F-ratio myth

[Peter Ward]

I have been puzzled by recent posts about Fast optics (eg F3.8) having the ability to capture data in less time than a slower optic (eg F8.0) .

This old chestnut was well addressed by Stan Moore....and I still have to agree with him.

Aperture is king.

Having recently dusted off my FSQ and its delightful F5 optics, I can say with certainty it does not gather data very quickly...its humble four inch aperture is not exactly a light bucket.

At F5.5 I have another optic I like to use (with five inches of aperture) which demonstrably gathers photons faster than my FSQ....despite being half an f-stop “slower”

I suspect the f-ratio myth persists as a carry-over from photographic lenses....which vary aperture at a constant focal length.

...not something you’d do with a telescope optic, unless you want to purposely reduce the aperture with a field-stop, then wonder when you removed the stop, why things got brighter!

This is not to say fast telescope optics with fine pixels is a folly.

Far from it!

Mechanically smaller (hence less taxing on the mount) plus they give good sampling and a wider field compared to the same aperture in a longer focal length. The big acreage CCD’s required to get the same field are not cheap! (or the filters/field correctors)

...but if you think more flux will fall into a smaller pipe...regardless of the F-ratio...

As they said in “The Castle”....tell ‘em their dreamin’. ;)

[RickS]

Well, aperture is half the story, Peter. The other half is image scale, combining focal length and pixel size. A small aperture scope can be faster than a huge aperture provided you're collecting the photons in big enough buckets (and are happy with much less resolution with a larger FOV as the upside.)

I do agree that the f-ratio of a scope is meaningless by itself...

Cheers,
Rick.

[Slawomir]

However, if I put a reducer on my modest 4" f/6 scope and make it work at f/4.5, while keeping the same camera, I should be capturing data more quickly. Or I do misunderstand something fundamental here?

Some other factors that come to my mind that might be worthwhile consideration are refractor vs reflector with similar apertures (loss of light at mirrors), and working at really fast f-ratios with 3nm filters (wavelength shift).

[Peter Ward]

I hear what you are saying Rick...my point however is a 10" can only gather 10" worth of flux.

Optimal sampling will help better detect what's hitting the focal plane...it just won't give you more photons down the pipe :thumbsup:

[RickS]

That's correct. You have kept the aperture the same but increased the image scale. You're stuffing more photons into each pixel. Thanks for illustrating my point perfectly, Suavi :)



Optical efficiency does matter (light loss through reflection vs lenses) but it's not a large effect. Central obstruction in some reflecting designs is a bigger consideration.

Really fast scopes and NB filters can be an issue, but a different one.

Cheers,
Rick.

[Peter Ward]

All you will get is a wider field of view over the same sensor. Unless you increase the aperture you will not be collecting more flux.

Google Stan Moore F ratio myth ;)

[Camelopardalis]

I noticed this recently while imaging with a mate with a larger refractor, ~2x the objective area. My scope f/5.5, his f/7, same camera (within manufacturing tolerances), same gain, same exposure time to hit "sky limited".

f-ratio had me fooled there for a while. It may work well for camera lenses, but comparing telescopes with differing light collecting areas has to be the way of the world, or should that be, the sky?

[RickS]

Hi Peter,

It is certainly true that the total number of pixels captured and funnelled into the image circle is determined by aperture.

If you want maximum resolution then you need large aperture to collect data quickly.

If you're looking for a wide field and are happy with much lower resolution then a smaller scope can do just as well or better.

Cheers,
Rick.

[RickS]

But you will be channelling more flux into every pixel and increasing the SNR in individual pixels more quickly.

[Peter Ward]

Nup. They don't.

Please google Stan Moore's analysis with a good look at the F12.4 vs F3.9 data.

[RickS]

I have read it before a few times. His idea of "object SNR" doesn't convince me. Simple maths and physics does...

Cheers,
Rick.

[Peter Ward]

The relationship is not that simple.

One can also sample appropriately with a bigger 'scope....I also suspect my view (and Stan's) is vindicated by the professional push is toward ever larger aperture optical telescopes (e.g. 30 metres) rather than trying to telecompress 4 metre class instruments.

