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Setup an off-axis adapter


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

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

2, Setup off-axis adapter, internal distances:

To connect the main camera, guide camera, off-axis adapter, field flatter (or coma corrector) and focuser there are a need of adapters.

Setup off-axis adapter

An over view of the adapters I needed to connect everything together with correct distances and threads, this is for a refractor.

This is a general guide to setup an off-axis guider. To setup it correctly you need to know some distances:

  • Main camera back focus
  • Guide camera back focus
  • Field flatter (coma corrector) optimal distance
  • Size of main camera's sensor
  • Size of guide camera's sensor
  • Size of prism in the off-axis adapter
  • Size of the hole in the tube of off-axis adapter
  • Built length of off-axis adapter

Examples, I don't take my own setup figures because it's a bit special. I chose the main camera to be a Canon APS-C DSLR which is a common camera, the back focus are 44 mm and the sensor size is 23.5 x 15.6 mm.

Table of common cameras back focus:

There are a lot of guide cameras, maybe the back focus are from 5 to 12 mm. Sensors sizes maybe from 6 x 4 mm to 12 x 9 mm. Check that the sensor size match the off-axis guider before you buy anything. The prism size and the hole in the connecting tube sets limits.

Filed flatter and coma correctors has normally it's optimal distance at 55 mm, the exact distance must be tested out because it depends on the telescope. Somewhere from 52 mm to 58 mm is common. It's the distance from the threaded end (or sometimes the base of it) of the corrector to the sensor in the main camera. Note: the sensor is protected by a glass window and filters, it sits deep behind.

When matching an off-axis adapter to a system above with a DSLR camera and a standard distance 55 mm field flatter you must have a thin off-axis adapter to get it to fit. Normally they have 10 to 11 mm built length. You in this case only have 55 - 44 = 11 mm to fit everything in and the off-axis adapter take all this space. The adapters / threads in both ends must already be built in the off-axis adapter. With a mirror less camera you have about 20 mm more to work with and much easier to adapt.

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