Off Axis Guiders, or OAG’s, can potentially be used with any telescope providing there is sufficient back focus. A lot of people use OAG’s with long focal length instruments such as SCTs or EdgeHD optical tubes where the most accurate guiding is needed during long exposure imaging without flexure being an issue (as would possibly be the case if guiding using a separate guidescope). Sometimes people are put off using an OAG as they may think they are complicated, so we thought we would briefly run through a general guide on how the Celestron OAG can be set up. We did a closer look at this OAG, its features and connections in an earlier blog here so we won’t cover these in this article.
The key to using an OAG is that you always start by focusing the imaging camera with your telescope and then focus the autoguider camera using the OAG’s (helical) focus adjustment.
First, connect the OAG to the telescope using the appropriate telescope-side connectors. Next, attach your imaging camera to the back of the OAG using the appropriate camera-side adaptors. Attach your guide camera to the top (guider) port, and orientate/rotate it as desired in the eyepiece/camera holder (for required framing) and secure it in place with the thumbscrews. In order to assist with focusing of the guide camera, the Celestron OAG features a smooth non-rotating helical focuser (the “non-rotating” part of the description means when the helical focuser is used to focus, the focuser barrel does not rotate i.e the guide camera and its field of view does not rotate). I usually rotate the helical focuser so that the OAG’s focus tube is about halfway between its innermost and maximum travel distance (which means you often don’t need to make as many rotations of the helical focuser to reach focus in one direction or another). Connect your guide camera (and imaging camera if required) to your PC so you can see the output images from each camera. Slew your telescope to a bright target. If the the Moon is visible it makes a good initial target but bright stars such as Vega, Betelgeuse, Sirius etc also make good focusing targets too as do bright star clusters such as Messier 13. Next, turn the telescope’s focus knob until the image of your target through the imaging camera is in focus. Once it is in focus don’t touch the telescope’s focus knob again!
Next check the output image from the guide camera and make any mount fine adjustments as required. You will likely find that your chosen initial bright target is out of focus and you will need to determine if you need inward or outward focus travel to obtain a sharp focused image with the autoguider. If the autoguider does not reach focus with the helical focuser extended as far outward as it will travel, and you find that the autoguider camera needs to move further outward (away from the OAG body) then you will need some extension or spacer. The type of spacer will depending on your autoguider. For example if the autoguider camera uses a 1.25″ nosepiece you could use 1.25″ extension tube (e.g. a barlow lens with the barlow removed). If the autoguider uses a removable nosepiece, leaving a T-2 female thread, you could use one or more of the three T-2 based threaded spacer extension tubes of 6mm, 11.55mm and 24.25mm lengths that are supplied with the OAG. The same applies if your camera uses a C-mount thread in which case you will need a male C-mount to female T-2 adaptor such as Baader Planetarium’s Baader C-Mount T-2 with Integrated 1¼” Filter Holder. You will need to remove the autoguider first, and then the autoguider locking screws, in order to attach these T-2 spacers.
If, on the other hand, you find that you need more inward focus travel, additional spacing will need to be added between the imaging camera and OAG. If you are using a T-2 threaded camera you can use the aforementioned Celestron OAG T-2 extension spacers, or if you are using an M48 based imaging device, the M48 spacer with the M48 male OAG camera adaptor.
Finally, tighten the Focus Lock thumbscrew when the autoguider is focused.
OAG rotation: Finding a guidestar
Finding a guide star is often said to be difficult – but not impossible. However it depends on the telescope being used and the particular area of sky you are imaging in. For example, imaging in the Autumnal and Winter Milky Way you can pick up lots of stars for guiding as there are more stars along that band compared to the Spring and Summer skies. However, modern cameras that can be used for guiding are very sensitive, so picking up a suitable guide star at anytime of year is not as difficult as people think.
If you find that you cannot see or select a suitable guide star when your telescope is (initially) “locked”/pointing onto your chosen celestial target, there may be ones elsewhere around the periphery of the field. Celestron’s OAG, with its large pick-off prism, also allows you to very easily rotate the OAG through 360º+, to find a guide star. To rotate the OAG guider, simply support the OAG unit and camera, and just ever-so-slightly loosen the 3 telescope-side thumbscrews that secure the front of the OAG to the telescope and then rotate the OAG as required to locate a guide star in your autoguider. Once a region of sky with suitable guide stars can be seen, re-tighten the 3 thumbscrews. If required, you can make a similar rotational adjustment to the camera side by loosening the 3 thumbscrews on the camera side and rotate your imaging camera to the desired orientation, then firmly re-tighten the screws again.
We should add that although the information above relates to the Celestron OAG, a similar workflow can be applied for any OAG.
For more information on spacing requirements and configuration examples for Celestron’s SCT’s, EdgeHD and other telescopes see Celestron’s OAG user manual here.