Guess we'll have to agree to disagree. :)

[Atmos]

It is a little more complicated than simple "Bigger is Better" because as has been mentioned, it is all about flux. Aperture, FR and pixel size all come into play.
The difference between the FSQ and AP.
AP captures 69% more photons but is only 21% slower (5 v 5.5).

The difference between the the FSQ and Alluna.
Alluna captures 16x the amount of photons but is only 2.56x slower (5 v 8).

Both of those examples are considering a similar pixel size. If the pixel size is changed things also change. Putting an ASI183 (2.4 micron pixels) on the Alluna for SUPER HIGH RESOLUTION imaging and putting that up against the FSQ (keeping the KAF-16803) changes things slightly.

The ASI183 has 14x smaller surface area (pixel size wise) than the KAF-16803. Forget the fact that the Alluna is imaging at 0.155"/pixel now, the FSQ is now getting 2.25x better signal than the 16" Alluna as the photons are now being spread very thinly among the tiny pixels. Realistically it's going to be closer to 2x SNR as the IMX183 sensor has a near 90% QE against the 60% KAF-16803.

It is not a great real world example but you get the idea. It is not just about the amount of flux entering the system (the raw aperture) but the way the photons are spread (f/ratio and pixel size).

[Atmos]

It all depends on what it is you're trying to accomplish. The perfect example of this is with the DragonFly Project. It is the only system on Earth that can go past Mag 30 and it only uses "small" Canon lens'.

[Peter Ward]

That's a bit of a stretch :)....DragonFly works not because of it's fast f-ratio, but because of the extremely low scattering from those beautiful Canon coatings ;)

[RickS]

There's certainly a lot more to it than the original premise that aperture is all important.



One of the main reasons for large aperture professional scopes is increased resolution (which is directly related to aperture.) If it was being done to increase the number of photons captured then there would be no point in optical interferometry.

[Peter Ward]

I ran similar numbers hence have no problem with the arithmetic you've presented...but the elephant in the room is: why would one sample at a bit over 1/10th of a pixel ? or even 1/100th of pixel?

Reduction to an absurd conclusion does not make a lot of sense when so many other factors will start coming into play by doing so (e.g. pixels sooo small to have well depth of 100 electrons :) )

Sure, a well sampled small "fast" system can give great results..but the same sampling rules can also be applied to larger (albeit optically slower) systems, that gather buckets more light with higher resolution.

[Atmos]

Moving to Chile any time soon? ;)

[Peter Ward]

Ah....yes....I wish.....Chart32 :bowdown::bowdown::bowdown:

[AstroJunk]

Fortunately for me I have two telescopes that I can compare directly. One is a 14" SCT using Hyperstar and has a focal length of 711mm. The second is a 4" Refractor with a focal length of 714mm so they are both effectively the same.

I happen to know that one of the scopes requires much less of an exposure to get a high signal shot of a 14th mag asteroid than the other using the same camera. That's simple physics, photons per pixel. I even had a paper published in the Journal for Occultation Astronomy on the subject :D

[Peter Ward]

Sooo... a C14 gathers more light than a 4" ?

I'd really like to hear from any Ceravolo dual FL owners on whether they see better S/N in the "fast" configuration.

[Slawomir]

I guess then the laws of physics somehow do not apply here, and my direct experience with these two f-ratios with my scope n camera, where I could easily see stronger flux per pixel per unit of time at f/4.5 as opposed to f/6 (confirmed by SNR measurements with PI) are either my imagination or camera n PI are “playing Jedi mind tricks’...google does not provide answers to all questions Peter ;)

So if we take a glass window (simulating a very large 1m aperture with loooong focal length), it clearly will be putting photons on a pixel behind the window much more slowly than a 4” strongly converging lens (small aperture fast f-ratio) placed in front of the same pixel.

Let’s create a new myth - “larger aperture is always faster”... :rofl:

[Shiraz]

No myth at all Peter. For a given aperture and camera, a fast scope will produce more signal than a slow one. The aperture determines how many photons get through from any point in the sky, but the focal length (and hence the Fno) determines how large an area of sky feeds photons through that aperture and into each pixel - for a given aperture and camera, a faster scope looks at more sky, resulting in more detected signal in each pixel. In more general terms, it is wrong to say that FNo by itself is the only determinant of image SNR quality, but it is equally misleading to contend that aperture by itself is the only determinant of image SNR. As Rick points out, sampling must also be included.

