Off-axis guiding is an accurate way to do long exposure imaging, especially when the telescope you are imaging through has a very long focal length. Any flexure of a separate guidescope can ultimately ruin your final image. With an off-axis guider (OAG), a pick off prism (usually a small sized one) picks off some of the light (coming through the telescope towards the camera sensor) outside/near the edge of the field of view and directs that beam to a guidecamera. As the OAG is using the light from the telescope itself, whatever the telescope and imaging camera “see’s” (i.e. movement) the guide camera will “see” too.
Celestron offer an OAG with a number of features to help with off-axis guiding. We’ll look at these in detail a later blog, but for the moment lets see what you get when you unbox one of their off-axis guiders.
To start with, here is the box the Celeston OAG comes in.
Once you open the box, the OAG and its components are nicely and safely stored in between soft foam. When you take everything out of the box and lay them all on a table you can see that Celestron have included a very wide range of adaptors for their OAG – nice one Celestron!
Celestron OAG with supplied accessories
So what have we got?
In the middle of the photo is the main body of the OAG. Easily visible is the (large) pick-off prism just above the centre of the body. Two sets of three locking screws for the telescope-side and camera-side adaptors 120degrees apart around the edge of the adaptor can also be seen. At the top is the helical focuser to enable the attachment, and focusing, of a guide camera (user supplied).
To the left of the OAG body are a set of female-threaded adaptors. Top left is the adaptor that allows the OAG to be attached to an SCT/EdgeHD telescope. The other two on the left are female threaded M42 (middle left) and M48 (bottom left) adaptors.
To the right of the OAG are male threaded M42 (top right) and M48 (middle right) camera adaptors, with a M48 spacer at the bottom right. On the far right are three T-threaded spacers each with different widths, which allows the camera’s sensor to be placed at the correct position (e.g. for use with EdgeHD telescopes).
A very comprehensive and useful user guide is also included (not shown) but it can be read here.
We will cover the OAG and the adaptors in more detail in a later blog – so stay tuned!