Aperture, F-ratio myth and telescope basics

[codemonkey (Lee)]

I've never really stopped to think too much about how telescopes work and how the combination of focal length and aperture work together to produce images of a certain brightness and field of view, so now that I am thinking about these things, I find I have questions.

Stan Moore has an interesting article on the F-ratio Myth basically saying that varying the F-ratio does nothing for exposure time, only varying the aperture makes a difference.

On the face of it, this makes sense... focal length controls FOV, aperture controls the amount of light. Bigger aperture means a bigger light bucket and thus more light.

But why does a bigger aperture mean more light from the same source? Shouldn't it mean more light, but from different sources? Take a point light source, it doesn't seem like it should matter whether you have a 80mm aperture or a 500mm, it's a point source, so it seems like you should either get it or not.

Does it work differently for point light sources and diffuse light sources?

[Atmos (Colin)]

Basically, point light sources radiate in all directions and not just directly at anything in particular. Because they radiate in all directions, a larger aperture is able to capture more of that "all directions". If I wasn't at work I would draw a diagram on my white board, could do that when I get home :-)

[Stefan Buda]

The author of the article you mention deserves a jail sentence for causing a great deal of confusion among astrophotographers.

There is no F-ratio myth. The laws of optics are clear and simple.

[glend (Glen)]

Originally Posted by codemonkey

.....saying that varying the F-ratio does nothing for exposure time, only varying the aperture makes a difference.

So why are there all these guys running reducers to get lower ratios, when they are obviously the same aperture on their one scope.

[multiweb (Marc)]

Originally Posted by codemonkey

... basically saying that varying the F-ratio does nothing for exposure time, only varying the aperture makes a difference.

Damn!... I'd better list my C11-hyperstar in the classified then. 30s subs at F/2 vs 15min subs at F/10 is not good enough

[RickS (Rick)]

The way I look at it is that only two optical parameters matter: clear aperture (amount of light collected) and image scale (how thinly that light gets spread over the sensor.) Seems pretty simple...

[astroboof (Steve McN)]

Hi there,
Figure aperture to light like you might a dam full of water- (the light source), and a hole in the bottom of a weir (the aperture), thus, the bigger the hole, the larger the flow of water out of it. I know that doesn't answer your query sorry.

Or perhaps, electricity supply and an appliances draw on it, if you'd rather.

Also apertures effect can be diluted by a FL, but the effect is far from linear or standard, or practical sometimes for loads of reasons.

Rough analogies, but things are rarely absolute or black and white, especially when dealing with the EM spectrum, where nothing is ever wasted or used, just converted into another form of energy. The laws of optics are only clear and simple if you like to order your mind with absolutes, for your own convenience. Truth is the many variables behind optical performance would be near endless is you stopped to consider all the interactions.

Years ago I had to do some of this with math for imaging optimising in post-grad Uni astro studies for our optical trains, it was a real eye-opener, and a pain.

If optical physics prompts you to ask more questions than it does answer them, than you are on a track to truth, which is a pandora's box to say the least.

As for us hobby imagers, if it does the job and we get satisfaction from our efforts, then it is good enough and worthwhile imVho

Cheers and all the best.

[Atmos (Colin)]

The only determinant factor is aperture for capturing photons. The focal length and therefore focal ratio just change the way that the captured photons interact with your camera.

[Peter.M]

Originally Posted by RickS

The way I look at it is that only two optical parameters matter: clear aperture (amount of light collected) and image scale (how thinly that light gets spread over the sensor.) Seems pretty simple...

I agree completely. Except then you need to consider obstruction, reflectivity or transmission of every surface, ect ect. I think it's best to just judge equipment on aesthetic value of the images they produce. I can find hundreds on fsq images that I think are high quality. The same is not true for c11 hyperstar systems.

[julianh72 (Julian)]

Originally Posted by Stefan Buda

The author of the article you mention deserves a jail sentence

The last two comparison images at the bottom of the page http://www.stanmooreastro.com/f_ratio_myth.htm are very misleading. If one was indeed taken at f/12.4 and the other at f/3.9, then they have been scaled differently, or were taken with different sized sensors. For example, if the f/12.4 image is the full capture over the sensor, then the f/3.9 image is roughly a 1/3 x 1/3 crop of the whole image. (Either that, or the sensor used for the f/3.9 image is only 1/3 the size of the f/12.4 image sensor.)

Using a focal reducer on a long focal length telescope lets you capture a bigger sky area on the same sensor, but doesn't alter the brightness of point sources.
[Edited to correct an overly general statement]

[Peter.M]

Originally Posted by julianh72

The last two comparison images at the bottom of the page http://www.stanmooreastro.com/f_ratio_myth.htm are very misleading. If one was indeed taken at f/12.4 and the other at f/3.9, then they have been scaled differently, or were taken with different sized sensors. For example, if the f/12.4 image is the full capture over the sensor, then the f/3.9 image is roughly a 1/3 x 1/3 crop of the whole image. (Either that, or the sensor used for the f/3.9 image is only 1/3 the size of the f/12.4 image sensor.)

