Questions
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1.  Diameter of existing pipes

2.  Feed originally mounted to back of scalar?  From the picture it appears that original feed was attached to the mounting flange on the back of the scalar ring. 

3.  Coax and power/control cables: how do you intend to route the cables?  Just curious.

4.  Diameter of feed mounting bolts (see drawing item #1)

5.  Measurement of the inner diameter of the feed flange (see drawing item #2)

6.  Linear distance between bolt holes (see drawing item #3 and #5)

7.  Width of the mounting flange (see drawing item #4)

8.  Before changing the shepherd's hook adjustment, measure the exact distance from the dish mounting plate to the feed mounting flange.  (If it is 45 inches, that indicates that your dish f/D is equal to 0.375 which is a typical TVRO focal/diameter ratio).

9.  Range of movement of the shepherds hook adjustment.