Dear experts,

 
I’m designing a mode-matching telescope that uses only a concave and convex mirror.
 
I have two questions about handling the gaussian beam in zemax.
 
1. The too large waist size and waist location deviation
Currently, I’m using skew-ray gaussian beam analysis and POP coupling to check the design. 
My design has ~3 degrees of incident angle to the mirror, so astigmatism of the gaussian beam occurs. But deviation of waist location is too large than expected in skew Gaussian beam analysis. in ray transfer matrix analysis, astigmatism of the curved surface is described like this. 

COS(3degree) is about 0.9986…~1 so I think the incident angle of my telescope will make small amounts of gaussian defocus. but zemax skew analysis shows ~40mm of waist position displacement.
 

I can't tell if I made an error in optical theory or if I made a mistake in the initial setting of the simulation of the zemax program.

2. Changes in Gaussian beams in a flat mirror

In skew ray analysis, Gaussian beam’s x waist and y waist deviated when reflect at the flat mirror. 
I wonder if this is changing the shape of the beam or is it a virtual deviation caused by an initial setup problem. And when placing a flat mirror, the reflecting surface faces a strange direction through the 3-D view. I want to know if it's right to look in this direction.

It may be an absurd question because I am an undergraduate with little experience in Zemax. I'd appreciate your help.

Thanks,

Sumin