Radio Astronomy issues

[kelly]

Hello, everyone!
I have a question that has confused a few of us.
When refering to a radio telescope, what is the "effective area", the dish or the aperture? I thought it would be the aperture, but there is a debate. I thought the aperture lets a certain amout of signal in, and this should be the effective area. I have an example our prof gave in class, and in some cases the effective area is the aperture, and in other cases the effective area is the dish.
Please help!
thanks,

[Kaptain K]

The effective area is pi times the aperture squared divided by 4.

[ngc3314]

Hello, everyone!
I have a question that has confused a few of us.
When refering to a radio telescope, what is the "effective area", the dish or the aperture? I thought it would be the aperture, but there is a debate. I thought the aperture lets a certain amout of signal in, and this should be the effective area. I have an example our prof gave in class, and in some cases the effective area is the aperture, and in other cases the effective area is the dish.
Please help!
thanks,

It depends on the feed horn (or analogous bits). For example, at least untl the Gregorian upgrade, the effective area of Arecibo was limited not by the size of the dish, but by the fact that the horn limited the received signal to what was reflected from only a part of it (the part most nearly normal to the incident radiation and hence giving the best beam pattern). It is not at all unusual for such a limitation to be built in on the receiving side - giving up some effective collecting area so as to reject ground radiation or interference more effectively.