Re Stan Moore's paper, ask why the two images he presents - from f4 and f12 scopes of the same aperture - have the same plate scale. Unfortunately Stan doesn't explain this, but I assume that he got there by binning 3x3 in the f12 configuration. That would give about the same scale and also restore the large SNR loss inherent in operating at f12 vs f4. If he used that approach, the only thing he ended up losing at f12 was ~90% of the field of view, but in the process he did not show the large drop in SNR at f12 vs f4 (with the same camera and no binning).


If anyone needs an illustration that aperture is not all that matters, try imaging with a 2x Barlow in the imaging train :lol:

Peter, your observation that you get better SNR results from an f5.5 scope than from an f5 could come about if you use different cameras on the two scopes.

[Slawomir]

Thank you Ray for explaining it so well.

I was also going to suggest making a test with the same scope and camera bin 1x1, but changing f-ratio with a barlow, imaging at native FL and then using a reducer. I tired it and it DOES affect the speed :thumbsup:

[Paul Haese]

Some direct experience.

12" f8 RC with SBIG STXL11000 which is about 0.76" per pixel

12' f4 Newt with QSI683 which is about 0.92" per pixel.

The image scales are not too dissimilar.

Same target to get similar looking results.

12" RC 39 hours
12" Newt 15 hours

Imaging speed does appear make a difference at similar image scales.

[Peter Ward]

Craig Stark also touched on this subject

http://www.stark-labs.com/help/blog/files/FratioAperture.php

[Shiraz]

Hi Paul

your experience could be explained by:

sampling - the imaging time scales with the square of the sampling, so your 11002 system required 1.5x as long as the 8300 due to this alone
QE - the 8300 has about 1.4x the QE of the 11002 at Ha, so there is another 1.4x in favour of the 8300
Read noise -for narrowband imaging, you will almost always be read noise limited and there will be a significant reduction in the required imaging time with the 8300 due to its lower read noise(how much depends on sub length).

So, multiplying these factors, your Newt with 8300 should be considerably more than 2x times faster than the old system was - but much of it is down to the limitations of the 11002, not the FNo of the scopes.

cheers Ray

[strongmanmike]

All hail King Ray, the god of unbiased reasoning :prey2:

:)

[Peter Ward]

That's the rub...I'm slumming it with just the one camera of late. :)

I have to say I've been tempted to try one of the new Sony chipped SBIG's..but suspect there will be no revelation in doing so with the various 'scope's I'm currently using.

[RobC]

Rick,

Don't you mean decreasing the image scale.

Rob

[ErwinL]

Hi Peter,

there have been lots arguments and it's hard to put it all together (and I hope I didn't miss a contribution already having said the following).
There is one of your statements that might have pushed you in a wrong direction:

The point is, that only a part of the light hitting your 10" mirror will also hit your sensor. All the light coming from outside your aperture angle will be lost.
Now, if you add a reducer like Suavi did and make your scope 'faster', you will increase the aperture angle and thereby also increase the total amount of light hitting your sensor. I.e. less of the light coming from your mirror will hit the camera housing.
(This is the exactly the same what Rick and others arged, only from a different point of view.)

[Peter Ward]

A 10" scope can only ever get 10 inches worth of light...much like a cookie cutter ...light is limited by the aperture of the mirror or lens....i.e. a cookie cutter can't gather pastry beyond its edge...angles simply don't come into it.

Hence I'm not sure what you mean by "aperture angle" as the wavefront from the sky is perfectly parallel to the telescope aperture (or "cutter" )

[Camelopardalis]

The light beam might be parallel as it enters the scope but as soon as it hits a lens or a mirror it is bent into a cone of light.

The steepness of the cone of light is directly related to its focal ratio.

[multiweb]

MBJ did a great talk years ago about this stuff. He used the water buckets and rain analogy. Rain falls down and in straight parallel lines, not in a cone pattern or at an angle so Peter's cookie cutter is on the money. The wider your bucket the more rain falls inside it. The deeper tbe bucket, the longer it takes to fill up. Pretty simple.

[Shiraz]

Hi Marc. I'd be a bit wary of the rain analogy - like all simplifications, it can be a little bit misleading.

