Focal ratio

[Darth Wader (Wade)]

Not sure if I understand this correctly so I thought I'd check... is the focal ratio just the focal length divided by the diameter of the primary mirror or lens, i.e. if my dob has a focal length of 1200mm and the primary mirror's diameter is 200mm does that mean that the f-ratio is f/6, or is there something I'm missing? Similarly, would the f-ratio of my 70mm, 900mm focal length refractor be f/12 (which doesn't seem right to me)?

Cheers
Wade

[Blue Skies (Jacquie)]

The f/6 is right, and so is the f/12.8 Does seem like a lot, but there it is!

[Darth Wader (Wade)]

Wow! I didn't expect a reply so soon, thanks Jacquie. So what is an ideal f ratio? Is it dependent on the type of scope/requirements (such as astrophotography)?

Cheers
Wade

[Blue Skies (Jacquie)]

Depends on what you want to do, really, plus the scope type, as you mentioned. I have an 8" f/6 dob and its a great all rounder. Traditionally you wanted a long focal length refractor to get good views of the planets, and if you wanted to go looking for comets or do variable stars a shorter focal length, which translates to a wide field of view, was better. Astrophotography also wanted a shorter focal length. But making a short focal length scope was a challenge to get right, so they often cost more. Things have changed a bit these days, though. Long focal lengths have fallen right out of favour with the advances in digital photography and videos combined with 5x barlows and the better availability of big scopes such as 12", but they were and still are made because they are cheaper and easier to get right. An f/15 refractor doesn't really need colour correction, as all the colours come into focus at the right point, so no need to add an extra lens in to make it an apochromatic. Long focal length dobs are easier to colimate but anthing longer than an f/8 in Newtonians starts to get difficult to handle, even if permanantly mounted. There are a lot of 4" f/8 at the cheap end of the market because the mirror is essentially spherical, and they are pretty easy to make by machine, they don't need extra work by hand to finish.

[Darth Wader (Wade)]

Thanks Jacquie, sorry for all the q's but I really need to understand!

I am right in saying that the higher the f number, the "faster" the scope?

[mental4astro (Alexander)]

A 'fast' scope is one with a ratio smaller than f/6, such a f/5 or f/4.5.
A 'slow' scope is a ratio is every ratio larger, like f/10 or f/12.

It is a relationship like the diaphram of a camera, a large diameter diaphram allows alot of light in, so in a way, a faster shutter speed. Conversly, a small diaphram diameter, a slower shutter speed is needed to allow the same amount of light in.

The f/ ratio is often overlooked, misunderstood & underestimated when selecting a telescope. A slow scope will deliver a dimmer image than a fast scope, often disappointing beginners. They do, however, disguise the poor quality of eyepiece usually sold with beginner, & even advanced, scopes.

I'll try to explain it with some practical examples.

The schmidt-cassegrain telescope, either by Meade, Celestron or whoever, has a focal ratio of f/10, very slow. This makes it a difficult beast to work with in photography because it needs really long exposure times to get a picture.

These same companies now make lens adaptors which can reduce the f/ratio to f/6.3 & f/3.3, making them 'faster', & reducing exposure times. They work like a negative barlow, where a barlow would increase focal length, these adaptors reduce focal length.

Another way to describe the visual effect of a fast scope vs. a slow one the comparison of an image of the same object with both a fast & slow scope using the same diameter of objective or mirror. The background brightness of the slow scope will be darker than the background in the fast scope. The slow scope has higher contrast in its image, ie, the blacks are blacker, but the fast scope can show fainter detail, though can be more diffiuclt to see because of the lighter background.

Refractors & reflectors deal with light differently, but each, as you may know has its own pros & cons which you need to balance up in choosing a scope, or assessing its performance. These then dictate what EPs & accessories you use & buy.

[gmbfilter (Geoff)]

Here is a link to some telescope formulas.http://www.televue.com/engine/page.asp?ID=141.they have some useful info

[renormalised (Carl)]

As Alexander was saying, Wade, the SCT's are slow scopes. However, for some brands of SCT's (mainly Celestron and many of the Meade models), you can replace the secondary corrector with a lens system that effectively allows the scope to operate at ultrawide fields and exceptionally fast f ratios (around f1.5-2). The reason why is that the main mirror of an SCT is actually figured to about f2 or thereabouts. It's the secondary corrector mirror which in effect slows the scope down to f10. When you replace the secondary, the scope can then behave like a system that operates at those fast focal ratios. You can get these lens system commercially....they're called a Hyperstar lens.

[Darth Wader (Wade)]

Thanks for your replies all, that makes more sense now.

Cheers
Wade

[Terry B]

Just note that almost all of the replies are commenting about f ratio for photography. For a Dob that is not designed for photography the f ratio is less important. F ratio just changes the image size not the brightness of the image. By changing the eyepiece the size of the image changes to your eye.
An example is 2 scopes of 200mm diameter, one at f4 and one at f8. The focal lenghts will be 800mm and1600mm.
Both collect exactly the same amount of light (200mm diam). If you put a 20mm eyepiece in both scopes the view in the f4 scope will display objects at 1/2 the size of the f8 scope. The f4 view of extended objects will seem brighter because they are 1/2 the size. By changing the eyepiece to create the same magnification the images will be identical in brightness.
The choice between the 2 scopes depends on other factors as stated above- ease of collimation, handling, physical size etc.

hope this helps

[mental4astro (Alexander)]

Terry B, I agree 100%, .

Good summery.