Can someone please explain ...

[AlistairHowie]

... in words of one syllable (I'm a newbie and I just can't "get it") why e.g. an f/10 focal ratio is slow. I realise this means longer exposure photography, but why? I can't get my head around why what I see visually in an f/10 SCT would take longer to image than a faster scope image of the same object.

I guess what I'm really asking for is an explanation of fast versus slow. I realise the implications, but I don't understand the why. This probably goes to the heart of my (limited!) understanding of optics - it's been some time since high school physics - so please try and provide me with a simplistic response!

Many thanks

[nightfisher]

here goes, but dont shoot me if im slightly wrong:

the longer the focal length, the dimer the object in view, so fast scopes, say f5 have a brighter image of an item than say a f10 on the same item in view

[brantuk]

An f-10 scope is slow and that would mean longer exposures Alistaire. An f-5 scope is fast and would give you shorter exposures.

At f-10 you are trying to get lots of contrast and sharpness on "near" objects like planets and moon so the long focal length helps that. But these objects are very bright so you need to take a very quick exposure to avoid swamping the sensor with bright light.

With an f-5 scope you are trying to gather very dim light which takes longer to collect with an f-10 e.g. distant faint galaxies and nebulae. The faster f-ratio will gather the light quicker but the shutter is held open for longer cos it really is very dim. If you did this with a slow (f-10) scope you'd need much longer exposure time.

I don't know the maths to describe it but that's my understanding - hope it helps

[brantuk]

Also - look at pics of Jupiter - you get nice definition on the surface (e.g. the stripes, storms, GRS etc). But you rarely see the moons in the same shot cos the light from them is too dim to see in a fast exposure from an f10 scope.

So when you see good planet definition and the moons too - sometimes with a little definition and colour - you know the imager is pretty expert and has done a lot of work to capture both planet and moons at the same time. Damian Peache's recent results on Jupiter are quite amazing

[MikeD]

Think of it like this.

Imagine you have a film projector projecting an image on an A4 sheet of paper, the image on the paper is the field of view (FOV).

If you move the paper further away from the projector the image gets bigger and at the same time dimmer and the FOV gets smaller because as the image gets bigger less of the whole image stays on the piece of paper. This is the same as increasing the focal length such as an f10 telescope.

Now if you move the paper closer to the projector the image gets smaller and brighter and the FOV gets bigger, because as the image gets smaller more of the whole image stays on the piece of paper.

This is the same effect as a shorter focal length such as F4.8.

Hope that makes scene and helps.

Mike.

[sabana]

A Fast Lens has a wider aperture than a slow lens. Therefore it has increased light gathering. The more light you can gatther the shorter the exposure time when taking a photograph.

But if you want increased Depth of Field (ie seeing more of the entire image) then you want a narrower aperture.

[JohnC]

Not scientific, but i always think of it like this. F10 it takes longer for the light to reach the camera therefore the shutter has to be open longer. F5 light gets there qicker so shutter does not have to be open so long :)

[cantab]

When it comes to imaging, the telescope alone has an inherent magnification. That magnification is greater for a a longer focal length, and thus greater per inch of aperture for a longer focal/ratio. You may know from visual experience that more magnification makes a dimmer image; this holds when imaging too, and a dimmer image requires longer exposures, so taking pictures is slower.

[Stu]

Mike, your explanation with the piece of paper/projected light makes it very easy to visualise.

Being purely visual, for me it is a simple matter in that if you view an object at the same magnification/fov in two scopes of the same aperture but very different focal lengths, the image will appear the same.

I guess in imaging, the focal length of the scope is fundamental to the size of image appearing on the sensor chip, it defines whether a large area of sky is concentrated on the chip giving brighter images, shorter exposures (fast scope), or if a small area of sky is spread over the sensor giving dimmer images which take longer to expose (slow scope)

Stu

[acey]

Photographers speak in terms of focal length and f-ratio, astronomers speak in terms of aperture and f-ratio. This causes confusion.