As Erwin suggested, light definitely comes in from a range of angles. The light that has a wavefront parallel to the aperture is focused by the optics into an on-axis point. Anything that is focused off-axis comes in with a tilted wavefront - it comes at a different angle from a different part of the sky and is focused to a point away from the optical axis in the focal plane.

An image with a million pixels samples the light coming from a million different directions - that's how an image is formed. The pixel size and focal length determines how big an area of sky is looked at in each of those directions - for a given aperture and pixel size, the faster the scope, the bigger the solid angle of sky looked at by each pixel. So the faster scope will collect more light in each pixel (for a given aperture and pixel size). Thus, the amount of light getting to each pixel is determined by both the aperture size (how many photons can get through it from any direction) and also the area of sky sampled by each pixel (how many photons you start out with from any direction). The rain analogy completely misses the second bit and wrongly leads to the conclusion that aperture is all that matters - aperture is fundamentally important, but it is definitely not the whole story..

[ErwinL]

The field of view displayed on your image has a diameter also on the sky.
The angle between the outer bounds seen from the point of observation is what I called "aperture angle".
Photons from (a few diameters) aside of this field still hit the mirror, but not the sensor. Adding a reducer (between mirror and sensor) redirects photons from outside the original field to your sensor. The total count of photons you earn is now those from the original field plus the ones from the additional part of the sky your image shows.

(Btw.: Making this angle too wide will lead to shadowing by your tube, but more serious is the aberration introduced by the reducer.)

[Peter Ward]

This very confusing/misleading. Starlight does not come in at angles. The wave front is perfectly parallel to the aperture...any photons left or right simply don't enter the telescope.

Once the light is past the aperture of the telescope, then the telescope optics do what they do..reflect/refract to form the image.

Sure, faster telescopes put the focused image over a smaller area....nailing down how much a difference that makes is the sticking point.

[ErwinL]

It does! The angle of the light from a single star is too small to resolve (except Beteigeuze I believe), but the angle between the different stars on our images can be resolved easily.

As a result, the star images will not get brighter with F-number, but extended regions like nebulae will!

[Shiraz]

maybe this will help. from https://www.ifa.hawaii.edu/users/gruff/default/Astrolab/07BasicTelescopeOptics.htm An image requires that the light come from different angles.

"A lens which could only focus light rays striking the glass head-on .... would be fairly useless for astronomy. Fortunately, most lenses can also accept rays which come in at a slight angle to the optical axis, and bring them to a focus as well. This focal point is not the same as the focal point for rays which are parallel to the optical axis; depending on the angle of the incoming rays, their focus lies on one side or the other of the optical axis, as shown in the diagram below. But if the lens is well-made, all these focal points will lie on a plane which is parallel to the face of the lens; this is called the focal plane."

[JA]

As per Erwin and Ray's comments on the field of view (acceptance angle, solid angle of view or however named), it is like considering a ray diagram for a convex lens with an object at infinity with on-axis parallel light rays(BLUE) and off-axis parallel (GREEN and RED) rays from infinity. The field of view being formed by the angle between the GREEN and RED rays as shown.

This is shown in the attached ray diagram, with the 3D effect forming a cone, or more correctly a truncated cone at the lens surface.

Best
JA

[Slawomir]

Such a great thread and excellent discussion - looks like quite a few myths are being busted :thumbsup:

[glend]

Craig Stark's article on this topic ( was written in 2008):

http://www.stark-labs.com/help/blog/files/FratioAperture.php

As Craig notes: "... that once we’re well above the read noise, the effects I’ve mentioned here become weaker. "
I wonder how the new generation of ultra low read noise CMOS cameras, would change Craig's article if written today.

[multiweb]

Ok, so you guys are talking about a lens/corrector in the front of the aperture to focus all ligh rays into the pipe. I thought we were comparing fast newts vs. RCs designs where the first optical element is located at the end of the tube.

[Shiraz]

no, there is nothing in front of the aperture. The ray diagrams just use a simple lens as the optical element to illustrate how an image is formed. The same thing happens with more complex real optical designs, but it is harder to visualise. Will try to put together something.

