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DIY Laser Guide Star

MEDDER

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I do astrophotography and have a huge fascination with adaptive optics and laser guide stars. I have a basic tip/tilt adaptive optics module on my telescope. But what grinds my gears as well as million dollar observatories, is when there is not a bright enough star in the guide camera's fov to get any benefits from the adaptive optics. Turn towards laser guide stars (LGS).

So now to the diy side of it, I am interested in building a rudimentary laser guide star. I have learned a fair bit of the process behind them but I am stuck at the actual laser side of it. Specifically type and output power (budget). There are two types of LGS, one being Rayleigh scattering and the other is sodium. Sodium you have a 589.2nm beam that then interacts with a layer of sodium atoms in the mesosphere, it glows and looks like a star. However the latter is much more complicated both in terms of the physics getting the sodium atoms to illuminate as well as building a 589nm laser.

So Rayleigh it is. You can use whatever wavelength you want but green is the most common because it provides a good middle ground with atmospheric scattering and camera wavelength sensitivity. With my specific camera sensor it would be best somewhere between 440nm-540nm. There are a handful of options I could use, from basic green/blue diodes to argon to a frequency doubled Nd:Yag. In terms of power they use 20W-30W lasers, but they are also taking around 1000 exposures a second so they need something pretty bright. My camera on the other hand maxes out with on roughly 20 fps so I can take significantly longer exposures, typically 0.05-0.1 seconds, which in turn means my guide "star" does not need to be nearly as bright. I am going to guess the absolute minimum I can get away with is 5w but would be pretty happy if I could do 12W.

To sum up the reason I am here, whats the best bang for your buck 420-520nm laser. Also to be clear, I wont be using this as an actual tool for astrophotography like the big observatories. I just want to build a 'proof of concept/working prototype' sort of thing. Sorry if parts of this are confusing, I am terriblw at writing thread starters. Here are a couple links that might be helpful.


Untitled-1.jpg
 



Arctos

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Lowest I have seen true LGS are 5-10W range and using multiple beams. But you have to lock frequency to sodium D2 (something like 589.182nm or something) and a stupidly narrow linewidth, either one of those factors in itself is beyond most if not all DIY'rs. It's hard enough for me to get lasers to 20-40pm with commercial budget and high power. They are just that uncommon at high powers needed for this.
Someone built a dye cell based sodium laser for this as part of a university. It might have been on one of the laser places on faceberg a few years back, can't remember who. Dave IIRC?
Another thing to note they are often coupled with adaptive optics for imaging. Again, crazy expensive stuff.

Also the beam telescope to do this will likely cost you 50k or more. Optical surfaces in UK often make them. You need something in the 200-400mm range depending on beam quality as you have to hit a layer 91km up in the air.

So, as an alternative Rayleigh approach as you said, I would go for a 532nm system as it'll have best beam quality and brightness for lower budget and ease of accessibility for such sources. Probably a decent used DPSS or other pulsed YAG might do the trick if it has the beam quality. Used beam expander will still set you back a lot of dosh still. I really think this project is going to be one hell of a difficult + complex one so would recommend to really spend even more time researching before going down any particular route.

520nm beam quality at 12W is not so good, from memory 8mm @ 1.2mrad or so is best we can do and that's using the best possible components and design.

Great link though thank you.

Good luck...
 

MEDDER

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Lowest I have seen true LGS are 5-10W range and using multiple beams. But you have to lock frequency to sodium D2 (something like 589.182nm or something) and a stupidly narrow linewidth, either one of those factors in itself is beyond most if not all DIY'rs. It's hard enough for me to get lasers to 20-40pm with commercial budget and high power. They are just that uncommon at high powers needed for this.
Someone built a dye cell based sodium laser for this as part of a university. It might have been on one of the laser places on faceberg a few years back, can't remember who. Dave IIRC?
Another thing to note they are often coupled with adaptive optics for imaging. Again, crazy expensive stuff.

Also the beam telescope to do this will likely cost you 50k or more. Optical surfaces in UK often make them. You need something in the 200-400mm range depending on beam quality as you have to hit a layer 91km up in the air.

So, as an alternative Rayleigh approach as you said, I would go for a 532nm system as it'll have best beam quality and brightness for lower budget and ease of accessibility for such sources. Probably a decent used DPSS or other pulsed YAG might do the trick if it has the beam quality. Used beam expander will still set you back a lot of dosh still. I really think this project is going to be one hell of a difficult + complex one so would recommend to really spend even more time researching before going down any particular route.

520nm beam quality at 12W is not so good, from memory 8mm @ 1.2mrad or so is best we can do and that's using the best possible components and design.

Great link though thank you.

Good luck...
Wow sounds like you know a lot about them.

And yeah, when I learned more about the sodium ones, specifically how narrow they are and the need for them to be tunable, I realized it would be a pointless attempt.

From everything I have looked at sodium are typically 20w and the rayleigh at the william herschel telescope is 25w. I found a small article on one using a 30w and it was some weird wavelength around 445nm, but like you mentioned I would have a hard time with beam quality. They could very well typically be lower wattage like you said, I am only finding info on the big ones like keck, VLT and WHT.

