How do radio telescopes form images?

[Drakkith]

Alright, I know about optical telescopes, but I haven't a clue how a radio telescope forms an image. I'm used to pixels on CCD's measuring photon counts, but how are the radio waves detected and an image created from them?

 

[mathman]

http://www.nrao.edu/index.php/learn/radioastronomy/radiotelescopes

The above may help.

 

[davenn]

Hey Drakkith
Have you moved ranch yet ? ;)

the link by mathman helps :) but is a really long winded way of saying ...

at radio wavelengths, the resolution is obviously much lower that at light wavelengths.
Hence the use of Very Large Array VLA and Very Long Baseline VLB systems.
In the simpliest of explanations, the radio telescope is panned across the object and the signal strength of the received signal is measured at each point. from all those points and strength readings you can produce a image/map of the object that would look similar to the isobars surrounding a low or hi pressure region on a weather synoptic chart
The more aperture you have ... using VLA or VLB line measurements then the better resolution you have of those strength readings. Ignore the wonderful colours you see in those images, they are basically just false colour representations of intensity of the radio signal.

Radio astronomy can be done relatively easily by the amateur radio astronomer
with reasonably modest equipDave

 

[davenn]

further to my previous post

found this reasonable site
it has a number of different chapters on frequencies to use etc other than this page

Basic requirements for a respectable amateur radio telescope operation ...

1) good wife and neighbours that will let you put up as big an antenna as you can get away with ... Aperture fever counts for everything! You're an optical astronomer ... I don't have to tell you about aperture fever haha

2) if can get the scope built in a RF quiet location away from suburbia and industry all the better. You need to get the man-made noise floor as low as possible.

3) very low noise preamps ahead of the receiver --- nitrogen cooled is awesome, peltier cooled is better than nothing. You will be aiming for 1dB or less of noise from the front-end electronics.
This is just the same as the thermal noise when doing CCD imaging, the cooler the better

4) Decide what objects you want to listen to ( observe) find out what frequencies to use and build/buy appropriate receivers for those freq's... if you have the money ... a decent quality spectrum analyser is great with data outputtable to a PC.

5) remember radio astronomy can be done day or nite, as long as the object you are studying is above your local horizon

there's a few thoughts probably forgotten others

ohhh before trying to observe pulsars etc .. there are lots of other things closer that require the minimum of receiver and antenna gear...

Solar flares
Jupiter noise
Atmospheric whistlers

for a couple

cheers
Dave

 

[Drakkith]

Ah, so they pan the antenna around to different points and record the signal? That's actually what I was thinking. Thanks guys.

 

[codex34]

They use interferometry.
There is a video somewhere of them showing a single signal pattern, then adding a second signal pattern to form an interference pattern, then they add more and more signal patterns to create better images.

 

[toycoma]

Google the Itty Bitty Radio Telescope and build it. I did all thanks to Directv. Matter of fact I'm building a second one four miles away and linking them together via the internet. Talk about interferometry. Why didn't we think of this before? Lol

 

[Drakkith]

Google the Itty Bitty Radio Telescope and build it. I did all thanks to Directv. Matter of fact I'm building a second one four miles away and linking them together via the internet. Talk about interferometry. Why didn't we think of this before? Lol

Nice. I may try my hand at building one. I say *may* because I'm terrible at building things.

 

[Chronos]

As I recall, ancient radio scopes were basically chicken wire, aluminum foil and a receiver.

 

[Solon]

Space based radio astronomy is something I thought would have been used more, but it seems very little has been done. Presently the ony unit in use is the Spektr-R.
http://en.wikipedia.org/wiki/List_of_radio_telescopes#Space-based

The telescope is intended for radio-astrophysical observations of extragalactic objects with ultra-high resolution, as well as researching of characteristics of near-Earth and interplanetary plasma.

The unit is in operation now, but it may be a while before any 'images' are available.

 

[Drakkith]

Space based radio astronomy is something I thought would have been used more, but it seems very little has been done. Presently the ony unit in use is the Spektr-R.
http://en.wikipedia.org/wiki/List_of_radio_telescopes#Space-based

The unit is in operation now, but it may be a while before any 'images' are available.

The issue is that at radio wavelengths your dishes have to be enormous in order to get good resolution. We get around this using interferometry. I find it kind of funny that interferometry allows us to get far higher resolution on our radio wavelength telescopes than our optical ones. (For now at least) So until recently we really haven't needed a radio telescope in space, not when our resolution has already been milliarcsecond or less.

Also, at radio wavelengths the Earth's atmosphere is less detrimental than it is at other wavelengths, and much of the radio band can penetrate the atmosphere, which is one of the primary reasons for having space telescopes, to see those wavelengths that are blocked by the atmosphere.

 

[syhprum]

I do not think there is any possibility of setting up an interferometry system coupling the antennas via the internet the delay time needs to be very stable although a microwave link might work if you have a direct line of sight, the other possibility is to use synchronized recorders and post processing of the signals

 

[davenn]

I do not think there is any possibility of setting up an interferometry system coupling the antennas via the internet the delay time needs to be very stable although a microwave link might work if you have a direct line of sight, the other possibility is to use synchronized recorders and post processing of the signals

not at all why would you say that ? ... its used all the time with the VLB systems

Dave

 

[syhprum]

It might well be possible with a dedicated circuit back surely not with the normal internet where the route is often variable, I may well be out of date but when VLB first started they used highly stable profesional TV tape recorders

 

[Enthalpy]

You won't transmit your signals in real time over the Internet !
You use one reference clock at each receiver, timestamp the signal, then transmit it with any delay over the Internet.

Then, either you have fabulous clocks that are precise to << a quarter signal period, OR you use a common signal source within the observation field (a quasar), and compute the cross-correlation of the received signals to deduce what the time lag is between the received signals. Then, you compute different cross-correlations (or possibly sums) with other time lags to look to other directions.

 

[syhprum]

This reminds me of the navigation system that was used before chronometers became available where the moon moving across the star field was used as a time keeper.

 

[SpaceDreamer]

If you want a small radio telescope I found this: http://www.haystack.mit.edu/edu/undergrad/srt/index.html, it gives details on how to build a radio telescope along with some projects for it and instruction Manuel and parts list is on the site. Would be interesting to build.

 

[sophiecentaur]

Google the Itty Bitty Radio Telescope and build it. I did all thanks to Directv. Matter of fact I'm building a second one four miles away and linking them together via the internet. Talk about interferometry. Why didn't we think of this before? Lol

This is not trivial to do because you will need to provide some common phase / timing reference for each half of the array if you want to make an interferometer. An interferometer only works if you can combine signals in a known relative phase. There is an unknown time delay on the internet so you would need to compensate for this.

 

[hcorbett391]

If you want a small radio telescope I found this: http://www.haystack.mit.edu/edu/undergrad/srt/index.html, it gives details on how to build a radio telescope along with some projects for it and instruction Manuel and parts list is on the site. Would be interesting to build.

I can second this suggestion, if you happen to have a spare $6,000 burning a hole in your pocket. I've had the pleasure of playing with a couple of the 'old' SRT antennae; noise readings ~70 K, potential for 'aperture fever' satiety resolution as a VLBI, and - supposedly - relatively flexible electronics.