Subject: Re: A Few Shots From South Africa
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Subject: Re: A Few Shots From South Africa
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On Thursday, March 1, 2018 at 11:34:10 AM UTC-5, Whisky-dave wrote:
> On Thursday, 1 March 2018 13:47:41 UTC, -hh  wrote:
> > On Thursday, March 1, 2018 at 7:19:12 AM UTC-5, Whisky-dave wrote:
> > > On Thursday, 1 March 2018 11:17:52 UTC, -hh  wrote:
> > > > On Thursday, March 1, 2018 at 5:33:38 AM UTC-5, Whisky-dave wrote:
> > > > > [...]
> > > > > Can you tell me which apollo mission (or any other) that 
> > > > > took car headlights to the moon. ?
> > > > 
> > > > Actually, they did take a corner reflector up there (at least once),
> > > > so headlights on Earth which happen to shine light that way would
> > > > have a direct path reflected back to Earth.  
> > > 
> > > There's no headlight on earth that could do that, what they left 
> > > there was for a laser to be shone to measure the distance to the
> > > moon oer time.
> > 
> > True, it was for a laser (which is light), but it was also designed
> > in the 1960s using 1960s technology for the receive sensor. 
> The apollos were 1960s technology too, well most that went to the moon were.

True, and as such, the only relevant technology question that this
poses is to what degree the technology has changed in the fabrication
of pentaprisms for the retroreflectors.  Simple answer there is that
it relies on mechanical geometry (and optical quality) which certainly
has improved over the past 50 years, but not by orders of magnitude.

> > > There;s no way a headlamp would be bright enough or be able to
> > > produce a beam bright enough to reach the moon and reflect back,
> > > it's difficult enough with a high power laser who beam only reflects
> > > back a very small amount of light in fact the reflected light is
> > > too weak to see with the human eye. 
> > 
> > Human eye?  Oh, sorry:  I thought this was about the ability of a
> > machine (Hubble's replacement) to have adequate senor resolution.
> It does but it still won't see headlamps on the moon any more than
>  it'll find the Spaghetti mosnster on the moon.

Except for how the whole "headlight on the moon" was simply a layman's
explanation of how much better the replacement is over Hubble.

> > >Out of 10^17 photons aimed at the reflector, ...
> > 
> > which is another way of saying ~0.25 Joules worth...
> so not a lot then.

Depends on context.  Here, the context is in raising the 
power on a terrestrial transmitter to go do something.

> > > only one is received back on Earth every few seconds, 
> > > even under good conditions.
> > 
> > Sure, but that's at the power level you specified. 
> A power much higher than the aveage car headlight.

There's been mere automotive headlights for sale that
are in the 100-200 Watt range for decades, and for a 
good halogen, figure 35lumens per watt...or use a HID
because their conversion efficiency is ~twice that. 

Similarly, I still own a couple of old camera strobes 
whose outputs were 200 Watt-seconds, which when you 
consider the short time duration of a strobe means that 
their peak output was easily 20 kJ (@ 10msec) if not
more (100 kJ @ 2msec).  True, this isn't narrow beam
focused like a laser, but more of that's an optics
based divergence question.

> > In contrast, Class4 laser today (minimum of 500mW) sell for as
> > little as $100 today, and 2W & 5W versions are pretty commonplace
> > (and why they're such a safety threat to aviation).  
> Yes well aircraft don't fly at the same distance from the 
> earth as the moon orbits.

Considering that 99% of that distance has no atmosphere for signal
attenuation, the distance only matters from the perspective of the
optical path's divergence angle.  

> > Case in point, here's a 10W green Class4 for only $250:
> I bet they can;t get that to reflect off something on the moon,
> and I bet such a laser if on the moon couldn;t be seen from earth.

Oh, it will definitely reflect ... that's basic physics.  The 
question is if you have a good enough sensor technology to detect
(reliably) the return signal.  Point is that our sensor tech is a
lot better today than it was back in 1969. 

> > <
> > 
> > ...that's 20x the power level you picked, and its an off-the-shelf 
> > commercial product.
> I didnl;t piuck any power level and it;s not really the power
> that is all important, at the distance the moon is the small beam
> would be about 5 metres in diameter, spreading the light out.

The light still gets there, and that which isn't absorbed will be
reflected, some of which back in the direction of Earth.  And because
atmospheric attenuation also isn't 100.0%, some *will* get through
to where it could be detected, given a good enough detector.  That's
all basic & immutable physics.  

> > Plus we can similarly look at the sensor side, to see how much that
> > technology to detect the return signal has improved over the last
> > 50 years...
> and how much the reflector has deterioded over that time.

Yes, there's been some deterioration that has been detected, but
that a performance shift has been detected is also illustrating 
that contemporary sensor tech is better today than in decades past.  

> > Case in point:  Nikon D5 goes up to ISO 3,280,000, which has 15x 
> > the light sensitivity of classical old ISO 100 film. 
> So, it still won't be able to see the laser beam.

Can't make that determination for sure without more literature research,
but it is a damn safe bet that that modern sensor is more sensitive than 
what they were originally using back in 1969.