Laser Beam to the Moon

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Summary:

Requirements to send and see a laser beam to the New Moon

Main Question or Discussion Point

Hello,
i am going to buy a laser and a telescope powerful enough to see the laser spot on the moon.
The lunar phase i choose for the experiment is New Moon, for obvious reasons.
When i move the laser, i should see the light spot move on the moon surface after about 2.5 seconds, due to the light speed.
I need to know the minimal hardware specification that are enough to do this experiment.
For advertising purposes, i would like to choose a laser with visible light radiation, so that the beam can be optically seen on the surface.

First of all, i would like to understand which are the parameters that must be taken in account for the calculation, for example:
- laser output power
- laser beam refraction and attenuation in atmosphere
- laser location altitude
- telescope limit resolution
- telescope limit magnitude
- lunar surface reflectivity

for example, with output laser power about 1000W , telescope with 16.3 limit magnitude and 0.14 arcsec resolution, lunar surface light reflectivity 15% , laser altitude 6000mt (Tibet) how far i am from the minimum requirements to achieve my goal?

Let me know!
Thank you!
 
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Answers and Replies

  • #2
Vanadium 50
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i am going to buy a laser and a telescope powerful enough to see the laser spot on the moon.
I don't think you are. You need something like this.
 
  • #3
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... and you shouldn't live anywhere near air traffic routes!
 
  • #4
phyzguy
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I think the point that you are missing is that a lunar surface reflectivity of 15% means that 15% of the incident light is reflected in all directions. The fraction of this light that is reflected directly back at you is extremely small. Try calculating the solid angle of your telescope aperture as seen from the moon. That is why the lunar ranging experiments make use of the lunar retro-reflectors left by the Apollo astronauts, which reflect most of the light back in the direction it came from. And as you see from the link @Vanadium 50 posted, even with a laser with a peak power of over 1GW, they only get a few photons coming back. I don't think there is any way you are going to see a laser spot on the lunar soil with any reasonable telescope you can own.
 
  • #5
berkeman
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@lovicodo -- Are you a HAM radio operator? Or do you have an active HAM radio club in your area? If so, consider a "Moon Bounce" experiment instead. A moderate size 2m station can establish a low datarate CW link with just a few hundred watts of Tx power...

https://en.wikipedia.org/wiki/Earth–Moon–Earth_communication
 
  • #6
tech99
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I remember reading, when at school and before space travel, that a kilogram of burning Magnesium on the Moon's surface would be visible to the largest telescopes on Earth. Unfortunately my Science teacher asked for the calculations, which I could not provide!
 
  • #7
Vanadium 50
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my Science teacher asked for the calculations, which I could not provide!
The best answer would be "what is it using for oxygen?"
 
  • #8
Vanadium 50
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how far i am from the minimum requirements to achieve my goal?
At least a factor of 500,000,000.
 
  • #9
davenn
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Summary:: Requirements to send and see a laser beam to the New Moon

i am going to buy a laser and a telescope powerful enough to see the laser spot on the moon.
The lunar phase i choose for the experiment is New Moon, for obvious reasons.

The obvious reason is a very bad one. The Moon is going to be within a few degrees of the Sun.
Not a good direction for pointing a telescope unless it has a good solar filter ….
But then that will stop any weak reflected light from the moon
 
  • #10
Vanadium 50
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@davenn , that's an excellent point. Even if you looked immediately after sunset/before sunrise (as appropriate),
  1. there's still a lot of scattered sunlight
  2. you're shooting your laser through a very thick column of air
  3. you don't have much time to make your measurement before the sun rises/moon sets
So what phase do you think is best?
 
  • #11
davenn
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@Vanadium 50

totally agree with your 3 points

So what phase do you think is best?
well as other earlier said, making a spot on the moon with a laser is very difficult considering how
much the beam will spread out on it's way to the moon. The beam spread is likely to be much
wider than the angle the Moon subtends in the sky, anyway. Using the reflectors that NASA Apollo
mission(s) left is a much better idea and much more likely to produce a successful result.

I am not personally aware of the quality of lenses etc for laser collimation is these days ?
But just getting an illuminated patch on the moon, a better time in the lunar month would be
around first or last quarter or a few days earlier/later (respectively) where there is a large dark
area of the moon to "aim at" and it is much higher in the sky where the atmosphere is thinner

Dave
 
  • #12
sophiecentaur
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i am going to buy a laser and a telescope powerful enough to see the laser spot on the moon.
That OP reads, to me, like a typical Christmas Puzzle, set by a mischievous tutor for bright students. Would I be right?
Best time to do the experiment: Well before first quarter, I would think. Moon not quite overhead but Sun well below the horizon. Imaging the spot would make a big difference to the detection. Many astro objects are only visible by photography. A long exposure and some masking over the sensor could help. But distinguishing individual pulses would not be as subjectively satisfying, perhaps.

Did you consider using RF signals for this? This link discusses problems and successes for amateur operators.
 
  • #13
Vanadium 50
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considering how much the beam will spread out on it's way to the moon.
Typically a good laser's divergence will be around a milliradian. This is diffraction limited, so to do better you need a really large source - not easy with a laser. So that tells you will be able to make a spot around 250 miles around.

Earthshine is about 100 mW/m2, so that's 10-15 GW of power (and I am playing kind of loose with radius vs. diameter and spot size). You can tell right away that any sort of setup where one sees the spot will take enormous power on the earth to drive that laser.

The trick is to pulse the laser: the return time is spread by a few 10's of microseconds, so megawatt-class lasers will work if I compare on-time signals with out-of-time signals. But this requires instrumentation, not just eyeballs.

The five retroreflectors on the moon do a much better job of returning light to earth than the charcoal-like surface of the moon, but there are hundreds of square miles of lunar surface per meter of reflector.
 
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  • #14
davenn
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Did you consider using RF signals for this? This link discusses problems and successes for amateur operators.

he/she wants to see the laser spot on the moon's surface, a little difficult with a radio signal
and the beam spread with a radio signal would be 100's of times worse
 
  • #15
sophiecentaur
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and the beam spread with a radio signal would be 100's of times worse
Yes but you cannot deny that it's been done by radio amateurs. That means you need to do the whole link budget before accepting or rejecting feasibility of a method. RF techniques are very good at digging down into the noise.
"Seeing" is not necessarily believing and we all know that. One thing that the OP should be persuaded is that proof can come in a variety of ways.
And I still wonder about the actual source of the original question. (I have written similar questions for students. If the available money is not limited then the optical way is possible.
 
  • #16
Vanadium 50
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Three people have suggested radio/radar. That's a fine answer, and I agree 100% that it is more feasible, but not the answer to the question the OP asked.
 
  • #17
davenn
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Yes but you cannot deny that it's been done by radio amateurs. That means you need to do the whole link budget before accepting or rejecting feasibility of a method. RF techniques are very good at digging down into the noise.

Ohhh yeah, RF moonbounce is lots of fun :smile:
 
  • #18
sophiecentaur
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Three people have suggested radio/radar. That's a fine answer, and I agree 100% that it is more feasible, but not the answer to the question the OP asked.
In which case I think the answer is No, unless you have a lot of resources. The question is asked in the first person and, if it ain't from a millionaire, that's definitely the answer.
It wouldn't be the first time that an original question to PF has been modified in order to provide a reasonable solution.
 

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