My Radio Telescope Project

[Asur1984]

On ‎14‎/‎09‎/‎2016 at 09:11, SteveBz said:

I'd like to see what your connector cable looks like.  How does it all come together?

Regards

Steve.

Sorry it's took a while to get back to you, here I think is what you wanted to see.

Jon.

[SteveBz]

On 17/09/2016 at 12:57, SteveBz said:

 Great I'll try Wed or Thurs and post back.

Well it's been longer than I intended, but I got everything set up today.  After I posted last time I realised that because of the mounting the dish was pointing at right-angles to where a polar-aligned telescope would have been pointing and I had convinced myself that Solar tracking would not work.  I had all the old motorised equipment ready to step in.  I installed an EQ5 motor last week and the CG-5 worked perfectly first time with full Solar alignment.

After a bit of faffing with the software to receive the signal and got the signal mapping software working, I realised that there was no signal, by this time the Sun had gone behind a tree.  Que Sera.  If the weather is fine, I'll try again tomorrow.  Generally the forecast is good for the week, so hopefully, I'll get out one day for a drift scan.  Then I can use setting circles to find any other sources that might be in the Sky.  It really needs to be near the ecliptic, because my dec elevation can only be done be lengthening or shortening the North pointing leg!!

Steve.

[robin_astro]

Hi Steve,

Have you done a quick check that everything is working ok by just standing in front of the dish? The human body is warm enough to generate a good microwave signal

 

(EDIT :  sorry just catching up so ignore this, I see you are already detecting the sun)

[SteveBz]

31 minutes ago, robin_astro said:

Hi Steve,

Have you done a quick check that everything is working ok by just standing in front of the dish? The human body is warm enough to generate a good microwave signal

 

(EDIT :  sorry just catching up so ignore this, I see you are already detecting the sun)

Hi Robin,

Thanks for the reply anyway.

When I get another run, because of the RA tracking, I can run several driftscans one after the other.

I'll try that and then manually 'stack' them ie average them.

I'm hoping you can then help me calculate the beam resolution.

Regards

Steve.

[robin_astro]

I'll have to dig out my Jodrell Bank course notes but it is effectively like calculating the resolution of an optical telescope only at 12GHz the wavelength is about 50000 times longer so by my reckoning, to get the same resolution as say a 100mm aperture optical scope you would need  a  dish ~ 5km across !  

 

Robin

[SteveBz]

You went on a course at Jodrell Bank? Fantastic. Do you listen to jodcast?

I think the formula in radians is just wavelength over diameter but I wanted to calculate it from the width of the peak in minutes, then we can work backwards to calculate the wavelength being measured.

I also thought that a small base line array based on satelite dishes from eBay might be fun.  There are a lot that go for 99p.  Then I would only be limited by the width of the local golf course!

Regards

Steve.

[robin_astro]

9 hours ago, SteveBz said:

You went on a course at Jodrell Bank? Fantastic.

"Introduction to Radio Astronomy" back in 2004 (sadly no longer run).  It was a 12 week on line course ending with a residential weekend of hands on work using their "smaller" dishes (7m and 42 ft) . It was indeed a great course (run by Tim O'Brien in person) ,  particularly  the chance over the weekend to make some real measurements, talk to some of the people working there and see behind the scenes (Including the Lovell telescope control room)

Robin

[billyharris72]

On 05/09/2016 at 10:33, SteveBz said:

I don't know if you've done the sums, but resolution = wavelength over diameter (in radians) for a single antenna.  That's about 10 degrees for my dish. To get better resolution I'd need an array of dishes. 16 dishes spread over 100mx100m would be like a 100m dish. Ie bigger than Jodrell Bank and at less than the cost of my Newtonian (apart from the field to put them in, obviously!).

This is what has put me off trying something myself (that and suspecting the missus will not being impressed with a huge dish in the garden) though I still just might. Any ideas as to how to combine the signals to get an array?

I think the angular resolution might be 1.22 x wavelength / D (assuming the same formula applies as for visual wavelengths which I assume it does). In the 21cm band in particular that makes things very difficult.

[SteveBz]

28 minutes ago, billyharris72 said:

This is what has put me off trying something myself (that and suspecting the missus will not being impressed with a huge dish in the garden) though I still just might. Any ideas as to how to combine the signals to get an array?

I think the angular resolution might be 1.22 x wavelength / D (assuming the same formula applies as for visual wavelengths which I assume it does). In the 21cm band in particular that makes things very difficult.

Hi Billy,

Yes, that's the equation I used, I just didn't bother with the 1.22.

