Society Activities

Adding an Off-Axis-Guider to your Telescope

If like me you have had problems guiding but you can’t identify any problems with the guiding stats then the problem might be flexure between your guide set-up and the main imaging telescope. My guiding is generally ok, but I have had nights when the telescope in certain positions, but not always repeatable, simply fails, by this I mean the guide stats show really good guiding, but the main telescope shows trailed stars. The most likely cause of this is flexure between the guide telescope and the imaging telescope.

So in this video I tackle this problem by fitting an Off Axis Guider (OAG), this is a small piece of kit that uses a prism to divert some of the light from the telescope imaging train to a guide camera. This ensures that the imaging camera and guide camera are always aligned as they are using the same optics, thus minimising flexure problems between the two systems.

Two criticisms I’ve heard about OAG’s

  1. They are hard to set-up
  2. It can be difficult to find a star.


I didn’t find any issues at all, the key is to be methodical, the video below takes you through how I did this.

Finding a guide star:

I’ve used the OAG for 3 nights now (~21Hrs of imaging a dozen objects) and so far I’ve always had multiple stars to choose from, I generally guide with 5s exposures but experimented with shorter exposures and was still able to find stars, that said I need more experience with the OAG to completely conclude this isn’t a problem, but I’d like to offer a possible reason for this criticism, back in the early days of CCD the sensors were tiny and the guide cameras even smaller, they were also very inefficient, However modern guide cameras are efficient and much larger – my guide camera is a QHY5L II – this has a sensor that is 4.8mm x 3.6mm, giving a field of view of on a 1000mm focal length telescope of 16′ x 12′ (about half the size of the full moon) and a quantum efficiency of 74% (for 100 photons hitting the sensor 74 electrons are produced) .

The full guide on how to set-up an Off-Axis Guider is in the video below.