[multiweb]

Like a fisheye lens exposed to all the sky yeah but I just can't visualise how rays that are not near parallel to a closed tube optical axis will hit the mirror at the end of it, with baffles as well, etc... The few that hit the primary sideways would they even bounce to the secondary or miss it altogether? :question:

[skysurfer]

When a lower f/ratio were better than a high one, regardless of the absolute aperture, as some claim, then we all use iPhones / Galaxies or other Android phones to do AP, as these are usually a 'bright' f/1.8... f/2/2. And for regular photography as well and the DSLR market would be dead and all reporter and professionals use lightweight phonecams.
And there were no EELT or JWST or other monster telescopes in use.

I can see it when I do AP with my Canon 70-300L (f/5.6) which has 55mm aperture and with my ED110. First is 300mm f/5.6 and the latter 600mm f/5.6.
See the difference that the latter shows more detail and fainter stars with the same f/5.6.

[glend]

It is about the angle of incident, and angle of reflection, the ray does not have to be perpendicular to the mirror ( these mirrors are not flat). You can demonstrate this with a ray diagram, or a ray string trace running from the edge of the mirror to the opening of the tube on the opposite side. With parabolic mirrors the angle of reflection will be towards the secondary.

[Shiraz]

nobody is claiming that - aperture is fundamental, but it is not the whole picture



agreed, the angles are typically not large for a telescope (generally within about a degree of parallel to the axis). The point is though that they are definitely not parallel from different parts of the sky and the same mechanism as in JAs widefield example applies. Each point in an image is illuminated from an individual part of the sky and the ray bundle that forms it is not parallel to the ray bundles that form all other points in the image. Telescopes and fisheye lenses form images in essentially the same way - just over vastly different field angles.

The amount of light that gets into a pixel is determined by two things:
how wide each ray bundle is before it is focused by the optics (determined by the aperture) and
how much sky is included in each ray bundle (determined by the angular extent of the pixel).

Cheers Ray

[multiweb]

Jason (http://www.iceinspace.com.au/forum/member.php?u=7423)should draw us one of his cool raytrace diagrams to illustrate. :)

[JA]

100% on the money - That it the crux of it! Only rays that are within the field of view of the telescope or lens will make their way through to the focal plane (unless limited by a field stop/iris). In the case of a telescope these rays are near parallel to the lens axis, typically no more than a few degrees down to fractions of a degree off-axis at higher focal lengths. For much lower focal lengths typical of camera lenses say 14 to 300mm, the field of view is larger and so the angle off-axis is much larger.

The diagram I drew earlier was a general case for a convex lens, but in essence holds true whether a lens or telescope. All that differs is the field of view.

For example for a 36mm x 24mm full frame, the horizontal field of view may be:

104 degrees FOV for 14 mm focal length
40 degrees FOV for 50 mm focal length
6.9 degrees FOV for 300 mm focal length
1.8 degrees FOV for 1150 mm focal length
1 degree FOV for 2000 mm focal length

Consider these field of views in relation to the Convex Lens ray diagram, in terms of the issue you raised of rays "near parallel to a closed tube optical axis".

In terms of the closed tube you mentioned it's internal diameter will (should !) always allow for the telescope's field of view, otherwise vignetting would result.

Best
JA

[RickS]

As you know I have a C300. I also share another one at SRO. In my experience there is a significant SNR advantage in the f/4.9 configuration. This advantage comes at the cost of decreased resolution, of course, but provides a larger FOV.

One last thought on aperture... let's take the example of a 300mm aperture scope with 50% obstruction and a 50mm f/4 lens. The scope sucks down photons at 432 times the rate of the lens. If we assume that aperture is all important, then we come to the conclusion that all those DSLR astro images must be fake, or perhaps were taken over a period of several weeks :lol:

[multiweb]

My C11/Hyperstar combo is about the same FL as my FSQ106N. I've also noticed it is a *little* faster that the FSQ so it seems to support your argument. ;)

I wonder if it has better resolution too? Or is it the placebo effect? :question:

[RickS]

Red wine & cheese effect, Marc :lol:

More seriously, the larger aperture will give you a theoretical Airy disk size more than two times smaller than the FSQ, so this effect is quite likely real.