I haven't done the array, although it looks great fun, but from what I read it think it goes something like this  (and I'm making much of it up as I go along):

1) You need enough dishes in any one direction to avoid just getting interference patterns (eg two would just give you Young's slits in microwave).  The big arrays seem to use a minimum of 7 to 9 in each direction.  I think you could get away with four in a straight line, ie if you wanted to do a 2d mapping I guess you'd need 16.  I tried to count the Mexican array, it looks like they have about 27 in a triangular formation.

2) You'd need to choose a single wavelength so that you could get wavelengths all lined up in one place.  I think that means taking out the LNB and replacing it with your own microwave filter/amplifier (I hope the filter is just a cheap oscillator circuit with two or three capacitors and coils.  There are plenty of designs if you google it).

3) Then you'd need a coax from each dish.  I imagine they'd need to be all the same length, coming into a sort of mixer to combine the signals and allow them to interfere or not.

4) Then amplify and capture on your PC and there you go.

But the really exciting thing would be if you could set up one, and I could set up one, and Robin could set up one and Jon could set up one ....  (eg 16 or 25 or 36 participants or more).

If we could encode the signals and combine them over the internet, imagine what we could have.  The mind boggles.

However, back in the real world, I'm about to post my slightly more mundane results.

Regards

Steve.

[SteveBz]

Here are my latest drift scan results.  They not what I expected, the main issues seem to be:

1) It was a windy day and the dish was flapping around a bit, in spite of CG-5's best efforts.

2) I was using a mains 12v adaptor to power the signal finder which gave it a buzz.

3) The major issue is that I think that as the LNB gets focus, it warms up and the voltage rises.  You can see a similar effect on Robin's scan as the peak is not symmetrical.  When I did it by hand in my first version it was much quicker and didn't have time to warm up significantly. 

The sun is the peak between 10:22:30 and 10:24:30.

I also got a power failure (the meter timed out) at 10:33.

Here are some more pictures of the setup:

Dish

Back of dish shown tracking (not used for drift scan)

Signal finder plus connection left: 12v power, right: multimeter attached to display)

Connection to multimeter again

There is a usb connection from multimeter (not shown).

Multimeter

Computer displays

That's it really.

I need to try on a calmer day and use a car battery for power.

I'll use the RA controls to speed up the drift scan and avoid heating of the LNB.

It'll probably be at least a week before I get so much time to devote to the next stage of this.

Have fun and please let me have your thoughts.

Steve.

[Pippy]

22 minutes ago, SteveBz said:

1) It was a windy day and the dish was flapping around a bit, in spite of CG-5's best efforts.

Most all telescope mounts are not manufactured for the wind loading your dish has Steve

If you really want / need rigidity, then this is how they do it in Australia (notice the dish diameter to mount mass ratio) ...

[SteveBz]

12 minutes ago, Pippy said:

Most all telescope mounts are not manufactured for the wind loading your dish has Steve

If you really want / need rigidity, then this is how they do it in Australia (notice the dish diameter to mount mass ratio) ...

Fantastic Pippy.

Oh look, they even have people living in them!

I probably can't quite afford that yet.  Meanwhile, I'll just have to wait until hurricane thingummy has passed and the folk in Haiti can move back in again.

Regards

Steve.

[Pippy]

43 minutes ago, SteveBz said:

Fantastic Pippy.

Oh look, they even have people living in them!

I probably can't quite afford that yet.  Meanwhile, I'll just have to wait until hurricane thingummy has passed and the folk in Haiti can move back in again.

Well yes.

But hopefully you get the idea.

[SteveBz]

9 minutes ago, Pippy said:

Well yes.

But hopefully you get the idea.

Hi Pippy,

Well I do still have the very old and rusty RA driven satellite mount and motor that came with it.  I could use that, it would be more stable, but the advantage of the CG-5 is that I can use the known position of the sun to "polar" align it and then use setting rings to point to another radio source, say Tau A.

I think it just needs to be a calm day with no wind, as it was indeed yesterday.

Thanks for pointing it out, it's useful information to have.

Regards

Steve.

[Pippy]

Just now, SteveBz said:

Well I do still have the very old and rusty RA driven satellite mount and motor that came with it.  I could use that, it would be more stable, but the advantage of the CG-5 is that I can use the known position of the sun to "polar" align it and then use setting rings to point to another radio source, say Tau A.

I think it just needs to be a calm day with no wind, as it was indeed yesterday.

Mesh type dishes also help with the wind loading problem - one reason why sky have holey dishes (apart from the savings in material cost).

So long as the dishes mesh hole size is kept to 1/10th or less than the desired RF wavelength, you don't really loose out on RF efficiency.