I would agree the optics will be the hard part. I have a spare 8" sct telescope I am tempted to use as the launch telescope but have a feeling a laser that strong would damage the secondary mirror and I would lose a decent amount of power since the mirror coatings are pretty old.

Like I mentioned I plan on it all being very rudimentary, if i can get it to 'work' one full night I would be happy. Its not something I would want to use all the time because astrophotography is already filled with random bugs and I dont need to make it more complex. Also I would have to stay out every night on plane watch and would probably want to start registering with the air force's laser clearing house.

With my adaptive optics unit (active optics) being tip/tilt i can get away with a relativly low quality "star" because it is only correcting for up-down-left-right and not wavefront. Though I am worried about the electronics side of it having to sync up the laser pulses with my camera since it needs to be pretty darn precise.

Still have loads of research to do.
 

kecked

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The sct telescope is a pain I’ve tried it. A beam expander works just fine. 20mm with as high and large as you can find. I used a Meade ar6” refractor, problem is lack of power . Even at 10w once expanded it was not much to see. I used 445nm. 532 would be easier to see.

Ok so say you build this your not the Kecked. Hihi. So what do you do call the faa every time you use it? Around her3 the arestyou for a pointer in the sky.
 

MEDDER

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The sct telescope is a pain I’ve tried it. A beam expander works just fine. 20mm with as high and large as you can find. I used a Meade ar6” refractor, problem is lack of power . Even at 10w once expanded it was not much to see. I used 445nm. 532 would be easier to see.

Ok so say you build this your not the Kecked. Hihi. So what do you do call the faa every time you use it? Around her3 the arestyou for a pointer in the sky.
the point of the launch telescope is not purely to expand the beam. Its to have it focused at x altitude, I forget what it is exactly but something like 10-15km.

As for the pointing it at the sky, I have yet to find any restrictions on it federally or in my state. I would have to watch for planes myself and I would have my widefield camera taking somewhat long exposures so that I could see any dim/tiny ones. Observatories get no fly zones put around them both because of the lasers but also so a plane does not fly through the telescopes fov and ruin a hour long exposure. Things like laser light shows will also get no fly zones or at least a pilot warning because of how many lasers there are and that they are moving around so much. In my case, laser that moves 15 degrees over an hour would be pretty easy to keep track of and I would have no problem with shutting it down for a few minutes.

You can register with the air force laser clearinghouse but nothing requires you to do it, its voluntary. They basically make sure that you dont burn up a camera on a weather satellite and stuff. I already have programs that show me satellite orbits, but it would also not hurt to register because I dont feel like paying for breaking a ten million dollar satellite.
 

LSRFAQ

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Actually your possibly opening yourself up to a weak but possible felony charge if you don't take steps to register. GO read the lasers in airspace portion of the FAA Modernization and Reform Act of 2012 and tell me it's not a poorly defined catch all law. It scared some of us who have/had variances for laser display as it offered no relief if we have an incident and had filed the paperwork and done the procedures.

Contrary to popular belief, since you have a case for a semi-legit scientific use, FAA/CDRH can be down right friendly.

I'm one of the few who post on this board and have actually went through the process of getting a NOTAM for "Laser Light Activity" issued. Much of what you read on here is speculation from people who have never went through the process. I have a good friend who still does it periodically.

Generally you'll need a separate person as a "trained" spotter and two means of communication with a local TRACON. Two cell phones meet the redundancy rule. What that training is, is pretty much up to you, but "Port is the red light and green is for Starboard" and how to determine aircraft motion from the nav lights basically sums up the training.

At least buy a copy of Skyzan or similar software so you know what your NOHD and other stats are. I would not trust most of the online calculators. There is a look up table based on wavelength when you file the forms that is very interesting.

One of the things you really have to watch for are Medivac / Law Enforcement /SAR Helicopter pop-ups. They often live in their own little world of uncontrolled airspace.

The other issue is the odd Cessna pilot who turns off his/her NAV lights to come see what your doing. Turning off the lights is a legit legal judgement for the Pilot Flying, if they feel its a hazard, they can hit the switch, and they do, knowing it lets them watch the fireworks display or the laser show. It will not happen with an airliner, but it will happen with low speed GA... I've had them come and skirt the edge of the NOTAM to "See what is going on" while working as a spotter, more then once. Generally they don't kill the lights until they are in close proximity, so you have an idea that they were there.

FAA is not too bad to deal with, they will run their own safety software and "negotiate" with you based on airspace requirements, and DOD has been known to be very professional. In my case FAA asked for an increased divergence for beams near the horizon towards active airspace.

Two things about Rayleigh and Mie, They are really wavelength sensitive, and on cold winter nights with no humidity or dust in the air, their usefulness can fall off quickly. If the air is really clear some on some cold nights, all you will have is some scattering off Oxygen unless you can find a "layer". This is why everyone goes the extra mile to do sodium. I booked an XMAS show once in a very frozen place, and found out rapidly that I had nothing to scatter from. I was not expecting that, because beams there in the summer were incredibly visible to high altitudes. Granted I needed scattering at lower altitudes then you do.

