Hacking together a radio telescope? Maybe.

[cubinator]

I posted a while ago about wanting to attempt to build a radio telescope by hacking some electronics into an old television antenna. Today I poked around with a voltmeter and learned a bit about the degree to which I can detect things.

I have an elliptical TV dish plugged into a TV box through a little gizmo that I bought that's meant to help align the dish to satellites in GEO that provide sweet entertainment. The box normally is an integral part of the TV experience, but in my case it's just providing easy power. Here's a diagram:

The important box is the most important part, because that's where I can divert the signal into a computer, where normally it would just make a noise.

Quote

"EEEEEEEEEEEEEEEEEEEEEEEEE"

-This tiny box I spent $25 on

That important box is called a Satellite Signal Meter, and it's the most basic model that is the cheapest thing ever in terms of radio equipment. You can find them in a bunch of places online, and the cheap ones are probably pretty much all the same model. I popped the back off of mine (much more easily than I anticipated, which was nice) and found this:

Look at all those places I can put a voltmeter on! "PWR" is where the coax cable plugs into the TV box, and "LNB" goes to the LNB, which is the detector part of the radio telescope. There's a potentiometer in here, which on the front of the box is a knob that changes the sensitivity, which is kind of annoying because I have to know which sensitivity is good. The voltage across the potentiometer directly is also really noisy, so it's not much help to bypass it. Then there's the dial, which on the front of the box points from 1 to 10 depending on how strong the signal is. In my experience, the change in signal is actually really minute and only moving the potentiometer can make the dial change from 1 to 10. On the back, though, there's a couple really helpful pieces of metal where I can measure voltage. 

Then there's the buzzer, which seems like maybe the nicest place to use as an analog input on an Arduino. I found that pointing it at the Sun changed the voltage from about 2 volts to maybe 2.1 volts or something, a really small nudge on the voltmeter dial but still enough that it would probably produce a somewhat useful number in a computer program. I'm thinking that if I remove the buzzer and replace it with wires that plug into an Arduino's analog pins, I can get the radio signal as a number in the serial. I bet it would actually be way better if I could pump the numbers into a MATLAB script instead, then I could average a bunch of readings and croup them into a nice array and make a picture all in one place

Anyways, that's what I've learned so far. I still think I can make a picture of the Sun if I build a whole scanning mount, but I probably can't get a good picture of much else.

[Shpaget]

Do you plan on automating the scanning motion? If you're into spreadsheets, you can have Arduino output directly into Excel and look at your image appear in front of you.

[cubinator]

50 minutes ago, Shpaget said:

Do you plan on automating the scanning motion? If you're into spreadsheets, you can have Arduino output directly into Excel and look at your image appear in front of you.

Scanning motion definitely automated, otherwise the picture wouldn't be very precise. I definitely would have the numbers piped out to some other program, I'll have to see later what option is best.

[Shpaget]

You could store it to an SD card in CVS format and once you have that, you can use whatever you want. I suggested Excel, because I had a mental image of conditional formatting background color and the image just appearing out of the empty cells.

One more thing that occurred to me, depending on how fast your acquisition is and resolution you're after, you may want to track the sky on top of scanning it. Mount the entire thing on an equatorial mount, or for hard mode, combine the alt az tracking with the scanning. Even if it takes only one second to take a measurement, it will take a while to scan even a 100x100 pixel image, and the Earth will swing away.

[cubinator]

Yep, I'm aware of those things. I was also thinking more along the lines of 10x10 pixels, at least for the first attempts. By the point I am able to produce a picture at all I'd already have a fully rotating programmable mount anyway, so it would be a matter of converting RA/Dec to Az/Alt, which I can probably make a decent function for. Part of me wants to just buy a tracking telescope mount and build an adapter to the radio dish, but they are way more expensive than building my own, and it's also good experience to build it myself, and gives me something to do while avoiding coronavirus.

I'll have to see what CSV encoding looks like in text, to be able to encode the various columns of the picture.

[Nuke]

get you a software defined radio module. will give you a lot of good data acquisition. 

[Gargamel]

On 5/24/2020 at 4:39 PM, cubinator said:

I'll have to see what CSV encoding looks like in text, to be able to encode the various columns of the picture.

