Starship Asterisk*

Chris Peterson wrote:

alter-ego wrote:My calculations aren't getting anywhere near either of yours. Chris, your divergence is much larger than I calculate, and Art, your photon collection rate per m² is much higher than I calculate. The discrepancy is so large I'd like to find out why. Maybe I'm missing something. Regarding the photon count rate per m² I'm assuming one calculates it by taking the irradiance at distance (W/m²) ÷ Photon Energy which gives 1/[m²·sec]

Chris - What is the beam diameter your starting with to arrive at 4 ly diameter? Are there other assumptions you've made to arrive at that (e.g. interstellar extinction)?

I assumed 1mm, which would be about right for the cavity beam in a 100W gas laser.

I ran all the calculations off very quickly and didn't check them. It's entirely possible I made an error. Lots of very big and very small numbers; plenty of opportunities for exponent errors.

neufer wrote:

alter-ego wrote:
Art - Are you using 10.5cm wavelength / 2.86GHz?
I assume you're calculating antenna diffraction ≈ λ/Diameter (the Half-Power Beam Width)?

Yes to all that.

I get a beam an order of magnitude wider than Chris's laser beam
but with a power 3 orders of magnitude stronger so the beam intensity is ~10 times stronger.

I extrapolated from Chris's photon count based upon photons that are 200,000,000 times weaker
when they should have been 200,000 times weaker
so I really should have gotten 0.4 microwave photons per second. What do you get?

alter-ego wrote:
Art - Are you using 10.5cm wavelength / 2.86GHz?
I assume you're calculating antenna diffraction ≈ λ/Diameter (the Half-Power Beam Width)?

alter-ego wrote:My calculations aren't getting anywhere near either of yours. Chris, your divergence is much larger than I calculate, and Art, your photon collection rate per m² is much higher than I calculate. The discrepancy is so large I'd like to find out why. Maybe I'm missing something. Regarding the photon count rate per m² I'm assuming one calculates it by taking the irradiance at distance (W/m²) ÷ Photon Energy which gives 1/[m²·sec]

Chris - What is the beam diameter your starting with to arrive at 4 ly diameter? Are there other assumptions you've made to arrive at that (e.g. interstellar extinction)?

neufer wrote:

Chris Peterson wrote:
At a distance of 10,000 ly, the beam diameter will be at least 4 ly.

For a , 100W source, that means that any given square meter aperture will receive one photon every 39 days.

At a distance of 10,000 ly, the beam diameter from a 70 m radar dish (at half the 21 cm wavelength) will be about 37 ly.

For a 126kW source, that means that any given square meter aperture will receive about 400 microwave photons per second.

Anthony Barreiro wrote:If every person on Earth aimed a laser pointer at the Moon at the same time, would it change color?

Not if we use regular laser pointers.

Chris Peterson wrote:
At a distance of 10,000 ly, the beam diameter will be at least 4 ly.

For a , 100W source, that means that any given square meter aperture will receive one photon every 39 days.

BDanielMayfield wrote:I guess there’s just no way to get around the inverse square law then, is there? The intensity of any radiating source will always be four times dimmer with every doubling of distance, won’t it? Even if the source is a laser pointer.

BDanielMayfield wrote:I just meant “terrible” in the sense of terribly hard to detect because of attenuation of signal strength Chris. Thanks for your answer.

Chris Peterson wrote:

BDanielMayfield wrote:A beam spreading to 88 AU over just 4 LY is terrible

That's not terrible at all. That's the divergence you get with a perfectly collimated Gaussian beam. You can't do better- this is just the result of the way the beam interferes with itself. A geometrically collimated (parallel) beam can't exist in nature.

Whether radio or optical communications are better over a few light years depends on details of the technology. Certainly, we should be able to use either, depending on engineering tradeoffs.

BDanielMayfield wrote:A beam spreading to 88 AU over just 4 LY is terrible, but I trust Chris’ math. Please excuse my ignorance, but why wouldn’t it be possible to greatly lengthen a low wattage laser’s collimation (that is, beam tightness) using a lens or lenses to sharply focus the beam?

Chris Peterson wrote:

neufer wrote:
We can fix that

Still only a photon per square meter every few minutes at the center of the galaxy.

neufer wrote:We can fix that :!:

Chris Peterson wrote:

Boomer12k wrote:
Tell that to the millions of blinded Aliens about to SUE US!!!!!!!

Just to put the energy into perspective:

At a distance of 10,000 ly, the beam diameter will be at least 4 ly. For a 100W source, that means that any given square meter aperture will receive one photon every 39 days. At a distance of 4 ly, the beam diameter will be at least 88 AU. For a 100W source, that means that any given square meter aperture will receive 3 photons per second. So it's just possible that a very low noise, narrow band detector attached to a large aperture telescope at our nearest star could detect an optical signal from Earth. At the center of the galaxy? I doubt it's physically possible, regardless of technology.

• We can fix that

http://en.wikipedia.org/wiki/MIRACL wrote:

[b][color=#0000FF]SeaLite Beam Director, commonly used as the output for the MIRACL.[/color][/b]

---

<<MIRACL, or Mid-Infrared Advanced Chemical Laser, is a directed energy weapon developed by the US Navy. It is a deuterium fluoride laser, a type of chemical laser.