[multiweb]

Pfweeh... I already feel better. :drink:



That's it. I'll put my FSQ for sale then. I enjoy too much lifting my 26kg "big john" over my head trying to slide it and aim for that thin rail profile on the dovetail. :thumbsup:

[Camelopardalis]

Clearly surplus to requirements :lol:

[Peter Ward]

Great diagram. Certainly helps toward visualising an optic's field of view.

Then I pondered the "magnifying glass" concept for a bit...it was sort of a Eureka moment for me :doh: ...and considered the flux being converged by an optic.

With a BIG magnifying glass, and a sunny day, you can burn most things
with a concentrated image of the sun.

But a (very) long focal length lens gives you a nice big solar image, but no smoke.

Similarly, a tiny short focal length lens gives you a tiny bright spot, but again no smoke.

The simple physics is not enough concentrated flux in either case.

But this only applies to extended sources (eg Sun/Moon/Nebulae) that you can concentrate.

With point sources..eg stars.... aperture wins every time.

[Shiraz]

also applies to stars Peter. Faster optics produce smaller more intense focal plane spots from unresolved objects like stars - works just the same as with extended sources.

[multiweb]

Actually a mirror/optics maker should be able to put this one to bed. There are a few here who figure and test optics. Stefan Buda, Bratislav, Mark Sutching. Maybe they can chime in.

[skysurfer]

That depends on the physical aperture and FL has nothing to do with this.
From the Sun we get 1000W/m2 so a lens of given aperture will concentrate the same amount of energy, regardless of its diameter. So smoke will appear.

My 40cm Dob instantly ignites a newspaper when pointed at the Sun and no eyepiece in it.

[Shiraz]

which bit of it :lol:

[Peter Ward]

Well I've give you the Airy disc size.... is solely determined by F-ratio....but suspect the seeing will swamp any differences in practice. :thumbsup:

[Shiraz]

same with seeing-limited imaging - angular star size is fixed by the sky, linear size in the focal plane depends on focal length, so shorter scopes produce smaller spots

[Camelopardalis]

That would be determined by the focal length... an f/4 at 800mm may not be "seeing limited" on a half decent night, whereas the seeing that night might not support good results from a f/8 at 1600mm... same aperture, but different results :shrug:

[Peter Ward]

I'd expect a 40cm dob to just that!

Try forming a solar image with a 50mm aperture F50 lens and then a 50mm F1.0 lens and let me know how is goes ;)

[skysurfer]

Simple calculation: both burn the same. Same amount of energy.
(pi/4)*(0.050)2 * 1000 = 2W of energy, on 50mm distance or 2.5m distance. In the latter case you are (slightly) right: the air cone of 2.5m long which is heated by the two watts results in slightly less burn at the focal point, but it is the same power being captured.

But when used as an objective lens for light and the air is clear inside the 2.5m tube of the 50mm f/50 lens it does not make any noticeable difference.

[Peter Ward]

I'm still having problems with this..

With my system in its native F8.0 configuration I can expect around a 10
micron Airy disk. A good match for my 9 micron pixels.

If I reduce the focal length I might get 6 micron airy disks. Those big pixels won't notice any more photons....the aperture has remained the same...hence I'm unsure of what you mean by "more intense"

[Peter Ward]

Let's just look at the focal length.

At 24mm a lens will produce a solar image .22 mm across

At 2000mm the solar image will be 18mm across

Sure, the same power/flux is being captured by the same aperture, but in the short FL case all the flux is going into an area of just 0.4 mm, versus 254 sq mm in the long FL case.

[multiweb]

All of you lot. :lol:

[multiweb]

That's a good illustration (https://omlc.org/classroom/ece532/class1/collect_solidangle.html).

[Shiraz]

yep, undersampled images are the odd man out. The star spots will become more intense (ie have more spatially concentrated energy) as the fl decreases, but this change won't be detected. ie, what you see will be determined by the sampling as well as the optics. Which is actually a good example of why sampling was mentioned as being of prime importance way back in the early days of the thread. :thumbsup:

[Slawomir]

While we are answering great questions so effectively, perhaps someone could explain to me in simple terms how does a photon enter a telescope (mis)behaving as a wave and then it hits the sensor as if it was a particle. Maybe terms such as wave and particle are just illusory concepts that do not penetrate into the essence of the reality and are just simplistic ideas manufactured by our limited brains :question:

[RickS]

Hi Suavi,

Here's something to get started: https://en.wikipedia.org/wiki/Wave%E2%80%93particle_duality

Maybe Ray could explain it better but I can't :)

Cheers,
Rick.

