|
Post by kallewirsch on Aug 27, 2010 4:31:57 GMT -4
But can someone either explain this to me or refer me to a site that explains this? (see bold) Consider this scenario: You are in a valley looking up the mountains. Great panorama. You see a lot of mountains around you. Literally speaking: there is alot of information hitting your eyes. But then you miss the mountain climber located in one spot waving at you. He is so small (his optical information is so week) that you are not able to see that he is waving at you. In order to see him (because you know he is there), you use binoculars or a telephoto lens. And there he is, you can see him very clearly and full size in your field of view. All the other "optical information" around him is gone, you don't see it anymore and it no longer does disturb your image and distract your eyes (your antenna). And since the climber now completely fills your field of view you can make out much more details as with the naked eye. But: in order to see that climber waving, you have to point your binoculars or your telephoto lens exactly at the mountain climber. A slight movement left or right - he is out of your field of view and nothing of him is left in your image. Thats the way radio dishes work. And of course they get bigger dishes in order to get a bigger wave collecting surface. But in principle, they are just big telephoto lenses (not to be taken literally, the physical principles are different).
|
|
|
Post by tedward on Aug 27, 2010 4:50:28 GMT -4
Also you can measure the level of the signal out of the dish/receiver combination. Not sure what they used for apollo though. Upshot is move the dish and the signal level alters. Gives me an interesting side line for today.
|
|
|
Post by captain swoop on Aug 27, 2010 9:48:36 GMT -4
And take his fellow crewman with him. To my mind that's not the person you want as your crew commander. If he were flying solo then I have no problem, he can do what he wants.
|
|
|
Post by ka9q on Aug 27, 2010 10:36:00 GMT -4
I was thinking more about this, and yes it still makes sense to me. But can someone either explain this to me or refer me to a site that explains this? (see bold) They just did!
|
|
|
Post by banjomd on Aug 27, 2010 10:39:47 GMT -4
And take his fellow crewman with him. To my mind that's not the person you want as your crew commander. If he were flying solo then I have no problem, he can do what he wants. There is no problem ; it didn't happen and Shepard is dead! (This is hijacking; I'm gonna start a new thread.)
|
|
|
Post by ka9q on Aug 27, 2010 10:42:53 GMT -4
And take his fellow crewman with him. To my mind that's not the person you want as your crew commander. Well... I tend to suspect that each of the crews had an understanding about just how much they would risk their lives on a landing attempt. One doesn't become a {test,fighter} pilot, and an astronaut, and be selected for a lunar flight, and train hard for that one flight for years, and finally nearly arrive on the moon only to punch the abort button unless you really know you'll die if you don't. (Edited to add:) And maybe not even then!
|
|
|
Post by banjomd on Aug 27, 2010 11:31:53 GMT -4
(Edited to add:) And maybe not even then! ;D
|
|
|
Post by fireballs on Aug 27, 2010 17:36:10 GMT -4
Consider this scenario: You are in a valley looking up the mountains. Great panorama. You see a lot of mountains around you. Literally speaking: there is alot of information hitting your eyes. But then you miss the mountain climber located in one spot waving at you. He is so small (his optical information is so week) that you are not able to see that he is waving at you. In order to see him (because you know he is there), you use binoculars or a telephoto lens. And there he is, you can see him very clearly and full size in your field of view. All the other "optical information" around him is gone, you don't see it anymore and it no longer does disturb your image and distract your eyes (your antenna). And since the climber now completely fills your field of view you can make out much more details as with the naked eye. But: in order to see that climber waving, you have to point your binoculars or your telephoto lens exactly at the mountain climber. A slight movement left or right - he is out of your field of view and nothing of him is left in your image. Thats the way radio dishes work. And of course they get bigger dishes in order to get a bigger wave collecting surface. But in principle, they are just big telephoto lenses (not to be taken literally, the physical principles are different). Great analogy! So that's really how radio/tv broadcasts from the moon work? Thanks for explaining it, everyone (yes, I read the other responses ) Thank you!
|
|
|
Post by fireballs on Aug 27, 2010 17:50:48 GMT -4
At the S-band wavelength, a radio telescope beam width is about 1/3 degree. For comparison, the diameter of the Moon as viewed from Earth is about 1/2 degree. So you can see that the antenna beam width is narrower than the size of the Moon. If the signal was not coming from the Moon it would be immediately obvious. I'm slightly confused about this... But I'll take your word Just some background- I'm not a math person at all. I have been in remedial math in the past. I think I have a condition called dyscalculia. It's like dyslexia but with numbers. So I don't do well with numbers. Never have and probably never will. So that;s why I don't get some of the stuff you guys are trying to tell me with degrees and so forth. I guess I just never had it all explained to me. I don't know
|
|
|
Post by gillianren on Aug 27, 2010 18:33:31 GMT -4
But if you know you're not good at it, why assume you know better than people who do?
|
|
|
Post by theteacher on Aug 27, 2010 18:45:12 GMT -4
For comparison, the diameter of the Moon as viewed from Earth is about 1/2 degree. So you can see that the antenna beam width is narrower than the size of the Moon. If the signal was not coming from the Moon it would be immediately obvious. I'm slightly confused about this... I'll take your word. Think for comparison of a flashlight pointing at the Moon. The filament in the bulb is placed exactly at the focus of the parabolic reflector and thus sending the light in parallel beams towards the Moon. The degree numbers Bob mentions tell, that the flashlight lights up only part of the Moon as the spot it makes on the surface is of smaller diameter than that of the Moon itself. The antenna dish works in the opposite way than that of the flashlight but following the same principle, meaning that the signal from the Moon hits the parabolic reflector of the dish antenna and is reflected to the focal point of the reflector, where it is picked up and transferred to the electronics. So if the reflector is not pointed exactly at the Moon, the signal from the Moon will not be reflected in a way, that it will hit the reflectors focal point and therefore no signal is transferred to the electronic circuits -> no signal detected.
|
|
Bob B.
