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Focal Ratio

Started by buzzman, 10/16/2007 02:53PM
Posted 10/16/2007 02:53PM Opening Post
Can someone fill me in on this subject. If I have a scope of the same aperture but I have one scope at F10 and one at F4, what would the difference be visually? I need to clear up this issue to decide a purchase. I plan on doing both visual and astrophotography with this scope.
Posted 10/16/2007 03:36PM #1
F ratio is a mathematical relationship between the focal length and the aperture. Simply divide the focal length by the aperture, and the result is a "F Ratio."

Just to oversimplify:
Longer F ratios (Higher numbers like F10) are easier to make well. They are simply less demanding. They are easier to collimate. They generally give more magnification for any given aperture. The tubes will be longer (for any given design). At times so long, they will be difficult to mount.

Shorter F ratios (Lower ratios like F4) are harder to make well. They require tighter collimation and focus and show coma (an abberation where the stars away from the center have a gull wing shape instead of a pinpoint. They do have a wider field of view, though, for any given focal length.

Fact is, you are trading field of view for magnification.

Imagine the light at the primary as the base of a triangle. A short, wide triangle is a low F Ratio. A tall, narrow triangle is a high F Ratio.

As for which one to buy----tell us what you wnat to do with it.

Posted 10/16/2007 05:46PM | Edited 10/16/2007 05:47PM #2
Hi Robert,

It's not a simple thing.

Focal length controls the size of the image at the focal plane. For instance, the sun and moon are about 1/2 degree in angular size. The size of the image at the focus of a telescope is about .009 times the focal length. In a 1000mm scope the size of teh moon's image is about 9mm. You can view the entire moon in any eyepiece with a field stop 9mm or larger. In a 3000mm scope, the size of the moon's image is about 27mm and it won't fit in our little eyepiece with only a 9mm field stop. It follows that for any given eyepiece, a long focal length scope provides more magnification than a shorter focal length scope. However, you can always change eyepieces to get higher magnification to match what you get with a longer focal length scope. You cannot always change eyepieces to match the low magniifcation and wide field images that you get with a shorter focal length scope.

It is important to know the size of an image if you plan on taking pictures because you can calculate the focal length needed to fit the target onto a ccd chip (or film if you are so inclinded). For that, you need to know focal length.

Aperature of a telescope determines the faintest star that can be detected and the resolution that can be obtained. Given two scopes with the same focal length, the scope with the greater aperature will show fainter stars and will also separate closer doubles. An 8" Newt will show lots more than a 4" refractor of the same focal length. (Hence aperature rules.)

The F-ratio attempts to say something about a scope that relates the focal length and aperature together. It is determined by teh focal length divided by the aperature. Given two scopes of the same F-ratio, extended objects such as nebulae, galaxies, sun, and moon, will appear to have equal surface brightness if viewed at the same magnification. However, the longer focal length scope will be showing a much larger image and will show more detail in that image. The image in the shorter focal length scope will have the same surface brightness but the image will be smaller and contain less detail. So, if you are looking at a faint fuzzy object in two scopes of the same F-ratio and you choose eyepieces so the image looks about the same size in each scope, the larger scope will show a brighter image with more detail. (Images were the same brightness until you magnified the one in the smaller scope to appear to be the same size as in the larger scope.) If you are imaging, the exposure time for targets is dependant on the surface brightness of the image produced and this is directly related to F-ratio, not aperature. However, image size is dependant on focal length.

hope this helps,
Posted 10/16/2007 09:00PM #3
Robert Buzanowski said:

Can someone fill me in on this subject. If I have a scope of the same aperture but I have one scope at F10 and one at F4, what would the difference be visually? I need to clear up this issue to decide a purchase. I plan on doing both visual and astrophotography with this scope.

Is one an SN and the other an SCT??

Posted 10/19/2007 12:07PM #4
It all depends on how much money you want to throw into the mount. Also, Jon is not entirely right about the barlow solving all magnification problems. In the special instance of photographing planets, if you have an f/4 instrument you need double barlows (like a 5x and a 3x) to get up to the f/40 and f/50 ranges imagers sometimes use. It's much easier to work with a long fr scope like an f/10 with one 5x barlow (for example).

If you're shooting deep sky it goes the other way, it is easiest to break into astrophotography with a fast scope (such as a small fast refractor) because it is most tolerant of tracking errors.

If the f/10 is an sct there are reducer correctors which will cut the focal ratio down to f/6.3 and at the same time flatten the field which is helpful.

So, in any case, as you move to a faster scope you won't have to spend as much on the mount to get good results on deep sky; tracking periodic error correction doesn't need to be as good for using a web-cam on planets, but if you're at f/50 it certainly is helpful to have precise hand paddle control, and precision typically means higher cost.

Now, as for visual applications, the faster scope can have a decisive advantage in wide field.

Mechanical issues need however to be considered. An f/4.5 dob can shake quite a bit in the wind, I often find them difficult to use. An f/10 SCT is often steady where the similar aperture f/4.5 is shaking (most of my experience is with an Obsession 15"). In a newtonian or refractor longer fr means longer length but in the bizarre world of SCTs you get shorter, and more squat, evne though it's f/10. In conditions of a breeze or wind the shorter scope will also have an advantage for imaging applications.

In terms of view, my friend's f/4.5 15" Obsession offers 1.6 degrees which is pretty good compared to my C14's puny 0.73 degrees. Nonetheless for wide field applications my side-mounted f/6.5 102mm refractor is a very viable alternative even with the smaller aperture.

What to make of all this?

Not much, except that no one scope meets all engineering criteria.

greg n

"Scope will get you through times of no money better than money will get you through times of no scope." --Freewheelin' Franklin