Xaviers Latest Nonsense

[Bryanpoprobson]

Oh dear, Hunchie has gone totally insane. In his latest offering to the believers of WooWoo nonsense on WooTube, he claims that Apollo was fake because: -

During Apollo 14 they carried out laser ranging tests to the reflectometer while the astronauts were still there. He claims that no such laser light was seen in any of the videos and that it would have been hazardous to the astronauts. 

Just hilarious...

[benparry]

I'm not entirely sure whether this is a badge of honour but i have been blocked on facebook Pascal. Myself and Allan F kind of annoyed him in a group and he left and blocked me. Can this be considered an honour 

[molesworth]

Oh dear, Hunchie has gone totally insane. In his latest offering to the believers of WooWoo nonsense on WooTube, he claims that Apollo was fake because: -

During Apollo 14 they carried out laser ranging tests to the reflectometer while the astronauts were still there. He claims that no such laser light was seen in any of the videos and that it would have been hazardous to the astronauts. 

Just hilarious...

Showing once again that anyone with a basic understanding of science wouldn't believe in this nonsense in the first place.  Just ask him to calculate how bright it should have appeared in the videos...

[onebigmonkey]

Here are the terrifying lasers of doom as seen from Surveyor 7:



https://www.drewexmachina.com/2018/01/07/surveyor-7-the-mission-to-tycho/

I mean, there's no high pitched oscillating noise and pulsing red lights or anything, but it would obviously have them writhing around in complete agony while they begged Houston to stop.

[raven]

Add to the list of things Hunchback doesn't understand: the inverse square law.
Maybe because he follows an inverse square law: the closer you look, the dimmer he appears.

[JayUtah]

In theory a laser doesn't obey an inverse power law because the energy progresses along a straight, confined line.  The inverse square law -- and similar such laws -- are meant to capture the effect on the quantity in question of the geometry of its expansion, usually in all directions.  So normal radiometry employs an inverse power law because it deals with, in the general case, spherical expansion.

In practice a laser obeys the inverse square law because the beam is never perfectly cylindrical.  The operative portion of it is conical.  Although it starts out on Earth only a handful of millimeters across, by the time it reaches the Moon it has expanded to hundreds of meters across.  This is due to the imperfection of the relevant optics and, to a lesser degree, the scattering encountered on the way out of Earth's atmosphere.  As such, all the energy in the laser beam expands over a solid section of a sphere.  So a coefficient applies.  But the underlying law is still a power law in practice.

Now there is also a curious aspect to the study of radiometry in that we simultaneously consider the energy from the perspective of the emitter, where certain laws apply, and also from the perspective of the receiver, where certain other laws apply.  It's valid to look up at the sky and say that only a tiny speck of light is visible in the whole of the sky, from the laser on Earth.  But as it comprises only a tiny sliver of the dome of space around you, it's not a worry.

[bknight]

Add to the list of things Hunchback doesn't understand: the inverse square law.
Maybe because he follows an inverse square law: the closer you look, the dimmer he appears.

Being a degreed aerospace engineer, he should know better.  The more I see if his antics makes me believe that he is either hopelessly incompetent or as a few have observed, he is just being a troll.

[Zakalwe]

Add to the list of things Hunchback doesn't understand: the inverse square law.
Maybe because he follows an inverse square law: the closer you look, the dimmer he appears.

Being a degreed aerospace engineer, he should know better.  The more I see if his antics makes me believe that he is either hopelessly incompetent or as a few have observed, he is just being a troll.

[12oh2alarm]

In theory a laser doesn't obey an inverse power law because the energy progresses along a straight, confined line.  The inverse square law -- and similar such laws -- are meant to capture the effect on the quantity in question of the geometry of its expansion, usually in all directions.  So normal radiometry employs an inverse power law because it deals with, in the general case, spherical expansion.

In practice a laser obeys the inverse square law because the beam is never perfectly cylindrical.  The operative portion of it is conical.  Although it starts out on Earth only a handful of millimeters across, by the time it reaches the Moon it has expanded to hundreds of meters across.  This is due to the imperfection of the relevant optics and, to a lesser degree, the scattering encountered on the way out of Earth's atmosphere.  As such, all the energy in the laser beam expands over a solid section of a sphere.  So a coefficient applies.  But the underlying law is still a power law in practice.

This laser physicist feels the need to expand a bit on the above. Please wear your safety goggles to protect the healthy eye

Lasers making parallel beams is a popular oversimplification. All beams widen and this is quantified as divergence, which describes the (half) opening angle of the cone Jay refers to.

In theory (and practice) the inverse square law always holds. It's just that as the origin you don't use the exit aperture, but treat the laser as a point source located behind the laser cavity or resonant medium.

What is responsible for beam divergence is diffraction at the exit aperture. I would not call it imperfect optics. It's fundamentally wave optics, in much the same way you can't image stars as perfect dots, but Airy rings, due to the finite aperture of telescopes. Another coefficient > 1 increases ideal minimum divergence because laser light comes in transverse modes, which means the intensity is brightest on the optical axis and has the shape of a bell curve towards the aperture (for the "TEM00" mode), as opposed to constant intensity across all of the aperture. Higher transversal modes (TEM01, 11, etc) have even larger divergence. Thus you use mono-mode lasers.

When Mythbusters shoot at the Moon for LLR, the relevant aperture for determining beam divergence is not the few mm of the actual laser they use. The beam is expanded by the telescope optics and the effective aperture is that of the telescope (1.5m or so). This gives a much narrower cone. You really need the oomph of a high power laser and narrow cone to get the photon counts needed for LLR. A hand-held diode laser (abysmal divergence without beam-forming optics) will not cut it.