The MIRACL laser first became operational in 1980. It can produce over a megawatt of output for up to 70 seconds, making it the most powerful continuous wave (CW) laser in the US. Its original goal was to be able to track and destroy anti-ship cruise missiles, but in later years it was used to test phenomenologies associated with national anti-ballistic and anti-satellite laser weapons. Originally tested at a contractor facility in California, as of the later 1990s and early 2000s, it was located at a facility (32.632°N 106.332°W) in the White Sands Missile Range in New Mexico. The beam size in the resonator is about 21 cm high and 3 cm wide. The beam is then reshaped to a 14 x 14 cm square.

In 1997, amid much controversy, MIRACL was tested against a US Air Force satellite in orbit at a distance of 432 km. The satellite was disabled but the Air Force did not get the data from the satellite it had hoped for.>>

http://en.wikipedia.org/wiki/Hydrogen_fluoride_laser wrote:
<<The hydrogen fluoride laser is an infrared chemical laser. It is capable of delivering continuous output power in the megawatt range. Hydrogen fluoride lasers operate at the wavelength of 2.7-2.9 µm. This wavelength is absorbed by the atmosphere, effectively attenuating the beam and reducing its reach, unless used in a vacuum environment. However, when deuterium is used instead of hydrogen, the deuterium fluoride lases at the wavelength of about 3.8 µm. This makes the deuterium fluoride laser usable for terrestrial operations.

The deuterium fluoride laser constructionally resembles a rocket engine. In the combustion chamber, ethylene is burned in nitrogen trifluoride. This reaction produces free excited fluorine radicals. Just after the nozzle, the mixture of helium and hydrogen or deuterium gas is injected to the exhaust stream; the hydrogen or deuterium reacts with the fluorine radicals, producing excited molecules of deuterium or hydrogen fluoride. The excited molecules then undergo stimulated emission in the optical resonator region of the laser.

Deuterium fluoride lasers have found military applications: the MIRACL laser, the Pulsed Energy Projectile and the Tactical High Energy Lasers are of the deuterium fluoride type. An Argentine-American physicist and accused spy, Leonardo Mascheroni, has proposed the idea of using hydrogen fluoride lasers to produce nuclear fusion.>>

ta152h0 wrote:
Is this not an older program that was done years ago? That image looks familiar.

Boomer12k wrote:Tell that to the millions of blinded Aliens about to SUE US!!!!!!!

Chris Peterson wrote:

zbvhs wrote:
As I understand it, atmospheric irregularities cause the image of a star, say, to bounce around in the telescope's field of view producing a blurred image. The position of the laser spot is known, but it likewise is bounced around by atmospheric distortion. By distorting the mirror, the spot -- and a target in the FOV -- is held in one position for the required exposure time. The mirror distortions occurring are in the micrometer range. Correct?

Correct (although the mirror corrections may be submicrometer).

http://en.wikipedia.org/wiki/Deformable_mirror wrote:

<<Deformable mirrors (DM) are mirrors whose surface can be deformed, in order to achieve wavefront control and correction of optical aberrations. Deformable mirrors are used in combination with wavefront sensors and real-time control systems in adaptive optics.

A DM usually has many degrees of freedom. Typically, these degrees of freedom are associated with the mechanical actuators and it can be roughly taken that one actuator corresponds to one degree of freedom. Actuator stroke is the maximum possible actuator displacement, typically in positive or negative excursions from some central null position. Stroke typically ranges from ±1 to ±10 micrometres. Free actuator stroke limits the maximum amplitude of the corrected wavefront, while the inter-actuator stroke limits the maximum amplitude and gradients of correctable higher-order aberrations.>>

Chris Peterson wrote:
The ideal way of fixing the wavefront is to use a star for reference. But often, there isn't a bright enough star in the field to act as a reference (the correctable region is very small- just a few arcminutes at best). So in that case, a laser is used to generate an artificial star near or on the target. Some newer systems generate multiple reference stars in order to correct a wider field.

http://en.wikipedia.org/wiki/Adaptive_optics wrote:

[img3="Slow motion simulation of typical adaptive optics operation at a telescope.
The left hand side shows what a point source (e.g. small star) would look like through a ground-based telescope without adaptive optics correction. The right hand side shows what is seen after adaptive optics correction has been applied. Note that the adaptive optics corrected image is normally very compact, but occasionally it "breaks up". If long exposure images are taken, the adaptive optics correction produces a sharp point at the centre of the image, while the uncorrected image is just a large fuzzy blob."]http://upload.wikimedia.org/wikipedia/c ... _movie.gif[/img3]

<<A natural guide stars alternative is the use of a laser beam to generate a reference light source (a laser guide star, LGS) in the atmosphere. LGSs come in two flavors: Rayleigh guide stars and sodium guide stars. Rayleigh guide stars work by propagating a laser, usually at near ultraviolet wavelengths, and detecting the backscatter from air at altitudes between 15–25 km. Sodium guide stars use laser light at 589 nm to excite sodium atoms in the mesosphere and thermosphere, which then appear to "glow". The LGS can then be used as a wavefront reference in the same way as a natural guide star – except that (much fainter) natural reference stars are still required for image position (tip/tilt) information. The lasers are often pulsed, with measurement of the atmosphere being limited to a window occurring a few microseconds after the pulse has been launched. This allows the system to ignore most scattered light at ground level; only light which has travelled for several microseconds high up into the atmosphere and back is actually detected.>>

zbvhs wrote:As I understand it, atmospheric irregularities cause the image of a star, say, to bounce around in the telescope's field of view producing a blurred image. The position of the laser spot is known, but it likewise is bounced around by atmospheric distortion. By distorting the mirror, the spot -- and a target in the FOV -- is held in one position for the required exposure time. The mirror distortions occurring are in the micrometer range. Correct?