Light Beamer | Atmosphere

The atmosphere introduces two effects: absorption (or ‘reduction of transmission from unity’), and loss of beam quality (or ‘blurring of the beam spot’). The transmission of the atmosphere at a wavelength of 1 micron is extremely good, exceeding 90% at high altitude ground-based sites. Going to a high altitude site also significantly reduces atmospheric blurring, which would allow an adaptive optics system to achieve performance near to the diffraction limit.

The effects of atmospheric turbulence on the beam include a broadening of the beam footprint (equivalent to image blurring for telescope observations), random jitter of the beam spot, and intensity fluctuations (or ‘scintillations’). The blurring depends on turbulence and wind profile in the atmosphere. The turbulence amplitude is reduced by a factor of approximately 4 between sea level and an altitude of 5km.

The quality of an image is measured by the Strehl ratio, which reflects the ratio of the peak image intensity from a point source to the diffraction limit of an ideal optical system. The ratio measures phase deviations caused by lens aberrations and atmospheric turbulence. 10m-class telescopes, such as the Large Binocular Telescope (LBT), comprising two 8.4m telescopes, have demonstrated image resolution of 40 milliarcseconds and Strehl ratio of 80% at a wavelength of 1.6 microns.

Breakthrough Starshot aims to achieve the diffraction limit for an optical system of laser beams across 0.2-1km, which is 1-2 orders of magnitude beyond existing demonstrations. There are no fundamental physics limitations to achieving this improvement. A beacon on the nanocraft or near its launch point (for instance on the mothership) could be used to correct for phase variations in real time. The effect of the light beam on the atmosphere could be studied, and corrected for, by adaptive optics, again in real time. Additional beam focusing may also be explored to reduce the beam spot size using pulsed laser filamentation techniques.

Nov 05, 2016 03:19 Breakthrough Initiatives Posted on: Breakthrough Initiatives

RE"
"Aug 28, 2016 20:06Nathan BemisPosted on: Breakthrough Initiatives
A realistic idea for a dry high altitude location is perhaps the Nevada high desert 1200m(valleys)-3000m+(ridges) within the basin is Mt. Whitney in CA reaching 4400m. Elon Musk has his gigabattery factory located near Reno Nevada which could make for a convenient partnership in terms of nearby energy production and storage, battery research and development, cost savings, accessibility for people working on the project, easy access of resources to and from the location, and perhaps provide clear enough skies. Expected cost of land should be low, plots as large as desired, and it won't be an obstruction for cities/towns. This location should satisfy all the international security, supervisory, and regulatory conditions expected.
An idea to have the nanocraft return- Is to program the smart chip to activate an eventual "U-Turn" once it has completed its objective. If it can come back, we may be able to slow it for recovery with the same array we used to send it out. If this is possible, then opportunities for more advanced equipment could be used on these missions.
To minimize risk of space debris collision- Perhaps modify the chip into the form of a thin tube (like a dart/needle) or in the very least try to align the components to form into a smart stick rather than a square chip. When the craft has reached max speed and no longer receiving propulsion have the sails dispatch unless they provide further purpose."

Answer:
Great ideas. Our current targets are in the southern hemisphere. They are not visible from Nevada. On the other hand performing our development tests from such a location makes great sense. We have not figure out a way for the star chip to turn around.

- Avi Loeb, Breakthrough Starshot

Nov 05, 2016 03:21 Breakthrough Initiatives Posted on: Breakthrough Initiatives

RE:
"Sep 15, 2016 10:45Jacopo MaroliPosted on: Centauri Dreams
This is an amazing project and I wish to contribute to solving this challenge with my humble idea:
What about building a dyson swarm? https://en.m.wikipedia.org/wiki/Dyson_sphere#Dyson_swarm
We could place many collector-probes around our beloved star and re-directing the collected energy toward those which can "see" the exploration-probes. This would have the advantage of not moving the beamer if you need to redirect the beam, because you'll just have to reconfigure which target every collector-probe should aim. Anyway if really needed they could be moved using dynamic solar sails. Another advantage is that we can start with a low number of collector-modules (let's say 8 to cover every angle) and increase them over time. We can get very close to the sun to collect energy: the Solar Probe Plus should get at about 6,000,000 km in his perihelion.
https://en.wikipedia.org/wiki/Solar_Probe_Plus We could also output the energy from multiple probes so we don't overcharge one in particular. If I'm right every collector-sound in the half of the sun toward the exploration-probe should be able to "beam" it. I'd really like to hear your comments about this idea and your opinions about feasibility."

Answer:
Of course, this is the perfect solution but too costly. It solves many of the technical problems we are currently facing. We estimate that putting these components into space would increase the cost significantly, possibly by two orders of magnitude (note that the mirror in space called JWST costs of order $10B, similar to the total cost of the Starshot program.

- Avi Loeb, Breakthrough Starshot

Jan 07, 2017 19:15 michael.million@sky.com Posted on: Centauri Dreams

I think around 1.5 to 1.6 micron is quite a good place to laser through, for one the Rayleigh scattering is less than 1 micron but not a great amount, two if we use silicon, a well manufactured material, which has a very, very low absorption index around those wavelengths and three the atmosphere has a good optical window there.

http://www.pveducation.org/pvcdrom/materials/optical-properties-of-silicon

https://www.gemini.edu/sciops/telescopes-and-sites/observing-condition-constraints/ir-transmission-spectra

If we had these silicon pyramids or troughs which is then backed by a fibre optic quality glass which also has a low absorption co-efficient as well we would have a very good sail material to use, with very, very low absorption. Bare silicon also has a good surface reflection of around 30% and if combined with optic glass as in dielectric mirror arrangement it would also give a very good dielectric mirror at 1.5-1.6 micron. Silicon and fibre optic quality glasses are very strong in compression but may need thermally warming prior to the main power beaming to prevent cryogenic shock.

http://photonicswiki.org/images/thumb/9/96/Absorption_attenuation.png/400px-Absorption_attenuation.png


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