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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!

3, Setup off-axis adapter, drawings of internal distances:

Setup off-axis adapter

How my own setup looks with the 2.5" field flatter attached. It's very compact and the field flatter has a 0.75x reducer builtin. It can handle up to 35 mm image circle, not the full frame's needed 44 mm, it needs a 3" field flatter for that.

Setup off-axis adapter

A schematic drawing how the main camera sensor and the guide camera sensor are oriented in an optical view. All optics has some vignetting and other drawbacks at the edges. That's why we want to have the guide camera as close as possible to the main camera sensor. But not that close that the prism block the light to the main camera sensor.

This is how it looks if you looking into the off-axis adapter towards the main camera (and have the shutter open). The usable image circle of a field flatter can be difficult to find, but modern designed 2" x1 correctors normally handle an APS-C sensor (29 mm diagonally). If the corrector also is a reducer they normally has a much smaller image circle. Look at my collection of links with reviews.

Setup off-axis adapter

If you looking from the side with X-ray vision it looks something like this with the example of a Canon DSLR APS-C camera. As you see the prism pickup the light rays (red in drawing) that fall outside the sensor. The black line is the optical axis.

What you have to do:

  1. Find the correct distance between camera sensor and the field flatter (coma corrector)
  2. When above setup are correct, find the distance from the center of prism to the camera house, just about
  3. Adjust the prism in and out until you have the edge of it at half the main camera sensor height + plus 2 mm
  4. Set correct focus to main camera
  5. Set correct focus to guide camera


If this distance is not correct you get elongated stars at the edges, it could need a precision of +/- 0.5 mm. Take test photos and analyze them for round tight stars. A time consuming process.

If the camera is a DSLR you normally place the off-axis adapter as close as possible to the main camera. With a short back focus mirror less camera you maybe need distances between the the off-axis adapter and main camera, otherwise you maybe not reach focus on the guide camera.

You adjust the prism position by moving the "tube" inwards or outwards, this doesn't affect the focus of the guide camera. If you get signs of a shadow on your photos of the main camera, move the prism outwards a bit. If there is no shadow, try to move the prism inwards a bit, just until the shadow comes and then back off a bit. In this example, half height of sensor is 8 mm, then add at least 2 mm safe marginal to start with. You can use my filter & off-axis adapter calculator to get an idea about the distance.

Just find the focus of the main camera, note the position you have on the focus scale.

You normally focus the guide camera by moving the upper part with the guide camera holder, look in your manual for the off-axis adapter. Sometimes it has a helical focuser built in.

Focus of off-axis guider

The upper yellow arrow point to the lock screw of focus for the guide camera. The lower green arrow point to the lock screw for the tube which is used to move the prism in and out.

You maybe have to add some spacers between the guide camera and the off-axis adapter, the distance shall be the same as the center of prism distance to the main camera. The thread are normally the T2 standard.

Focus of off-axis adapter, extreme position

Note: the "tube" sometimes can hit the sensor of the guide camera, be careful to not let this happens, it will destroy the guide camera !

It can be difficult to find the focus of the guide camera, stars could be weak, have at least 2 seconds exposures, set a high gain in the beginning. You can also use the main focus, note the focus scale position when you find the guide camera focus. Compare this value with the value where the main camera had its focus. It will give you a hint if you need to add some spacers between the guide camera and off-axis adapter.

If the main camera has the focus draw tube 30 mm outwards (0 means full in) and the guide camera need it to be 40 mm and you can't adjust it with the built in focuser. Then there is a need of an extender of 10+ mm to the guide camera. If the opposite, then you need to put a extender between the off-axis adapter and main camera. Note, still you must have the correct distance between the field flatter and the main camera.

Tilt and rotate of off-axis adapter

I have rebuilt my off-axis adapter, it can now handle full frame sensors better. I also built in a tilt adjusting, the green arrows. The yellow arrows point to the locking screws for rotation of the main camera position. To the main camera there is a M48 thread, on this I screw the Canon EOS adapter. On the telescope side originally it was a M48 thread, I have replaced it with a M68 thread to fit my 2.5" field flattener.

See also my project: full frame off-axis project. or my project: medium format off-axis project.


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