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Old 22-02-2018, 01:00 PM
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Originally Posted by Shiraz View Post
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.
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.
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Old 22-02-2018, 01:09 PM
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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

Last edited by Shiraz; 22-02-2018 at 01:40 PM.
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Old 22-02-2018, 01:10 PM
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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.
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
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Old 22-02-2018, 01:21 PM
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Originally Posted by skysurfer View Post
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.
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
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Old 22-02-2018, 01:36 PM
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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
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.
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Old 22-02-2018, 01:45 PM
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Originally Posted by Shiraz View Post
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.
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"
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Old 22-02-2018, 02:06 PM
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Originally Posted by skysurfer View Post
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.
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.
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Old 22-02-2018, 02:08 PM
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which bit of it
All of you lot.
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Old 22-02-2018, 02:15 PM
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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
That's a good illustration.
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Old 22-02-2018, 02:21 PM
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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"
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.

Last edited by Shiraz; 22-02-2018 at 05:46 PM.
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Old 22-02-2018, 06:12 PM
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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
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Old 22-02-2018, 06:48 PM
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Originally Posted by Slawomir View Post
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
Hi Suavi,

Here's something to get started: https://en.wikipedia.org/wiki/Wave%E...rticle_duality

Maybe Ray could explain it better but I can't

Cheers,
Rick.
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Old 22-02-2018, 07:09 PM
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Gday Suavi
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how does a photon enter a telescope (mis)behaving as a wave and then it hits the sensor as if it was a particle.
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
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Old 22-02-2018, 07:57 PM
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Way above my payscale .

This might be useful? - cheers
https://theconversation.com/explaine...e-duality-7414

Last edited by Shiraz; 22-02-2018 at 08:08 PM.
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Old 22-02-2018, 09:23 PM
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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
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Old 22-02-2018, 10:47 PM
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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."

Last edited by doppler; 22-02-2018 at 11:00 PM.
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Old 23-02-2018, 02:13 AM
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Do we even actually know what a so called photon actually is? Google wasn't much help.
As a physicist I can tell you that the answer to your question is 42. Now what was the question again

More seriously, I think Slawomir captured our current state of knowledge with this sentence:
Quote:
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
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
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Old 23-02-2018, 03:23 AM
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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’
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Old 23-02-2018, 11:17 AM
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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

Last edited by markas; 23-02-2018 at 08:01 PM.
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Old 23-02-2018, 05:50 PM
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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.

An wave/particle in an optical train is well charasterised 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
All good. I've found many of the responses useful. Thanks to one and all for keeping the thread relevant and informative.

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