I forget details, but it's really simple, If you were to write a spreadsheet into a text file, it would be each coloumn is deliminated by commas, and a new line means a new row.  Very very easy to parse into a spread sheet.  But if you want conditional formatting, it would probably easiest to import it into one sheet/page, then have a script that sets the conditional formatting on a second sheet/page.  I do something very similar for woodturning.  I tell the spreadsheet the diameter of the vase I want to make at each increment along it's length, and the script spits out the size of the trapezoids I have to cut to make the rings, and then draws a picture of the vase by adjusting the formatting on a second page.

I'm really curious as to how this project turns out.   Betcha there's a lot of resources on the net for home brew radio telescopes. 

 

[Guest]

 

[Nuke]

On 5/25/2020 at 9:29 PM, Gargamel said:

I forget details, but it's really simple, If you were to write a spreadsheet into a text file, it would be each coloumn is deliminated by commas, and a new line means a new row.  Very very easy to parse into a spread sheet.  But if you want conditional formatting, it would probably easiest to import it into one sheet/page, then have a script that sets the conditional formatting on a second sheet/page.  I do something very similar for woodturning.  I tell the spreadsheet the diameter of the vase I want to make at each increment along it's length, and the script spits out the size of the trapezoids I have to cut to make the rings, and then draws a picture of the vase by adjusting the formatting on a second page.

I'm really curious as to how this project turns out.   Betcha there's a lot of resources on the net for home brew radio telescopes. 

 

i usually go with lua for parsing data. it has some awesome pattern matching capabilities. you can parse an entire line in one line of code where if you did the same in c or c++ you might have a dozen lines. but if you are using an arduino or something and sending data to a computer over serial, what you can do is just have your print commands put in all the formatting for whatever text format you were going to use and you dont have to bother with any intermediate parsing code. just cut and paste from the terminal. 

[cubinator]

I have a robot arm that I used with a DC motor shield for Arduino to program its movement many years ago. I don't remember how any of it works and I can't find the code I wrote that moves the joints in a sequence, so it looks like I'll be relearning it from scratch. The motors in the arm are probably too weak to haul the big metal dish around anyway (though I would like to test them, and a lot of the weight seems to be in the rusty old mount that I'd already have removed if it wasn't such a nice handhold ) but I will use them to get some practice before either modifying this robot arm even further, constructing a new platform and digging up some bigger motors, or some combination of both.

In any case, I'll be able to turn this motor control scheme into a way to scan areas of the sky with the dish.

[Nuke]

you can use a lazy susan as your main turret. those can hold quite a bit of load (especially the big ones you see in furniture). then find you an old bicycle rim that you can use as a belt drive and a small gear motor. for elevation make a jack screw out of some threaded rod perhaps. of course you probably want accurate feedback on both axes. an accelerometer might be able to give you good indication of azimuth, and a digital compass can give you your heading. then point it at a known radio source to calibrate away any error.  

[Shpaget]

Just how big is the dish you plan on using?

11 hours ago, Nuke said:

an accelerometer might be able to give you good indication of azimuth, and a digital compass can give you your heading. then point it at a known radio source to calibrate away any error.  

Closed loop seems a bit too complicated, not to mention that the affordable ones are not nearly good enough for the purpose. I'd go with two $1 ebay bubble levels and a compass for rough alignment, and optionally optical finder scope for more precise alignment. Once it's set up and north aligned, tracking can be done with steppers. With some gearing and microstepping, even the cheap ones can be suitable.

[cubinator]

The dish is a little less than 1 meter wide on the long axis, I think. I may substitute it for a circular dish of similar caliber - Just look at your neighbors' roofs for satellite TV antennas; that's exactly what I'm using. However, I would be happy to use a bigger dish (or construct one? ) if one were available. @Shpaget

I have a bin full of all kinds of stepper motors in the basement, so there won't be any issue finding something that will work. I've been toying with the motor shield today trying to get it to work. I haven't had any success yet, but I'm very happy to have this puzzle on my hands. I did find out that only two of the four batteries in my robotic arm are actually part of the power circuit, so it was running on 3V instead of 6V the entire time I've had it. How strange. I'll have to make my own battery circuit!

[Shpaget]

If you need help with figuring out the drivers and motor shield, I might be able to help. I'm no expert but I've been playing with them for a few years.

[cubinator]

I got a suggestion for something simpler I can do to get started - I can use Earth's rotation to scan, that way I only need to mount the telescope on one axis, scan a certain declination over one day, then go out every day and turn the antenna a degree or so. That way I could get a full sky survey, might even show some objects.

I also am not unconvinced that I can build a bigger and better dish with metal wire and some pliers.