Q:  For the HST optical telescope,

          wavelength  lambda =  500 nm
          diameter      D    =  2.4 m

     What is the diffraction limit for HST?

                                      500 x 10^(-7) m
           theta  =  (70 degrees)  *  ---------------  
                                         2.4 m

                  =  1.5 x 10^(-5) degrees


 Q:  For the Arecibo radio telescope,

          wavelength  lambda =   21 cm
          diameter      D    =  305 m

     What is the diffraction limit for Arecibo?

                                       0.21 m 
           theta  =  (70 degrees)  *  --------  
                                       305 m

                  =  0.048 degrees

 
So HST images should be about 3000 times sharper than Arecibo images.