[Obviousman]

In theory a laser doesn't obey an inverse power law because the energy progresses along a straight, confined line.  The inverse square law -- and similar such laws -- are meant to capture the effect on the quantity in question of the geometry of its expansion, usually in all directions.  So normal radiometry employs an inverse power law because it deals with, in the general case, spherical expansion.

In practice a laser obeys the inverse square law because the beam is never perfectly cylindrical.  The operative portion of it is conical.  Although it starts out on Earth only a handful of millimeters across, by the time it reaches the Moon it has expanded to hundreds of meters across.  This is due to the imperfection of the relevant optics and, to a lesser degree, the scattering encountered on the way out of Earth's atmosphere.  As such, all the energy in the laser beam expands over a solid section of a sphere.  So a coefficient applies.  But the underlying law is still a power law in practice.

This laser physicist feels the need to expand a bit on the above. Please wear your safety goggles to protect the healthy eye

Lasers making parallel beams is a popular oversimplification. All beams widen and this is quantified as divergence, which describes the (half) opening angle of the cone Jay refers to.

In theory (and practice) the inverse square law always holds. It's just that as the origin you don't use the exit aperture, but treat the laser as a point source located behind the laser cavity or resonant medium.

What is responsible for beam divergence is diffraction at the exit aperture. I would not call it imperfect optics. It's fundamentally wave optics, in much the same way you can't image stars as perfect dots, but Airy rings, due to the finite aperture of telescopes. Another coefficient > 1 increases ideal minimum divergence because laser light comes in transverse modes, which means the intensity is brightest on the optical axis and has the shape of a bell curve towards the aperture (for the "TEM00" mode), as opposed to constant intensity across all of the aperture. Higher transversal modes (TEM01, 11, etc) have even larger divergence. Thus you use mono-mode lasers.

When Mythbusters shoot at the Moon for LLR, the relevant aperture for determining beam divergence is not the few mm of the actual laser they use. The beam is expanded by the telescope optics and the effective aperture is that of the telescope (1.5m or so). This gives a much narrower cone. You really need the oomph of a high power laser and narrow cone to get the photon counts needed for LLR. A hand-held diode laser (abysmal divergence without beam-forming optics) will not cut it.

Thank you for that!

[smartcooky]

Probably not directly on topic... but worth keeping in mind...

The way LLR works is fundamentally misunderstood, especally by Hoax Nutcases like Hunchback and the Blunder. Its is usually described as..."they fire a laser at the corner reflectors on the moon and then see how long it takes to come back and the time delay allows them to work out the distance". This is the grade school explanation... reality is far more complicated.

The signal we receive on the earth from the laser beam that has reflected from those corner reflectors is so weak that it is all but undetectable. Only one photon in 3x10⁷ photons we fire at the moon actually strikes the LLR reflector, and of all the photons that do get reflected, only one in every 3x10⁷ of those strike the detector here in Earth....that works out to not very many received reflected photons. The laser sends micro-pulses of laser light, 100 picoseconds long (one ten-billionth of a second) at a frequency of about 20 PPS. Only 1–5 return photons per pulse are detected.

The only reason they are able to detect the signal at all is because the light from the laser is monochromatic, but its not just a matter of looking for these photons and timing their arrival. The pulses are about 2cm deep, and even when detected, there is no way to determine where in the pulse the detected photon is - the leading edge, the trailing edge or somewhere in between. 

Now, there have been many improvements over the 50 years LLR has been in operation, but how this was explained to me is as follows:

The detector (which is essentially something like a highly sophisticated photon counter) only looks for the signal very close to the time it is expected to return - it counts the total number of photons in a given time window, and compares them with a window immediately before and immediately after the expected arrival time. These measurements are integrated over a period of time, until eventually, a detectable increase in the number of photons inside the window, compared with outside the window, means the reflected laser beam has been detected, and the more photons are detected, the better the accuracy of the measurement (N_error~√N) - I'm sure someone will correct me if I have that last part wrong.


 

[molesworth]

This has turned into a very informative discussion, thanks.

As with most things in science, the reality of getting accurate, repeatable measurements or results is a lot more complicated than most people realise.  Although there is still a place for the "grade school" explanations to help keep the non-technical general public engaged in science - something badly needed these days.

[JayUtah]

The detector (which is essentially something like a highly sophisticated photon counter) only looks for the signal very close to the time it is expected to return - it counts the total number of photons in a given time window, and compares them with a window immediately before and immediately after the expected arrival time.

"Range gating."  We do the same thing for very sophisticated radar.

[JayUtah]

In theory (and practice) the inverse square law always holds. It's just that as the origin you don't use the exit aperture, but treat the laser as a point source located behind the laser cavity or resonant medium.

Thanks for providing the correct model.  The P.E. exam actually only covers lasers insofar as they can weld stuff together and holography used to inspect surfaces. 

[smartcooky]

The detector (which is essentially something like a highly sophisticated photon counter) only looks for the signal very close to the time it is expected to return - it counts the total number of photons in a given time window, and compares them with a window immediately before and immediately after the expected arrival time.

"Range gating."  We do the same thing for very sophisticated radar.

Yep. If I correctly recall my time working on TACAN ground stations, we used range gating to help detect interrogation pulses and separating them from things such as reference pulses, squitter pulses and reply pulse-pairs.