[AndrewJ]

Gday Suavi

This has always screwed with my head but being a mech engineer i try to equate it to something "real" like ocean waves
When they roll past a point etc as a total wave, they refract in a relatively well understood manner, but when they hit the beach, each particle in the wave has its own momentum and it disperses its energy based on what it hits.

Andrew

[Shiraz]

Way above my payscale :).

This might be useful? - cheers
https://theconversation.com/explainer-what-is-wave-particle-duality-7414

[Atmos]

Although it isn’t quite correct the best way to visualise it in your head would be to consider a photon has being a discrete particle that oscillates in waveform as it moves... much like a snake.

Although it glosses over many things it is easy to visualise :)

[doppler]

Do we even actually know what a so called photon actually is? Google wasn't much help.

"The photon is a construct that was introduced to explain the experimental observations that showed that the electromagnetic field is absorbed and radiated in quanta. Many physicists take this construct as an indication that the electromagnetic field consists of dimensionless point particles, however of this particular fact one cannot be absolutely certain. All experimental observations associated with the electromagnetic field necessarily involve the absorption and/or radiation process.
So when it comes to a strictly ontological answer to the question "What is a photon?" we need to be honest and say that we don't really know. It is like those old questions about the essence of things; question that could never really be answered in a satisfactory way. The way to a better understanding often requires that one becomes comfortable with uncertainty."

"The photon the experimenter in quantum optics (detection correlation studies) usually talks about is a purposely mysterious "quantum object" that is more complicated: it has no definite frequency, has somewhat defined position and size, but can span whole experimental apparatus and only looks like a localized particle when it gets detected in a light detector."

[luka]

As a physicist I can tell you that the answer to your question is 42. Now what was the question again :lol:

More seriously, I think Slawomir captured our current state of knowledge with this sentence:

Maybe not "limited brains" but limited knowledge... or maybe both... Basically we don't really know what a photon is. The question becomes not what they are but how we observe them. Depending on the experiment, they can show wave-like and particle-like properties but are neither of the two.

But, wave or particle, photons sure look pretty when coming from the night sky :astron:

[Slawomir]

Thank you all for helping me to get a better understanding of what light is. It may seem like I semehow got us off the original great topic started by the OP, but in fact I just wanted to remind us that there are great mysteries happening not only in the skies, but also right within our telescopes when we try to collect ancient light in the most effective way.

Luka - I agree that the phrase ‘limited brains’ should be replaced with ‘limited knowledge’ :)

[markas]

I'm buying into this one quite late, but here goes anyway;

Firstly, the "f-ratio myth":
The amount of light entering the telescope is a function of aperture. Hence the number of photons arriving at the sensor plane is basically a function of aperture.

BUT, the concentration of photons hitting a pixel is also a function of focal length.

If you take Steve Moore's own equations for Signal and S/N you can arrive at a relatively simple relationship that pixel S/N ~ (pixel size/f-ratio)*sqrt(sub time) - all other things such as quantum efficiency, optical efficiency etc being equal.


Similarly, the Signal per pixel ~(px/f-ratio)^2 .

These are very useful rules of thumb in estimating exposure times and signal intensity for your system based on images obtained by other systems of known (px/f-ratio).

Secondly, photons and waves.

Very hard to comprehend, but the quantum mechanical reality is that if you interrogate a wave/particle with wave instruments, you'll get wave answers. If you interrogate the same wave/particle with particle instruments, you'll get particle answers.

A wave/particle in an optical train is well characterised by wave behaviour, but the sensor asks particle questions and the particle explanation works.

Confused? We're not Robinson Crusoe - quantum mechanics is plain hard to understand. As the great quantum theorist Richard Feynman said "if you think you understand quantum theory, you don't"

With apologies!
Mark

[Peter Ward]

All good. I've found many of the responses useful. Thanks to one and all for keeping the thread relevant and informative. :thumbsup:

Now onto the subject of: life the universe and everything.... :D