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Re: Dish surface 'flatness'

At 03:00 PM 7/25/2008, Peter Ellis wrote:
>Now, I need to re-mesh the dish, but I need to decide what 'flatness'
>('true-ness') the surface should be. There has to be a formula, that
>encompasses things like: frequency of operation, curvature or focus
>distance, etc.

Focal length, f/d, etc are irrelevant.  The thing that is relevant is 
wavelength of your signal.  You want the variations in the dish's 
surface to be small relative to a wavelength.  1/4 wavelength is 
ok.  1/8th wavelength is better.  Better than that matters not much.

Its not to hard to get an intuitive feel for what is going 
on.  Suppose you have a dish that is a perfect paraboloid, except for 
one little dent that's a little farther from the focus.  Suppose the 
dent is 1/2 wavelength farther from the focus than the rest of the 
dish.  The signal that reflects off of that dent will be 1/2 
wavelength late, in other words 180 degrees out of phase with the 
rest of the signal, and will therefore SUBTRACT from the rest of the 
signal.  That isn't good.  That's a clue that 1/2 wavelength is too 
much variation.  You can go thru the same sort of thinking with a 1/4 
wavelength dent and see that the effect is much smaller, etc.

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