Bob the Excel Guru?
Posts: 3,072
|
Post by Bob B. on Aug 27, 2010 18:47:34 GMT -4
For comparison, the diameter of the Moon as viewed from Earth is about 1/2 degree. So you can see that the antenna beam width is narrower than the size of the Moon. If the signal was not coming from the Moon it would be immediately obvious. I'm slightly confused about this... But I'll take your word Just some background- I'm not a math person at all. I have been in remedial math in the past. I think I have a condition called dyscalculia. It's like dyslexia but with numbers. So I don't do well with numbers. Never have and probably never will. So that;s why I don't get some of the stuff you guys are trying to tell me with degrees and so forth. I guess I just never had it all explained to me. I don't know I'm not sure what part you're having trouble with but I'll try to explain. When describing the size of something in the sky it is meaningless to give linear measurement. Celestial objects are described by their angular size. For instance, if you were looking at the Moon you could project one line from your eye to leftmost edge of the Moon, and a second line from your eye to the rightmost edge of the Moon. The angle between those two lines is the angular size of the Moon, which happens to be about 1/2 degree. Another way to think about it is to pretend we can line up a whole bunch of moons side-by-side so the edges are just touching each other. Suppose we continue the line all the way around the sky in a complete circle until we come back to the starting point. It would take 720 moons to complete the chain. Since a full circle is 360 degrees, each moon has to span 1/2 degree. Regarding the beam width of the antenna... You can think of the beam as being a cone with the point of the cone being at the antenna and the base of the cone projected against the sky. The cone, however, really doesn't have a base as the beam goes on forever getting wider and wider as it moves out into space away from the antenna. If you were to measure the angle between the sides of the cone, that would be the beam width. A directional antenna is the same principle as a spotlight except it is radio waves instead of visible light. Unlike the spotlight, however, the radio antenna can be a receiver as well as a transmitter. When using a very large antenna like those used to receive radio signals from the Moon, the beam width is narrower than the width of the Moon. You don't just point the antenna in the generally direction of the Moon, you have to actually point it at the right area on the surface of the Moon in order to pick up the signal. Pointing the antenna is like shinning a flashlight at the Moon, only the flashlight beam is so narrowly focused that you can move it around and illuminate only part of the Moon at a time. If you could use the flashlight as a receiver, the transmitter must lie somewhere inside the beam or else you won't receive anything.
|
|
Bob B.
Bob the Excel Guru?
Posts: 3,072
|
Post by Bob B. on Aug 27, 2010 18:49:51 GMT -4
Think for comparison of a flashlight pointing at the Moon. The filament in the bulb is placed exactly at the focus of the parabolic reflector and thus sending the light in parallel beams towards the Moon. The degree numbers Bob mentions tell, that the flashlight lights up only part of the Moon as the spot it makes on the surface is of smaller diameter than that of the Moon itself. The antenna dish works in the opposite way than that of the flashlight but following the same principle, meaning that the signal from the Moon hits the parabolic reflector and is reflected to the focal point of the reflector, where it is picked up and transferred to the electronics. So if the reflector is not pointed exactly at the Moon, the signal from the Moon will not be reflected in a way, that it will hit the reflectors focal point and therefore no signal is transferred to the electronic circuits -> no signal detected. Crap, you used my analogy while I was still writing. (Great minds think alike.)
|
|
|
Post by theteacher on Aug 27, 2010 19:00:01 GMT -4
Think for comparison of a flashlight pointing at the Moon. The filament in the bulb is placed exactly at the focus of the parabolic reflector and thus sending the light in parallel beams towards the Moon. The degree numbers Bob mentions tell, that the flashlight lights up only part of the Moon as the spot it makes on the surface is of smaller diameter than that of the Moon itself. The antenna dish works in the opposite way than that of the flashlight but following the same principle, meaning that the signal from the Moon hits the parabolic reflector and is reflected to the focal point of the reflector, where it is picked up and transferred to the electronics. So if the reflector is not pointed exactly at the Moon, the signal from the Moon will not be reflected in a way, that it will hit the reflectors focal point and therefore no signal is transferred to the electronic circuits -> no signal detected. Crap, you used my analogy while I was still writing. (Great minds think alike.) Well, lucky punch on my part, I guess :-)
|
|
|
Post by theteacher on Aug 27, 2010 19:14:11 GMT -4
But if you know you're not good at it, why assume you know better than people who do? It must be "who are", right? Ok - it's late. Is there a t-shirt for correcting Gillian? :-) ;D
|
|