Dear Community,

I am working on the design of an off-axis parabola (OAP), the purpose of OPA is to collimate the light coming in the form of the cone from a laser diode to 10 mm beam width. At the first try I did the design by setting the entrance pupil dia 10 mm and focuses it by using an OAP for a particular F# (so that I can get the laser cone angle in OAP image space). 

2nd try I did in the opposite way, defining the source as an "object cone angle" and decentering it by changing VDX value in field data editor, for collimation I am using the same OAP. By doing this collimation is achieved but beam width is not same as previous design beam width i.e. 10 mm.

Where I am missing?

Zemax design files are attached for reference.

Thank You.