Speed confuses me

[danielson23uk]

Hi everyone,

I read The Warthogs sticky on Eyepieces and found myself confused about the speed of a focal ratio. Without getting too bogged down in the maths of arriving at the focal ratio (Warthog demonstrates that clearly) what is the impact in REAL terms of a faster scope over a slower scope?

Does the speed relate to increased frequency of light captured in being interpreted by the brain? pfwffff:icon_scratch: I'm lost!

[themos]

A faster scope will show a brighter image of, say, twilight sky background, than a slower scope.

[The Bat]

A fast telescope allows shorter exposure times for imaging, so an f5 would allow you to get an image with a quarter of the exposure of an f10.

For visual use, it's something to do with the shape of the light cone coming down. Slow focal ratios give you get more contrast and less aberration, I believe. This makes long focal ratio scopes better for lunar and planetary viewing.

Highly simplistic interpretation, I am sure. No doubt someone else with a slightly better understanding can phrase it better.

Rachel

[RobH]

Hello there.

Thius is an extract from my website about focal ratio, speed etc.

In this instance, it's talking about imaging, but the same applies visually.

Hope it helps

A bigger telescope and/or a faster focal ratio will gather more light in a given exposure time.

The light gathering power of a telescope is related to the square of its diameter. A 12 inch telescope will have 4x the light gathering power of a 6 inch telescope, and 9 times the light gathering power of a 4 inch telescope.

It can easily be seen why when you look at the area of the lens or mirror. The 12 inch telescope mirror has a surface area of 73062 square mm, a 6 inch telescope 18146 square mm, and a 4 inch has 8171 square mm.

Focal ratio is similar. The exposure that you will need from a given telescope at F8 will be 4x as long as with the same telescope at F4 to achieve the same brightness. So a 6 inch telescope imaging at F4 will produce the same brightness of image as a 12 inch telescope at F8. This is because at F8, the image the telescope produces covers 4x the area of the image produced at F4.

An easy way to envisage this is to imagine you have a film projector, and you want to make the projected image bigger, so you move the screen away to double the distance. This makes the image twice as big, but it also covers 4 times the area, and is therefore 4 times fainter as the same amount of light is spread over an area 4 times as large.. To get the same image brightness as before, you now need a projector 4 times as powerful.

Cheers

Rob

[great_bear]

A faster scope will show a brighter image of, say, twilight sky background, than a slower scope.

Well... erm... depends how you look at it.

- for any given magnification, the two are actually the same brightness, although you would, of course, need different eyepieces to get the same magnification - take 75x as an example, in either of the Sky-Watcher 150 OTAs:

- SW150P = 750mm(scope fl) / 10mm(eyepiece) = 75x

- SW150PL = 1200mm(scope fl) / 16mm(eyepiece) = 75x

Brightness levels (by exit pupil size):

- SW150P = 10mm(eyepiece) / 5(focal ratio) = 2

- SW150PL = 16mm(eyepiece) / 8(focal ratio) = 2

- so as you see, the brightness is the same.

[michael.h.f.wilkinson]

Well... erm... depends how you look at it.

- for any given magnification, the two are actually the same brightness, although you would, of course, need different eyepieces to get the same magnification - take 75x as an example, in either of the Sky-Watcher 150 OTAs:

- SW150P = 750mm(scope fl) / 10mm(eyepiece) = 75x

- SW150PL = 1200mm(scope fl) / 16mm(eyepiece) = 75x

Brightness levels (by exit pupil size):

- SW150P = 10mm(eyepiece) / 5(focal ratio) = 2

- SW150PL = 16mm(eyepiece) / 8(focal ratio) = 2

- so as you see, the brightness is the same.

You are right. Visually, there is little difference, except that the wider light cone of the faster objective is harder to correct for in the eyepiece, and therefore a fast scope is better off with expensive EPs. The big difference is in imaging, where extended sources can be imaged in a shorter time with fast lenses.

[themos]

The very fact that you have to use different eyepieces to get the same brightness (to the eye) means that the two scopes produce images of different intensity (to the focal plane).

[great_bear]

Highly simplistic interpretation, I am sure.

Not really - I'd say you were bang on with that one.

I've never seen an explanation of why the contrast is better on slower scopes for a given magnification, but I reckon it's something to do with the fact that the portion of image you're looking at, occupies more of the barrel width, so the ratio of "used" to "unused" image at the focal plane is higher.

