Focal Ratio

[kd001]

Can somebody please explain exactly what effect the focal ratio has? I understand that higher focal ratios are easier on the eyepiece. Looking at the Skywatcher Dobsonian range, those with larger aperture tend to have a smaller focal ratio. So I was wondering is it better to have a telescope with a large focal ratio (ie the 8" dob with F/5.9) or one with a smaller focal ratio but a slightly larger aperture (ie the 10" dob with F/4.8).

Thanks in advance for clearing this up,

Kevin

[jnp]

The focal ratio provides an indication of the brightness of images. Mathematically it's the focal length divided by aperture. All numbers in mm. e.g a 5inch (130mm) Newtonian with 650mm is 650/130 =5.

You are correct that high focal ratios are easier on the EP, this is because the angle of light is shallower. Some EP are not designed with short ratios in mind so may have aberations at the edge of view.

I can see that the focal lengths are roughly the same for the two scopes you mentioned (4.8 = 1200/250 5.9=1180/200 ). This means you will get about the same magnification, but the 10" mirror will be some 56% brighter ober the 8".

Once we start talking about large mirrors, having high focal ratios makes them harder to build,mount and use. (and a step ladder may be needed to look through the EP). If the 10" mirror had the same 5.9 focal ratio as the 8" then it would need to be much longer to accomodate a 1475 focal length. This can make the difference when it comes to portability.

[SteveP]

Kevin

As jnp says, focal ratio is a means of combining two of the critical parameters in scope design, ie aperture and focal length. The first determines the light gathering capability of the scope and the second its magnification capability. The bigger the aperture, the more ickle photons will be collected and typically the brighter the image will be. The longer the focal length, the larger the image will be for the same sizeeyepiece. If you're considering a dob, I'd be looking at the aperture as the more important ... together with the size/weight of the scope. If you can't lug it around easily, it won't get used.

In your example, the 10" has about 50% more light gathering capacity than the 8" (5"squared/4"squared) ... wish I knew how to do mathematical symbols!

Does that help?

[albedo0.39]

The focal ratio provides an indication of the brightness of images.

Are you sure ? I knew this was the case with camera lenses, but specifically i.r.o. telescope I understood that F ratio had no relationship to image brightness. It does however give an indication of the amount of "work" the mirror has to do to form an image on the eyepiece.

Long F ratio - shallow angles everywhere, long tube [everything else being equal]

Short F ratio - steep angles, short tube [more convenient ?]

Happy to be corrected.

Steve

[jnp]

The analogy with a camera is valid. If you keep the same focal length and reduce the aperture, the f ratio increases and the image becomes darker as only a smaller part of the lens is being used.

Same applies with mirrirs. You can convert a 10" f4.8 to an 8" f5.9 if place a piece of card with an 8" hole over the 10" mirror. Alas it wont work th eother way around

[arad85]

Can somebody please explain exactly what effect the focal ratio has?

...

Thanks in advance for clearing this up,

Kevin

OK.. my take on this. Think of a telescope as a cardboard tube. The focal ratio is the ratio of the length of tube to the diameter of the tube. Lets take for example a toilet roll and a kitchen roll. They both have approximately the same diameter, but one is twice as long as the other. Now, imagine holding each over your eye and pointing them at a picture. The shorter toilet roll would allow you to see more of the picture as the shorter length would allow your eye to see more. The longer kitchen roll would see less of the picture. It's the same with telescopes... a short focal length will give you a wider field of view when compared with a longer focal length scope...

Additionally, the wider field of view of the shorter scope means that light is coming at your eye is from wider angles. In order to get these into your eye, they have to be "bent" further. A short ratio scope means that the rays have to be bent further therefore the eyepiece is doing more "work" to bend them, therefore it is more difficult to do this well the further from the centre of the eyepiece you are (at the centre, you're not bending light at all...). This si why people say you need good eyepieces for fast scopes.

Assuming the apeture is the same and using the same eyepiece, a short focal length scope will give you wider field of view and lower magnification compared to a similar sized higher focal length of scope.

At least, that's my view on it...

[The Warthog]

The f/ratio makes more difference in prime focus photography than in visual observing. An f/8 scope will behave as an f/8 lens, and you will have to adjust your exposure for an f/8 lens. If you move to an f/5 scope. you will adjust your exposure time accordingly. In astropnotography it is only the exposure time that is adjusted for exposure, as the aperture is usually fixed.

For visual observing, the f/ratio is less important in terms of the brightness of the image, as for a given aperture the brightness of the image for a given magnification will always be the same, regardless of focal length. Longer focal ratios will be more forgiving of cheap eyepieces than will short focal ratios, however. If you are looking through an 8" scope at a magnification of 100, the brightness of the image will be the same, whatever the focal length. The focal length of the eyepiece that gives you that magnification will be different from one scope to another.