Tutorial:
Setup an off-axis adapter

Contents:

1. Introduction
2. Internal distances
3. Drawings of distances

Note:
I take no responsibility or liability for what are written here, you use the information on your own risk!

1, Setup off-axis adapter, introduction:

Having an off-axis adapter get some advantages over a separate guide telescope. Less weight and no flex between the main telescope and the guide telescope because only one telescope needed.

You get the same focal length to your guide camera as the main telescopes has, one disadvantage of this is that you get a smaller field of view compare with a shorter focal length guide telescope.

An off-axis adapter consist of a prism that direct part of the light beam 90 degrees to the guide camera. Between the guide camera and the prism is a tube. Because of the limited space where the off-axis adapter is mounted the hole in this tube can be of small size and block some light.

The mount for the guide camera are normally either a 1.25" tube or the T2 thread. I prefer threads because it gives a more stable connection mechanically.

This is how it looks in my earlier setup. A Canon full frame camera and a TS OAG-9 thin off-axis adapter. The guide camera is a QHY5 CMOS camera. The field flatter optics sits inside the focal draw tube.

In this photo you see that the main focuser draw tube block the travel distance of the off-axis adapter focuser of the guide camera. It sets limits on the guide camera's focus range.

This off-axis adapter has a rectangular hole in the tube that connect to the guide camera, good because it let more light pass through. The opening is 9 x 5.5 mm, fit the sensor chip better which normally are of rectangular shape. Other off-axis adapters could have a circular hole instead.