The photographer might compare a 40mm f10 lens and 40mm f4 lens. The 40mm refers to focal length: the f10 has aperture 4mm, the f4 has aperture 10mm. So the f4 gathers more light and you can use a shorter exposure: it's "fast" while the f10 is "slow".

If those lenses were telescopes, the astronomer would call them a 10mm f4 and a 4mm f10, but would still call the f10 a "slow" scope, simply because we've inherited the termninology.

For fixed aperture and magnification, the f-ratio makes no difference to image brightness. Set up an 8" f4 telescope beside an 8" f10 telescope. The lowest useable power of either is fixed by the observer's eye pupil and is the same for both scopes (about x30). Provide eyepieces that yield this lowest power in either scope, and aim them at a galaxy. The image brightness is exactly the same.

But suppose we use the same eyepiece in both scopes. Now the f10 will yield higher magnification, spreading the same amount of incoming light over a larger area. Now the "slower" scope has a fainter image.

[robbieince]

Another way of looking at it might help

F ratio is simply the focal length divided by the aperture. Let's think of two scopes of identical aperture = 100mm

One has a focal length of 1000mm making it F10 and the other has a focal length of 500mm making it F5

Both let the identical amount of light in being the same aperture so the total amount of light falling on the image plane is the same but the images are not the same size.

The f10 scope gives a more magnified view (the focal plane image is larger) so that same amount of light is spread over a larger area.

So per unit of area at the detector, ie light per pixel or photons per mm on the eye the brightness is lower.

In photographic terms the f10 scope would need a 4 times longer exposure to record the same light level (number of photons per pixel) so it is called slower.

Regards

Rob

[Luke]

I think the thing is that the speed will come more into play when you do imaging at prime focus.

When you are doing observing, the focal length of the eyepiece also comes into play.

Say I'm looking at something in an F5 dob or an F10 SCT, if they have similar aperture I won't notice much difference between them when observing, because I will use different eyepieces and ultimately end up at the same magnfication in both scopes.

With prime focus imaging, it's then down to the focal length of the telescope. My SCT is much more "zoomed in" than the dob, with a much longer focal length, so although things are more zoomed, they are also fainter, because you have less light per camera pixel.

Think of it like a torch beam, which is of course just light too. If you shine it on a nearby object, the beam is more concentrated, so brighter. Shine it onto a faraway wall though and the light is more spread out and dimmer.

I hope that helps, as you zoom in, you've got less light per same area of surface, that's how I think about it anyway.

[Stu]

The main confusion which seems to happen between photographic and astronomical principles are that photographic lenses involve a variable aperture which changes the f number and thus the required exposure, whereas astronomical scopes have a fixed focal length and aperture so the exposure time is related to these two fixed things.

Stu

[Astro Imp]

Think of it like this.

Imagine you have a film projector projecting an image on an A4 sheet of paper, the image on the paper is the field of view (FOV).

If you move the paper further away from the projector the image gets bigger and at the same time dimmer and the FOV gets smaller because as the image gets bigger less of the whole image stays on the piece of paper. This is the same as increasing the focal length such as an f10 telescope.

Now if you move the paper closer to the projector the image gets smaller and brighter and the FOV gets bigger, because as the image gets smaller more of the whole image stays on the piece of paper.

This is the same effect as a shorter focal length such as F4.8.

Hope that makes scene and helps.

Mike.

I have read various explanations on this topic and this is by far the easiest to make it clear.

Thank you.

[great_bear]

An f-10 scope is slow and that would mean faster exposures Alistaire. An f-5 scope is fast and would give you longer exposures.

[...]

The faster f-ratio will gather the light quicker but the shutter is held open for longer cos it really is very dim. If you did this with a slow scope you'd still wash the sensor out

Ooops! This is the wrong way around

[brantuk]

I'm sure you're correct GB but would you point an f10 sct at the moon and do a long exposure of five+ mins?

