Has anyone encountered this before.

When I used a 15mm TV Plossl in a Celestron 70mm f/8
flourite. The image at 37.3 is ok. But when I insert a shorty
celestron ultima 2x barlow for 74.6. There is a vignetting
in which the outer 45-50 degree view is blocked by the tube
of the refractor such that I can see the inner tube of the
scope in the outer 45-50. Got this. How come?

Ch.