Imaging Cygnus a at 8.45 GHz with ATA
destevez.net
destevez.net
Can somebody explain why it is interesting to get a much-lower resolution image than the VLA provided? Is it sampling some property the VLA image does not? (E.g., 8+ GHz samples higher-energy processes.) Or is this an exercise in technique (one person doing the work of an institute?), or in extracting usable data from antennas not built for the purpose? Background and motivation would be helpful.
[Edit:] The previous paper is helpful: "Here we show a complete description of the interferometric pipeline, covering from the IQ output of the SDRs to the CLEAN images. Therefore, this experiment is a valuable resource for anyone interested in learning about interferometry in radio astronomy."
The ATA has less and shorter baselines, each made of much smaller collecting elements than the (E)VLA. Put simply, baselines bring resolution, collecting area brings sensitivity, and both are intricate (the longer the baseline, the more sensitivity you need).
Have a look also at VLBA techniques, which allow for even higher resolutions. Such a technique was used with the EHT to "image" the M87 blackhole.
Just the jets of Cygnus A are 40000 light years long (nearly half the diameter of the milky way). Active galactic nuclei like Cygnus A have power outputs on the order of 10^35 watts, this is 10(!) orders of magnitude more than the power output of the sun, and it's a highly collimated beam rather than radiating in all directions, so the usual inverse square law doesn't apply.
If such an object were in our vicinity (< 1000 light years), a single sweep of the jet would obliterate all life on earth. I suspect it would be enough radiation to re-melt the rocky planets in the solar system and strip them of their atmosphere. Some of these objects are estimated to have consumed hundreds of stars every single year. Even outside of the jet, the amount of ionizing radiation emitted in every direction around such an object would likely be incompatible with life as we know it.
So, luckily the object is actually very far away :)
Would that still be true for a light (or w/e) beam that is perfectly collimated?
I was thinking of a hypothetical case.
>In a vacuum.
Read more.
> Achieving a truly collimated beam where the divergence is 0 is not possible, but achieving an approximately collimated beam by either minimizing the divergence or maximizing the distance between the point of observation and the nearest beam waist is possible.
In practice the situation is even worse and you can't even get the performance that a perfect Gaussian beam would allow. We often express the performance of real beams by a thing called M^2 or beam quality parameter [2]. In some sense is measures how much wider the beam is than it needs to be, and this number is never less than one.
[1] Btw such a beam would have infinite brilliance. [2] https://en.wikipedia.org/wiki/M_squared
That's not written in stone yet, it's a good approximation but still falls down under specific conditions.
EIRP is describing that the total flux through a sphere at a given radius is constant regardless of the beam shape, but we are only interested in a specific cross section and are not accounting for the rest of it.
And yes, the inverse square law is strictly speaking only true for point sources. But it is an extremely good approximation once you are much further away than the size of the source (a distance ten times the diameter of the source is usually totally sufficient).