Star Track: A Lesson in Positional Astronomy With Lasers

[gocivici] threatened us with a tutorial on positional astronomy when we started reading his tutorial on a Arduino Powered Star Pointer and he delivered. We’d pick him to help us take the One Ring to Mordor; we’d never get lost and his threat-delivery-rate makes him less likely to pull a Boromir.

As we mentioned he starts off with a really succinct and well written tutorial on celestial coordinates that antiquity would have killed to have. If we were writing a bit of code to do our own positional astronomy system, this is the tab we’d have open. Incidentally, that’s exactly what he encourages those who have followed the tutorial to do.

The star pointer itself is a high powered green laser pointer (battery powered), 3D printed parts, and an amalgam of fourteen dollars of Chinese tech cruft. The project uses two Arduino clones to process serial commands and manage two 28byj-48 stepper motors. The 2nd Arduino clone was purely to supplement the digital pins of the first; we paused a bit at that, but then we realized that import arduinos have gotten so cheap they probably are more affordable than an I2C breakout board or stepper driver these days. The body was designed with a mixture of Tinkercad and something we’d not heard of, OpenJsCAD.

Once it’s all assembled and tested the only thing left to do is go outside with your contraption. After making sure that you’ve followed all the local regulations for not pointing lasers at airplanes, point the laser at the north star. After that you can plug in any star coordinate and the laser will swing towards it and track its location in the sky. Pretty cool.

31 thoughts on “Star Track: A Lesson in Positional Astronomy With Lasers

    1. This project looks like a recipe for problems with the FAA and your local police department.

      I’m not sure how you would prevent it from inadvertently pointing a laser at an aircraft, given that it doesn’t know anything about aircraft positions. I think, given the risk of an aircraft laser strike, that, at a minimum, you should be using a low power laser, not a “high power green laser”, and you would be better off not building it at all.

      1. Your giving me ideas…

        They could add a light sensor and disable the laser if it detects a reflection. The moment the laser strikes a plane, bird, or someone who walks in front of the device the laser shuts off.

        P.S. I agree that the laser looks overpowered. I think a suitable red LED spotlight would work good enough for locating stars.

        1. Realistically, the energy reflected back to you will be a very small fraction of the radiated energy, so you’d need a fairly expensive detector to compensate for path loss and the reflection coefficient of whatever you hit.

          If someone handed me this problem, and said “make it safe to use a high power laser with airplanes around”, my first thought would be to look for signals on 1090MHz (the transmit frequency for aviation transponders) and shut the laser down if I detected them (but not all aircraft have transponders, and not all aircraft with transponders have them switched on, though MOST do). So, that’s not a 100% solution, and you DO need a 100% solution here.

          For this project, the risks appear to outweigh the benefits.

      2. The risk is insanely low unless you are near the end of an airport approach path. You have a higher chance of throwing a rock and hitting a boat while standing in the middle of a corn field than you would having this hit an aircraft with the beam if you are out away from airport approach paths.

  1. I agree with the disturbing-other-astronomers issue, but admit to having used a green laser myself at least at light-polluted sites to show newcomers where to look. But there may be a fix for the airplane issue – add in a cheap SDR receiving ADS-B signals, and and you can turn off the laser when any are within some tolerance. It will work once every aircraft is required to emit the signals – when is that again?

    1. Good idea. You might have better luck with transponder signals (kill the laser if you detect any). But the risk of a laser strike is too great for me to build something like this without some pretty good assurance that there aren’t going to be any issues. Especially, since in the USA at least, you’re looking at jail time on the first offense, regardless of whether it was intentional or not.

      1. The problem with the transponder idea is that everywhere you go except Antarctica, Siberia and maybe some ocean areas you will constantly pick up transponder signals.
        This telescope might be smart enough though to run dump1090 on a rtl-sdr get the ADS-B and if it the scope has a GPS it can compute the in-between and being airplane aware of the xmitted GPS+alt data can intentionally refuse to fire on airplanes.

