The 3/8" is below the edge of the tube or the primary? If it is the former, there is absolutely no problem as the EPs that tend to use the shorter lengths are the very short FL EPs and thos have a very small FOV.
If the latter, you probably have a tube which is too small for the primary. For example, a typical 8"F6 (50" FL) will want to have almost an inch of space around the primary in order to obtain a 2 deg. FOV. If you are using a tube that is only 9" in diameter on the inside, you have dropped that FOV to about 1 deg. without blockage from the end of the tube.
As to the focuser tube actually impinging on the optical path, the reduction in light will be pretty minimal and the aberration of the image will be very small, ending up being a very faint pair of lines from a bright star like a spider does only a lot fainter. Basically, nothing that I'd worry about.

• Focuser mounting/Tubing - ryarb - January 16, 2003 - 00:53 UTC
  • Focuser - bobmay - January 19, 2003 - 19:57 UTC
• Focuser mounting/Tubing - ryarb - January 16, 2003 - 01:10 UTC