- but that's just a guess... I could be completely wrong!

On the abberation side of things it's a lot more obvious. For a given magnification, as you say, the light rays are bent less (suffering less errors from optical imperfections), and also (for a given magnification) the eyepiece lenses will be larger (assuming "traditional" eyepiece design) large lenses being (naturally) easier to manufacture accurately.

[great_bear]

The very fact that you have to use different eyepieces to get the same brightness (to the eye) means that the two scopes produce images of different intensity (to the focal plane).

Sure - it's just that when stated unqualified as "A faster scope will show a brighter image of, say, twilight sky background, than a slower scope" it does tend make it sound like - to newbies - that you'll get brighter views through the faster scope, which - for a given angular view - isn't the case at all.

I'm aware that you know your stuff - I'm just clarifying the statement.

[michael.h.f.wilkinson]

The very fact that you have to use different eyepieces to get the same brightness (to the eye) means that the two scopes produce images of different intensity (to the focal plane).

You are right that the intensity at the focal plane of the objective is higher, but it needs to be magnified more to achieve the same visual magnification. This cancels out any difference in brightness. The depth of field (region in focus) in a slow scope is typically higher, which means that field curvature, residual chromatic aberrations etc, are less pronounced (the latter is not important in a reflector). In fast reflectors, off-axis coma and the like are far worse than in slow ones.

Generally, it is more difficult to produce a low aberration fast lens (the same holds for camera lenses, try buying the 50mm F/1.0 Noctilux from Leica:eek:). Thus many slow scope are better in terms of optical quality.

A key difference between fast and slow scopes is that a fast scope allows you to see a wider field of view more easily.

[themos]

That's ok, great_bear! To attack the original question from another angle:

Does the speed relate to increased frequency of light captured in being interpreted by the brain?

No. The "speed" relates to the shorter exposure time you would need with a "fast"scope to make the same image.

[Merlin66]

The depth of focus also varies with the focal ratio...this is why it's easier to focus with a higher f ratio scope.

[danielson23uk]

No. The "speed" relates to the shorter exposure time you would need with a "fast"scope to make the same image.

So it's much the same way as in photography when lens with apertures of f2.8 are considered 'fast glass'? Because they take less time to let a particular amount of light in due to their aperture size. I think I've got it, and even if I haven't just smile and nod! ha ha

[themos]

Yes, it's the same concept. The bigger the aperture (or the shorter the focal length), the more light gets in (or the more concentrated light gets) but the more light has to bend and the harder it is to make light bend accurately. f-ratio is a measure of how much light-bending the optics do.

[kniclander]

practically, you can geenrally use cheaper e/ps with a slow scope and your collimation can be ropier

[dph1nm]

So it's much the same way as in photography when lens with apertures of f2.8 are considered 'fast glass'? Because they take less time to let a particular amount of light in due to their aperture size. I think I've got it, and even if I haven't just smile and nod! ha ha

Careful here. With terrestrial photography a smaller f-ratio really does mean a larger aperture and therefore more photons - hence the better performance in low light. In astrophotography, folks will tell you that an f4 scope is faster than an f8 scope of the same aperture. Here the number of photons in unchanged, they merely get distributed amongst fewer pixels (so more per pixel) in the f4 case.

NigelM

[BazMark]

Does the speed relate to increased frequency of light captured in being interpreted by the brain? pfwffff I'm lost!

Hi Daniel,

The way I understand the 'speed' thing is that the 'speed' of a scope is related to the length of the light path between the objective lens / primary mirror and the focal point of a given aperture scope.

ie:. In a short focal length 6inch scope the light reaches the focal point quicker after it passes the primary mirror or objective lens than it does in the same aperture scope with a longer focal length. Therefore it is known as a 'faster' scope than one with a longer focal length.

If I've got this wrong then I'm sure someone will correct me.

[themos]

I'm afraid that is wrong, BazMark. What you describe is just the focal length, short or long. The f-ratio describes the relationship of the aperture to the focal length. Scopes of the same f-ratio have apertures that are the same percentage of the focal length, so that the aperture subtends the same angle at the focal plane. "Slow" scopes look long and thin (if you were to "unfold" the optical path in case mirrors are used) and "fast" scopes look short and fat.