function of the focal length and some people incorrectly attribute the change in magnification and FOV when the F-ratio changes, to the change in F-ratio, but it actually happens because the focal length changes.

eg. a 10"/F5 telescope with a 10mm eyepiece has the same magnification and FOV as a 5"/F10 telescope, with the same 10mm eyepiece. Same focal length = same magnification and FOV, but not the same F-ratio.

Cheers,
John B

toryglen-boy (Duncan) · #12 14-02-2009, 09:36 PM

Quote:

Originally Posted by ausastronomer

Hi,

This is not correct. It is a very common mistake. These two things are solely dependent on the focal "length" of the telescope, not it's F-ratio. The F-ratio is a function of the focal length and some people incorrectly attribute the change in magnification and FOV when the F-ratio changes, to the change in F-ratio, but it actually happens because the focal length changes.

eg. a 10"/F5 telescope with a 10mm eyepiece has the same magnification and FOV as a 5"/F10 telescope, with the same 10mm eyepiece. Same focal length = same magnification and FOV, but not the same F-ratio.

Cheers,
John B

i know all this,

thats why i was very careful to say F-"size" meaning focal size, and length, and made sure i didnt say F-ratio