[ronin]

Fast and Slow are photographic terms, if you are not imaging then easier to forget it all.

If you work through the geometry as in euclidian geometry then two f/5 scopes give an image that is equally bright/intense. Assuming they are different diameters and so focal lengths the image will be different sizes but each should be the same brightness/intensity. You end up with a small bright image and a larger bright image.

This is true for all equal f numbers.

Fast in photographic terms means you can use a faster speed at the shutter to get a correctly exposed image. So on a fast scope you can use a faster (shorter) exposure time to get your image.

This shorter exposure time means that tracking errors are not more apparent. Also means you do not get quite as cold.

If you have a fast scope with 100x magnification and a slow scope with 100x magnification then if the apertures are the same the image brightness/intensity will be the same. In this visual case fast/slow is irrelevant.

[great_bear]

I'm sure you're correct GB but would you point an f10 sct at the moon and do a long exposure of five+ mins?

Nope. Nor would you point it at the sun for 10 hours. Not sure where you were going with this...

The terms "fast" and "slow" relate to the shutter speed required when a scope or lens is pointed at a specific reference object. It's not related to the subject matter in any way.

[brantuk]

Hmmm..... just trying to understand why I got this the wrong way round again lol

You see I was "corrected" when I used to think of it the opposite way (I used to relate it to shutter speed like yourself) - but I was "corrected" on that and was told I had it the wrong way round then as well lol.

[Stu]

Hi Brantuk

I think you are just confusing the fast/slow conversation with how the different configurations are normally used.

A fast scope by definition does not give very high magnifications, 'sees' a wide field of view and gathers more light ie it is fast. It is suitable for larger, dimmer objects and because of its low magnification is easier to guide for longer exposures (subs) to bring out detail in fainter objects.

A slow scope, because of its long focal length, is suited to smaller objects requiring high magnification. It 'sees' a smaller portion of the sky so is not suited to faint objects, aswell because it gives higher magnification it is harder to guide accurately. However, taking many very short exposures of bright planets and stacking them is what they are good at, avoiding the atmospheric seeing effects through the shorter exposure and also avoiding the need for accurate guiding. The long focal length gives good image scale on planets which would otherwise look tiny through faster scopes.

That represents the sum total of my knowledge, hope it is of some use :-)

Stu

[acey]

Surely a faster f-ratio scope (f-6 or lower) is the one that gathers dim light quicker during a long exposure (ie slow shutter)?

If you keep focal length fixed, then lower f-ratio equals bigger aperture equals shorter exposure time. Been said lots of times in lots of different ways in this thread which just goes to show how confusing astronomers have made things for themselves by adopting photographers' terminology.

For visual observers the only difference between fast and slow scopes (of equal aperture) is the field of view, along with factors like accuracy of collimation, coma, size of scope etc. Image brightness has nothing to do with it. Only way you change that is by changing aperture.

[Ags]

The focal length determines the image scale at the focal plane. This is irrelevant visually as the eyepice magnifies this image (changes the scale).

The total amount of light in the image at the focal plane is determined by the aperture. If two scopes have the same aperture they will have about the same number of photons coming through the focal plane. But...

If you take two scopes with the same aperture but different focal lengths, the image at the focal plane in the slower scope is stretched over a greater area, so it is dimmer per square mm of image.

[brantuk]

"Been said lots of times in lots of different ways in this thread which just goes to show how confusing astronomers have made things for themselves"

First thing that makes sense lol

Lot of stuff to digest there for an allready re-fried brain - so I'm gonna have to read again a few times. Hope this is helping the OP.

Thanks Stu, Acey and Ags for taking the time

[AlistairHowie]

Wow - now that IS a lot of responses. Many thanks for the input, particularly Mike for your projector analogy which I found very helpful.

I decided to surf the internet for photography 101 type information and found a few good sites explaining aperture and f numbers. So with that information to hand, and what's in this thread, I think I've got it!

Cheers

[brantuk]

Looks like you'll have to explain it to me then Alistair lol