Stargazing Does a Radio Telescope Have Aberration and How Does it Affect Its Performance?

[Clear Mind]

I was studyng my exam of astrophysics laboratory, while treathing the optical aberration I've wondered: does radio telescope have aberration? Is a paraboloid antenna in a single dish affected by coma? and the radio telescope made by arrays of resonant structures?

Maybe those are silly questions, nobody seems to treat this argument (even the sacred bible "Tools of radioastronomy" does not talk a lot about that)...

Thanks in advance for the answers

 

[Chronos]

Factors like aberration and coma are typically not an issue with radio telescopes due to the immensely long wavelengths of interest. Only something gross, like a serious misalignment between dish and antenna should matter.

 

[Merlin3189]

I think a radio dish does not form an image. It samples a single point or measures intensity in a single direction.

The response of the whole atenna assembly will not be perfect, so the received signal will contain energy from wrong directions, which will reduce the signal to noise ratio. When this is mapped to the image created by scanning, I don't know whether there are systematic aberrations analogous with optical telescope faults.

Since one is essentially trying to look at a plane wave perpendicular to the axis, a parabola would be perfect. But they are much smaller in terms of wavelength than optical devices, so diffraction becomes more significant.

 

[JMz]

I assure you, the Arecibo telescope was producing images of radio sources decades ago. Also some of the other radio telescopes. Yes, the wavelengths are long, but that's one reason why the dishes are so large, many wavelengths wide: to provide angular resolution for the images.

As for the OP, aberration has indeed been an issue in radio-telescope images. The techniques are sometimes different than at optical frequencies, though. In particular, it is feasible to digitize the radio waveform in time at sufficiently high speeds that some methods (signal processing algorithms) become practical that aren't practical for optics. For instance, adjusting phases in time (even nonlinearly) makes this more like acoustics than optics, in that regard. But still, the output can be images, something that is rarely produced in acoustics.

 

[Clear Mind]

Thanks for the reply!
So, summarizing up... if a radio telescope is capable to build directly an image (didn't know they can do it, i mean, not in a directly way), they may suffer of aberration (indeed I've found someone speaking about spherical aberration in the Arecibo radio telescope). But in generally they don't seems to be affected by aberration, at least not in a heavy way.
My first though before this discussion was that a possibile aberration would result as a deformed antenna pattern, which may be true, but it's a neglectable effect.

 

[sophiecentaur]

I think a radio dish does not form an image. It samples a single point or measures intensity in a single direction.

Yes. There is only one receiving element so it will only look in one direction (one pixel) - plus its resolution will be diffraction limited. To build up an image, you need to look in a range of directions (scan). This has to be by moving the feed point about for a fixed reflector like the one at Arecibo. [Edit - or using the Earth's rotation, of course]
Diffraction applies to any imaging system but some of those effects can be corrected for if you look at a 2D array of image points. Given enough time (to get adequate signal to noise ratio) there is no fundamental limit to resolution but practicalities and the time available will always limit the performance for faint objects.
I don't know how important this is for radioastronomy but the polarisation resolution off-axis is a feature of reflector antennae. (Relevant for comms applications where polarisation discrimination is used)

it is feasible to digitize the radio waveform in time at sufficiently high speeds that some methods (signal processing algorithms) become practical that aren't practical for optics

The great thing about radio signals is that we can measure phase as well as amplitude for some sources. We have much more modest expectations of radioastronomy images and they never will be as sexy as what Hubble has to offer the general public. A bit of a Cinderella branch of astronomy, unfortunately, as a result.

 

[JMz]

Thanks for the reply!
So, summarizing up... if a radio telescope is capable to build directly an image (didn't know they can do it, i mean, not in a directly way), they may suffer of aberration (indeed I've found someone speaking about spherical aberration in the Arecibo radio telescope). But in generally they don't seems to be affected by aberration, at least not in a heavy way.
My first though before this discussion was that a possibile aberration would result as a deformed antenna pattern, which may be true, but it's a neglectable effect.

I wouldn't say it's not important: It's precisely what limits the resolution. Yes, Arecibo has/had a spherical antenna, so its dominant aberration is spherical, but that's largely compensated by the boom "microphone" that hangs down overhead. (People apparently spent a lot of time on the subject of shaping the beam in the early days, because it was basically all analog, as with optics -- though not necessarily always linear.)

 

[JMz]

Yes. There is only one receiving element so it will only look in one direction (one pixel) - plus its resolution will be diffraction limited. To build up an image, you need to look in a range of directions (scan). This has to be by moving the feed point about for a fixed reflector like the one at Arecibo. [Edit - or using the Earth's rotation, of course]

No, a 2-D range of directions can be collected simultaneously, exactly like we would say about an optical photograph. The dish is many wavelengths across (at the wavelengths selected), so it truly images the radio sky. At its best, the resolution is/was much better than we could get from ground-based optics, at least before real-time atmospheric compensation. And it lent itself to interferometry with distant antennas, providing even better resolution, especially if you gave it time for the Earth to rotate.

Also, of course, radio can see to interesting places like the Galactic Center, which are forever inaccessible to Hubble (though not Spitzer).

Diffraction applies to any imaging system but some of those effects can be corrected for if you look at a 2D array of image points. Given enough time (to get adequate signal to noise ratio) there is no fundamental limit to resolution but practicalities and the time available will always limit the performance for faint objects.

Exactly right. However, for resolutions beyond the classical diffraction limit, the required signal/noise are enormous, as I recall.

I don't know how important this is for radioastronomy but the polarisation resolution off-axis is a feature of reflector antennae. (Relevant for comms applications where polarisation discrimination is used)

It's relevant in some observations, just as it is in optical astronomy, but I don't know how often it's used.

The great thing about radio signals is that we can measure phase as well as amplitude for some sources.

Exactly: as with acoustics, but not with optics. (The best of both worlds? :-)

We have much more modest expectations of radioastronomy images and they never will be as sexy as what Hubble has to offer the general public. A bit of a Cinderella branch of astronomy, unfortunately, as a result.

Aesthetically, I tend to agree. OTOH, radio provides a look at objects that are invisible at all wavelengths except X-rays (which have their own resolution issues, no possibility of nonlinear signal [temporal] processing, and AFAIK no prospects for polarimetry).

 

[sophiecentaur]

No, a 2-D range of directions can be collected simultaneously,

OH? Multiple feeds then. I am impressed.

 

[sophiecentaur]

for resolutions beyond the classical diffraction limit, the required signal/noise are enormous, as I recall.

Yes. You take the difference between two numbers to get a small one. Useful for solving two close sources.