[amluto]

> a) How much more can this accomplish in comparison to the well known Arecibo Radio Telescope?

It's bigger and therefore collects more light. This lets users see dimmer targets. It could also have a different field of view, different wavelength sensitivity, and/or different instrumentation. I don't know details of this new telescope. FWIW, Arecibo is also a radar (it can transmit pulses and look for reflections). I don't know whether the new telescope can do that.

> b) How is a fixed parabolic dish radio telescope different from a radio telescope array like Karl. G. Jansky Very Large Array? What are the relative pros and cons of one over the other?

Very generally, single dishes will have much nicer point spread functions, so the images are more like camera pictures. Aperture synthesis images can have weird artifacts. Huge dishes like this are also huge and this have more collecting area than many small dishes combined, anthough the big arrays, in contrast, have much, much better angular resolution.

> c) How do you 'steer' the telescope to look at different parts of the sky? I understand the dish is fixed, but the feed horns can be repositioned, but I don't really understand the physics/math behind it, other than the focus is changed.

Imagine a big mirror on a wall. If you stand in a different place relative to the mirror, you see a different image in the mirror.

[gjkood]

Thank you. My high school physics classes around lenses and mirrors with the parallel light sources from infinity and the lines coming to a focus at the focal point are coming back to me now. Didn't think about the parallel lines hitting the curved surfaces at any angle and reflecting to the focus. The steering mechanism is much more clearer to me now.

[saboot]

Could you clarify on the difference between a single radio telescope having a better PSF, and an array having better angular resolution? What's the difference between those two qualities?

[amluto]

There are a few ways to have good angular resolution with a weird PSF. Even with just optics, you could build an instrument with a PSF that has a narrow central peak with a big ring around it. This would be great for separately resolving nearby objects but bad for resolving a single object against a background of many nearby objects.

For aperture synthesis, particularly strange things happen. If you take a quick (no rotational synthesis) exposure with a huge 3-antenna array, for example, you only get 3 choose 2 = 6 degrees of spatial freedom, but you get very fine angular resolution. In practice, you do "map making", in which you try to extract specifically the parameters you care about, but if you try to make an actual picture, you certainly can't fill in your whole field of view with tiny pixels, since you can't get past the small number of degrees of freedom.

My personal favorite example of aperture synthesis is the images of Saggitarius A*. You can get incredible detail, but the "images" are made under certain assumptions and don't represent actual individual pixels with reasonable PSFs.