Things change periodically on how airspace is managed, and very much depends on where you are. If your in the middle of Montana on a mountain, that will be far different then being on the outskirts of the Class B in Phoenix.

At least listen to the local UNICOM or Tower if you try this. Airspace is like an onion, it has layers. Sectional charts are great, at least until FAA's new toys allow for direct flights without airways and waypoints, and that is coming soon.

Human nature being what it is, eventually some one will ask what you are doing, or a pilot will see /detect your beam and radio in.

I'm not saying I approve of putting high power beams into modern airspace, but if your going to do it, do it right.
Why? Because it creates less problems for the next person who wants to do something similar.





Steve
 
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MEDDER

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Thank you for all of that, some of this stuff is hard to find without somebody telling you about it first. I hope I dont seem relaxed about it all. Wether I do this project or not is still up in the air and the regulation side of it, although a pain, is in a way the easy part and not needed until I have a setup ready for testing. I wont be turning it on unless I know for certain I I am safe from prosecution.

Luckly the two major airports nearest to me are far enough away and at the right angle that I dont get any low altitude jets even the slightest bit near me. Even high altitude ones are hardly an issue, in my 6 years of imaging through a telescope I have only had one fly through a picture. Of course that still means I have to be safe about it and do it all properly. I would probably just be pointing it straight up, both to keep scattering at too low of altitudes down and to keep the amount of airspace it is going through to a minimum.

Rayleigh is probably the best option for me because my cameras peak sensitivity is between 420-520nm like I kinda mentioned. Winter its pretty much always cloudy here and during the summer humidity I dont think ever drops below 50% especially at night, however I have no idea what it is like at altitude lol. Though I imagine having a lake next to me will help. Would love to go the extra mile for sodium but my camera is only about 40% efficient with 589nm light. And like discussed a bit, the needed linewidth and beam specs are insane for an amature, let alone building a powerful 589nm laser.

I am leaning towards 445nm-ish even though it wont be perfectly suited for my camera. Will keep beam visibility down a bit compared to 520/532nm and it will be refracted by the atmpsphere a bit more which is the whole point of a LGS. Main concern with that is beam quality.

Thanks for the info again, will give me a few more hours of reading to do.
 

RB astro

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Good luck in your project, sounds challenging though.

I do astrophotography as well but I just use a basic 20mW 520nm on my scope as a pointer only.
I enjoy mucking around with some of my other lasers though just for the sci-fi look in the photos.
Here's one I did of the milkyway and a 450nm.

65430

And another with my 589nm going for that Keck look, LOL.

65431

RB

:)
 

kecked

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I want 100kw of 10.6um in a beam 1mm with 0.0000001mrad so I can pop balloons on the ISS. This way even if cloudy it pops a hole in the clouds.

What you want in yellow exists but the medical dicks are sitting on it making it expensive. A fiber laser is not hard to produce. There just isn’t a market.

Have you tried traditional adaptive optics using a guide star? I hear they are really very good and even affordable.


 
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MEDDER

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What you want in yellow exists but the medical dicks are sitting on it making it expensive. A fiber laser is not hard to produce. There just isn’t a market.

Have you tried traditional adaptive optics using a guide star? I hear they are really very good and even affordable.
yes, I have the SBIG AO-8T (now branded as diffraction limited). It makes life easier since it corrects for imperfections in the telescope mounts tracking ability as well as light wind, but it is nothing close to the "before and after" article you sent. Professional observatories have deformable mirrors that can correct for the wavefront, where as mine really only corrects for if the star moves in the x or y direction.

I am still considering a 589nm, the more I learn about them the more I see it as being possible to build one. But at the same time actually creating a guide star with it is really quite hard. It would be sacrificing peformance for coolness level. I am likely just going to diy a dpss nd:yag, but even those are not exactly cheap.
 
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MEDDER

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Good luck in your project, sounds challenging though.

I do astrophotography as well but I just use a basic 20mW 520nm on my scope as a pointer only.
I enjoy mucking around with some of my other lasers though just for the sci-fi look in the photos.
Here's one I did of the milkyway and a 450nm.

Very cool, here is one of mine from a recent trip but I am quite bad at editing wide field milkyway photos. The second one is an image from my ccd camera on the telescope, sadly the 'infinity point' is not in the frame because that was in my guide camera.
Untitled-1.jpgCCD Image 1158.jpg
 

paul1598419

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I see a couple of objects that were moving during your exposure in your first photo. Maybe a couple of satellites or some other objects. Thanks for sharing it.
 

RB astro

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I see a couple of objects that were moving during your exposure in your first photo. Maybe a couple of satellites or some other objects. Thanks for sharing it.
Nah Paul, it's actually the Empire striking back.

:LOL:

Great setup Medder (y)

Have a look through the thread (linked below) that I posted a few years ago and you'll see a photo of one of my scopes too.
At the top of the scope, you'll see my little 520nm pointer.

RB

:D
 
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GSS

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Oh wow I remember seeing that thread. I actually posted on it when I was like 4 months old on the forum:)
 




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