In order to decenter the middle window, click anywhere on surface 4 (it has the comment Front Window2) and press the INSERT key on the keyboard. A new surface will be inserted, and all surfaces from the old surface 4 onwards will have been renumbered (so that surface 5 now has the comment Front Window2). 

Now double-click on the new surface 4 and set it to a Coordinate Break surface:

Set the Surface Type to "Coordinate Break"

Then scroll along to the y-decenter and enter a value of -5 mm. The Layout Plot and Prescription Report show the consequence. All surfaces from the Coodinate Break onwards are decentered by -5 mm.

 The y-decenter can be clearly seen  Note the y-coordinate of the surfaces

The effects of a CB last until another CB is encountered, therefore two coordinate breaks are usually required: one to implement the tilt/decenter and the other to restore the original axis.

To demonstrate this, click on Surface 8 (the front of window 3), press INSERT again and make the new surface a CB. Make the thickness of this surface 10 mm, and make the thickness of surface 7 zero, so that the second CB is colocated with the back of the middle window. Give this CB a decenter in y of +5. You should now have the following (open intermediate step.zmx from the zip if you got lost). {Note that because of the rectangular apertures on the surfaces, rays that miss the middle window are terminated. This is an essential part of the sequential model. If you wish to model a system in which rays that miss the middle window carry on to hit the third window, you must use non-sequential ray-tracing.}

 A second CB restores the original coordinate system  

The restoring CB restores the original coordinate axes so that subsequent surfaces are back in their original positions.

Setting the values of the restoring coordinate break surface by hand is not good practice, as its easy to forget that the second CB needs to be adjusted whenever the first one is. However, ZEMAX makes it easy to automate: just double-click on the y-decenter of the second CB and choose a Pick-Up solve to lock the value to the first CB, as follows:

How to set up the pickup solve

Do this with the decentration in x, and for the tilt parameters also, but leave the order flag = 0 for both CBs at this time. (This is an error, but we will see why in the next page).

You should be able to set any value of decenter in x and y, and ZEMAX will restore the original coordinate system at the second CB. If you got lost, open intermediate step2.zmx from the attached zip archive. We will now move onto tilts.