Solve Optics Problem: Diameter of Star Imaged w/ 18" Telescope

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Optics Problem( pleasez help me)

Consider a telescope with an aperture of 18 inches and focal length of 120 inches.
what is the diameter of the star imaged in focal plane of telescope? Take the image diameter to be associated with the first minimum in the diffraction pattern of the image. Use wavelength of 550nm.

Hint: By Using lens diffraction theory.
 
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What is the equation for diffraction at a circular aperture?
 


hiiiiiiii friend.,

for the circular aperture

U= C exp(ikr) * integral of Exp(ikysin@) 2sqrt(R^2-Y^2) dy
bet ween the limits (-R,R)
 
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The nice Mr Rayleigh solved it for you
 


In other words 1.22 times wavelength divided by aperture diameter for the half angular width of the central maxima.
 

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