 

[SteveBz]

3 hours ago, Pippy said:

Mesh type dishes also help with the wind loading problem - one reason why sky have holey dishes (apart from the savings in material cost).

So long as the dishes mesh hole size is kept to 1/10th or less than the desired RF wavelength, you don't really loose out on RF efficiency.

 

Nice, of course.  I was going to drill holes in the mounting plate at the back to reduce its weight.  I could even, if I felt confident, do the same with the dish.

[The Admiral]

5 hours ago, SteveBz said:

But the really exciting thing would be if you could set up one, and I could set up one, and Robin could set up one and Jon could set up one ....  (eg 16 or 25 or 36 participants or more).

If we could encode the signals and combine them over the internet, imagine what we could have.  The mind boggles.

An interesting thread. I think the problem with multiple dishes in an array is that as these signals have to be dropped in frequency, all the local oscillators would need to be phase-locked, otherwise it would be impossible to measure the phase difference between the different signals in order to improve resolution. I'm not involved at all in the practice of this, but I did speak to a Jodrell Bank man and gleaned that this is what they have to do with MERLIN and the Square Kilometer Array.

Ian

[SteveBz]

13 minutes ago, The Admiral said:

An interesting thread. I think the problem with multiple dishes in an array is that as these signals have to be dropped in frequency, all the local oscillators would need to be phase-locked, otherwise it would be impossible to measure the phase difference between the different signals in order to improve resolution. I'm not involved at all in the practice of this, but I did speak to a Jodrell Bank man and gleaned that this is what they have to do with MERLIN and the Square Kilometer Array.

Ian

Well the SKA is a magnificent undertaking.  It'll be fantastic, but of course there's just too much sky to cover, this is why amateurs are still making such an impact.  Not only do we draw people into astronomy like a portal, but those of who have funds can make real contributions with their setups.

But the remainder of us.... well maybe we can club together with our 99p eBay satellite dishes (actually mine cost £5 as I probably say earlier in this thread).

There are plenty of ways of synching information.  If we are synching at 21 cm, we don't need to send the carrier wave (ie 1.4GHz) we can just send regular packages showing the phase offset and amplitude and combine them across a peer-to-peer network.  Everyone would have access to everyone else's information.

Steve

[robin_astro]

Yep it is all about very accurate clocks (you need to sync the signals to a fraction of a wavelength so at say 1.4GHz you need  agreement between observers to perhaps a few tens of picoseconds) .  A phased array aerial farm in one field connected by cables is easier and you can get some nice fringes eg as here at 400MHz   

http://www.britastro.org/radio/RadioSources/extragalactic.html    Virgo A  (fig 11.11)

http://www.britastro.org/radio/RadioSources/supernova.html        Tau A  (fig 8.4)

(Not mine, though I am thinking about perhaps doing something along these lines)

Robin

 

[SteveBz]

15 hours ago, robin_astro said:

Yep it is all about very accurate clocks (you need to sync the signals to a fraction of a wavelength so at say 1.4GHz you need  agreement between observers to perhaps a few tens of picoseconds) .  A phased array aerial farm in one field connected by cables is easier and you can get some nice fringes eg as here at 400MHz   

Hi Robin,

Obviously you're right.  Thanks for pointing that out.  I wonder if there is a precise satellite signal that people could all share.

Then all you'd need is precise location.  I guess to maybe a tenth of a wavelength (1 or 2 centimetres).  The location would then be used to adjust the clock signal and calculate relative distances between dishes.

Should this be another thread?

Maybe I'm going to start a new thread for this discussion.

I'll cut and past part of the discussion to start it off.

Regards

Steve.

[SteveBz]

I thought I would leave this thread for the originally stated objective of building a radio telescope.  I've started a new thread for the baseline array here:

I hope that's OK with everyone.

Regards

Steve.

 

[SteveBz]

Getting the dec and RA working properly has been a bit of a problem.  I now have them both motorised and working, see here:

You can see that the axis aligned with the central radius of the dish is now parallel with the polar axis.  It is now even more amazingly heavy and I had to cut a niche with an angle grinder in what passes for the dovetail bar to avoid it slipping apart, see here:

There is also a 25 m cable that came with the dish which is both Coax and 4 core control for the satellite tracking motor, I am going to re-purpose it as an RA control cable by putting RJ9 (4P4C) & RJ11 (6P4C) plugs on each end.  Plugs arrive tomorrow (I hope).

This way I can have the computer screen inside (avoiding the glare of the sun and any rain there might be) and not have to run in and out to operate the RA.

Watch this space...

Steve