Using a focal reducer on a long focal length telescope lets you capture a bigger sky area on the same sensor, but doesn't alter the brightness of the image which is captured.

You are wrong. It does alter the brightness of extended sources. Because now at a lower sampling rate more area of sky fits into each pixel

[gregbradley]

A reoccurring question.

I imagine all a focal reducer is doing is focusing more of the collected light onto the sensor that normally misses it and is wasted and so there is a gain and also gives a wider field of view where more light sources are emitting.

Greg.

[multiweb (Marc)]

Originally Posted by gregbradley

A reoccurring question.

I imagine all a focal reducer is doing is focusing more of the collected light onto the sensor that normally misses it and is wasted and so there is a gain and also gives a wider field of view where more light sources are emitting.

Greg.

2 degrees field worth of light on a chip is worth more than 30min field in the bush

[rustigsmed (Russell)]

I guess it gets confusing when the FOV changes all the time, generally when you change aperture the FL generally changes too.

However in this is perhaps a good example?

an 8" f5 newt (FL 1000mm) vs a 10" f4 newt (FL 1000mm) same field of view an extra 2" light gathering power hitting the sensor in the same spot, makes sense the 10" (or f4) is "quicker".

Russ

[Stefan Buda]

Looks like you guys missed the recent and excellent post on this topic by Ray:
http://www.iceinspace.com.au/forum/s...d.php?t=136008

I said post rather than thread because there is no need to read past the first post.

It should have been nailed to the top but it hasn't even been pinned..

[Shiraz (Ray)]

Originally Posted by codemonkey

I've never really stopped to think too much about how telescopes work and how the combination of focal length and aperture work together to produce images of a certain brightness and field of view, so now that I am thinking about these things, I find I have questions.

Stan Moore has an interesting article on the F-ratio Myth basically saying that varying the F-ratio does nothing for exposure time, only varying the aperture makes a difference.

On the face of it, this makes sense... focal length controls FOV, aperture controls the amount of light. Bigger aperture means a bigger light bucket and thus more light.

But why does a bigger aperture mean more light from the same source? Shouldn't it mean more light, but from different sources? Take a point light source, it doesn't seem like it should matter whether you have a 80mm aperture or a 500mm, it's a point source, so it seems like you should either get it or not.

Does it work differently for point light sources and diffuse light sources?

Hi Lee. Stan Moore's article is very misleading, because in debunking one myth he inadvertently created a new and even more insidious one. The problem seems to be that he neglected to mention the key part of his argument which should be "F-ratio makes no difference to sensitivity for a given aperture IF THE PIXEL SIZE IS ADJUSTED TO COMPENSATE FOR THE CHANGE IN FOCAL LENGTH". The bit in caps is the bit he left out and now people continue to quote the first part of the argument and assert that F-ratio makes no difference - even experienced photographers who surely must know that it clearly isn't true. Suggest that you ignore that paper.

the things that matter are the aperture, which determines how many photons get in and the angular size of the pixels, which determines how many of the available photons end up in each pixel. if you fix the focal length and pixel size, the sensitivity increases with reducing FNo as Russ' example shows. This applies to extended objects and stars for most systems (except highly undersampled ones). The main point is that you cannot consider aperture, focal length, pixel size (or FNo) on their own - you must consider them all to get the system sensitivity.

re the way a lens works, please forgive me if I have misunderstood your question, but here goes. If you hold up your hand and look at it, you get almost no information on where the light that you see came from - it could have been from many different sources in the room, but you can't tell anything much about those sources by looking at the light reflected from your hand. A similar scrambled pattern of light falls all over the aperture of a lens or mirror, but the lens or mirror has the ability to unscramble that light pattern and develop a map of the angles that the light came from and how bright it is - that is the image that you detect. Light from a point source falls all over the lens aperture and it is then transformed (unscrambled) by the lens back into a point in the focal plane. Light from an extended object also falls all over the aperture, but it is transformed into a 2D representation of the source object. If the aperture is bigger, more light from both point and extended sources gets into the system.

[codemonkey (Lee)]

Originally Posted by Atmos

Basically, point light sources radiate in all directions and not just directly at anything in particular. Because they radiate in all directions, a larger aperture is able to capture more of that "all directions". If I wasn't at work I would draw a diagram on my white board, could do that when I get home :-)

That makes sense, thanks mate.

Originally Posted by Stefan Buda

Looks like you guys missed the recent and excellent post on this topic by Ray:
http://www.iceinspace.com.au/forum/s...d.php?t=136008

I said post rather than thread because there is no need to read past the first post.

