Single-dish radio telescope aperture

[shirin]

hello everyone!
this is a new single-dish radio telescope, FAST, under construction:
http://nextbigfuture.com/2011/06/chinas-five-hundred-meter-aperture.html
The aperture is 500 m. But the illuminated aperture is only 300m. So what is the advantage of bigger aperture?
is the angular resolution 1.22*wavelegth/300 or 500? if it is 300 so what is the preference of bigger aperture?

 

[TumblingDice]

The aperture is 500 m. But the illuminated aperture is only 300m. So what is the advantage of bigger aperture?

Looking at the diagram and picture, I would say... The dish is 'fixed' (unmovable). The feed system above the dish moves to allow pointing at different locations. The 500m aperture size allows the feed system to be positioned/pointed over a range of locations while illuminating a 300m 'aperture' size of the dish area underneath.

How does that sound?

 

[shirin]

Ok, thanks. Now I see it.
So is it correct to say that angular resolution of a telescope is dictated by the illuminated diameter not the aperture itself?
what about arecibo? Is the illuminated diameter of arecibo exact 300m or less? I read just now that it has a fixed dish and moving feed but smaller fov than that of FAST.

 

[TumblingDice]

Ok, thanks. Now I see it.
So is it correct to say that angular resolution of a telescope is dictated by the illuminated diameter not the aperture itself? what about arecibo? Is the illuminated diameter of arecibo exact 300m or less? I read just now that it has a fixed dish and moving feed but smaller fov than that of FAST.

The area of the surface used to illuminate the feed is most significant. In the case of FAST, it appears that the entire 500m dish exists to support the directionality, while only 300m diameter is captured by the feed horn (receiver). I couldn't find info to confirm for Arecibo, but I suspect it only uses a portion of its 300m diameter, too, being that the feed horns are designed to accept a specific signal 'cone' from the dish.

But there's more difference between FAST and Arecibo. Most radio telescopes use a parabolic surface that provide a crisp focus on distant objects at a single focal point. This works for moveable dishes that can be aimed. Arecibo uses a spherical reflector that introduces an error factor in focus, but it's the same in all directions and can be corrected for as the feed horn is moved to capture illumination from different targets. FAST, however, incorporates moveable panels that adjust to create a parabolic reflecting surface for different targets.

Here are the main wiki links:
http://en.wikipedia.org/wiki/Arecibo_radio_telescope
http://en.wikipedia.org/wiki/Five_hundred_meter_Aperture_Spherical_Telescope