      2. I think a long pointer stick might be good enough. Not as fancy or cool as a laser of course. Oh and Mr. Argaiv, American law just changed recently. If you didn’t intend to commit a crime then there is no crime. (See Comey’s comments on a certain politician)

  2. I built something similar to this a few years ago and, after playing with it for one evening, realized it was a really bad idea.

    a) It’s an arrow in the sky pointing down at your house acting as a beacon for neighbors, police, FAA, and who knows what other three letter agencies.

    b) Worse, unless mounted above eye level and shielded from accidentally pointing downward, it’s too easy to point at someone’s eye and do serious damage.

    Cool idea, and fun to implement, but not practical unless you’re way out in the boonies.

  3. “…the only thing left to do is go outside with your contraption. After making sure that you’ve followed all the local regulations for not pointing lasers at airplanes…”

    Yeah-Right! Otherwise go directly to JAIL; do NOT collect $200. And there’s no “Get Out of JAIL Card” – in Real Life. Another “Maker” == FAIL…

  4. IANAA(stronomer), but why couldn’t you just use a cheap red dot sight (like they use on firearms) instead of projecting a laser beam out there? It would avoid all the issues with the laser (airplanes, eye safety, ticking off other people, etc) and it seems to my ignorant mind like it would give a more accurate bearing as well.

    1. Actually that’s a pretty good idea. I never thought about that. This way the pointer will be visible just for the observer and as you said it would avoid all the issues with the laser pointer. I’ll buy a cheap red dot sight and try it with the tracker. I will also update the tutorial with this option. Thanks!

  5. Why use a laser at all. Why not just use a spotting scope for alignment. It’s what is normally used. The only reason to use a laser is if you are doing atmospheric anomaly compensation and you would need a complicated focusible primary mirror to do that.

    1. I shoot a green laser down my eyepice to aim my telescope, laser sighting works fast and it is only a second or two for a rough swing. Like transmitting a few seconds of tone to tune your antenna.
      Lasers are not supposed to be fired at airplanes fortunately there are laws to help stop that behavior, but it is only a problem if you have a real steady hand laser sniper at the end of the runway during final, and except for a blacked out grass strip it is annoying but the airport is brighter unless we are talking a genuine blinding beam. Dont get me wrong I didn’t appreciate it, green laser over an urban area, but I just looked away despite it distracting me for a moment while getting ready to enter the landing pattern.

  6. I bet gocivici is among the best hopes for a reborn automated sextant, reverse this hack and add a clock and video sensor and it is over 90% there I think. AUtomated sextants were used in SR-71s, B-52s, and Polaris missiles to get an accurate locational fix in the days before Transit, GPS, or accurate inertial navigation. I think it would be cool to have this as a backup to GPS even today, not to mention the cool factor of building one.
    Commercial airliners and multi-crew aircraft included a dedicated navigator with an aircraft sextant which poked out a hole in the cockpit into the slipstream, but the SR-71 had no room for a navigator. Polaris had to be it’s own navigator as it sailed through space toward the scrap pile, fortunately never fired in anger.

    1. You are right. Today I added a variable called latitude to the code, I also edited the tracking code for the southern hemisphere. This way a simple if-else statement can find which hemisphere you want to track. Thanks for the feedback.

  7. Is it really necessary to input the angles down to hundredths? As there is no visible backslash compensation, this rig will most likely struggle to be repeatable to even whole degrees, let alone sub-single minutes of angle…

    1. This was the first problem I encountered during the build. Because of that, I used a gyro to measure degrees instead of moving the steppers to a specific angle. This way I think there is no need for backlash compensation.

    1. I did not test their accuracy so I can’t talk about their accuracy. I considered using a continuous servo or hacking a normal servo for using it continuously. But using a stepper was cheap, and it had a library so it was easy to code. That is why I used a stepper.

      1. I was thinking something along the lines of http://www.sbara.org/presentations/Satellite_Antenna_System_with_Arduino_by_Umesh_Ghodke_K6VUG_%28FARS%29.pdf except with a small, light, laser pointer instead of heavy antennas, and I had assumed a couple of light hobby servos would be cheaper than even a stepper driver…
        … but I hadn’t realised how damned cheap you managed to get some stepper motors WITH driver boards. Nice!
        I may still have to try a servo version with a couple of spares I have here :-)

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