It should have been nailed to the top but it hasn't even been pinned..

Nope, I got that, but that doesn't answer my original question about how aperture relates to more light instead of different light. Cheers for the link though, it was a good thread.

Originally Posted by Shiraz

"F-ratio makes no difference to sensitivity for a given aperture IF THE PIXEL SIZE IS ADJUSTED TO COMPENSATE FOR THE CHANGE IN FOCAL LENGTH".

Spot on Ray. He's technically correct, but his presentation is a bit misleading. I actually got that part and my main question didn't really relate to his article all that much, it's just what I was thinking about at the time. I think I should have omitted that from my post as it's largely unrelated to my main question and has served only to confuse people.

Originally Posted by Shiraz

re the way a lens works, please forgive me if I have misunderstood your question, but here goes. If you hold up your hand and look at it, you get almost no information on where the light that you see came from - it could have been from many different sources in the room, but you can't tell anything much about those sources by looking at the light reflected from your hand. A similar scrambled pattern of light falls all over the aperture of a lens or mirror, but the lens or mirror has the ability to unscramble that light pattern and develop a map of the angles that the light came from and how bright it is - that is the image that you detect. Light from a point source falls all over the lens aperture and it is then transformed (unscrambled) by the lens back into a point in the focal plane. Light from an extended object also falls all over the aperture, but it is transformed into a 2D representation of the source object. If the aperture is bigger, more light from both point and extended sources gets into the system.

The main thing I was confused about is why does a bigger aperture not mean light from different sources rather than more light from the same sources. To make that extreme, imagine I had a mirror the size of my thumbnail. Obviously it could only reflect light from a small area... but then if I sold my thumbnail-sized mirror and bought one the size of Queensland, it would reflect light from a much greater area simply due to its larger size, but (assuming all else is equal) the intensity of the reflected light would remain constant.

Now if the bigger aperture means that more light is focused into an area of the same size, I can see why the intensity, and resolution would increase... but how does this not also increase the "area" of light gathered, i.e. an increased FOV. Unless it does but because the recording medium was already fully illuminated (presumably), the extra FOV just bypasses it? In which case if you had a system where the sensor was not fully illuminated, keeping everything constant except the aperture would result in a sensor that is more fully illuminated with data that was previously not recorded...?

[gregbradley]

Originally Posted by Shiraz

Hi Lee. Stan Moore's article is very misleading, because in debunking one myth he inadvertently created a new and even more insidious one. The problem seems to be that he neglected to mention the key part of his argument which should be "F-ratio makes no difference to sensitivity for a given aperture IF THE PIXEL SIZE IS ADJUSTED TO COMPENSATE FOR THE CHANGE IN FOCAL LENGTH". The bit in caps is the bit he left out and now people continue to quote the first part of the argument and assert that F-ratio makes no difference - even experienced photographers who surely must know that it clearly isn't true. Suggest that you ignore that paper.

the things that matter are the aperture, which determines how many photons get in and the angular size of the pixels, which determines how many of the available photons end up in each pixel. if you fix the focal length and pixel size, the sensitivity increases with reducing FNo as Russ' example shows. This applies to extended objects and stars for most systems (except highly undersampled ones). The main point is that you cannot consider aperture, focal length, pixel size (or FNo) on their own - you must consider them all to get the system sensitivity.

.

I think you got a good grasp of the subject and have cleared that area up nicely. I did not get it at first but then that F-ratio myth was blocking my understanding of it. I agree its missing that exact component as you have just stated.

You've also highlighted the importance of correct camera pixel size for a certain focal length.

Well done Ray. You've made a very good contribution here. Your article should be exported around other sites.

Greg.

[Bassnut (Fred)]

Yes,excellent Ray. I always found Stans paper unintuative and confusing, something wasn't quite right. You have put it simply.

[barx1963 (Malcolm)]

Originally Posted by codemonkey

The main thing I was confused about is why does a bigger aperture not mean light from different sources rather than more light from the same sources. To make that extreme, imagine I had a mirror the size of my thumbnail. Obviously it could only reflect light from a small area... but then if I sold my thumbnail-sized mirror and bought one the size of Queensland, it would reflect light from a much greater area simply due to its larger size, but (assuming all else is equal) the intensity of the reflected light would remain constant.

The way I think about this is a light source such as a star emits light in all directions. If it simply emitted a single pulse of light there would be a sphere of photons heading out into the universe expanding all the time. If you were looking at the star at the time that sphere hit, you would collect s many photons as would fit into your pupil and that would determine how bright that the star appears at that moment. All a telescope does is collect photons from a larger part of the sphere and concentrate them into your pupil, hence making the star appear brighter. I think your reference to a star being a point source is confusing you, the light which is emitted is definitely not a point.

Malcolm