Let’s suppose you’ve got a legacy optical system that produces a beam that comes to a focus at f/8, and you want to collimate this beam at a diameter of approximately 80mm. 

In this case, we will need a 640mm focal length collimator (80mm beam diameter multiplied by the f-number of 8), and a good choice would be SORL’s 25-055-04 mirror.  This OAP has a focal length of 25 inches (635mm), an off-axis distance of 5.5 inches (139.7mm), and its diameter of 4 inches (101.6mm) is large enough to collimate an 80mm diameter beam. 

Note that when SORL gives a value for off-axis distance, they give the distance to the inside of the OAP’s clear aperture, whereas a Zemax decenter measures the distance to the center of the clear aperture.  Thus, to figure out the amount of decenter needed in Zemax you simply add half the OAP diameter to SORL’s OAP off-axis distance.  So when SORL says the OAP has an off-axis distance of 139.7mm and a diameter of 101.6mm, the Zemax decenter value will be 139.7 + (101.6/2) = 190.5 mm.

We will enter to parabola into ZEMAX as a Standard Surface with a Radius of Curvature of 1270mm (the radius of curvature equals double the focal length, because it's a mirror) and, because it's a perfect parabola, a conic constant of -1 (for details on the definition of the conic constant, see the Zemax User's Guide in the "Standard" section of the "Surface Types" chapter).