Last week we presented a quick un-boxing for the Celestron Off-Axis Guider (OAG) with a short overview of the components that come with it. You can read it here. In this blog we’ll take a little closer look at the OAG and how it is attached to a telescope. We’ll also look at an example of attaching an imaging camera and a guide camera too. For this article, the telescope in question is a Celestron EdgeHD 925 with a Canon EOS 60Da imaging camera and Starlight Xpress LodeStar X2 guide camera.
To start off, if we look at the back of the Celestron EdgeHD telescope OTA, there is the (Celestron-)orange-and-black 1.25″ EdgeHD visual back. This needs to be removed first before we can fit the OAG.
With the visual back removed the male SCT-thread on the large black adaptor plate (see below) which is where the OAG will be attached.
We can now attach the Celestron OAG to the telescope. However before we do, it’s best to see what telescope-side and camera side adaptors we need. The detailed user manual for the OAG (see here) shows different configurations for different equipment so check the user guide for your set up.
In the image above, the following components are laid out – from right to left – as follows: Celestron C925 Edge HD with SCT-male thread adaptor attached ; OAG SCT telescope-side adaptor ; main OAG body ; M48 camera-side adaptor ; 11.55mm M48 spacer ; M48 wide (Canon) T-ring. All the items, except the T-ring, are supplied with the OAG “out of the box” (see previous blog post about supplied components here). The telescope-side and camera-side adaptors each have a dovetail channel (can be seen in the image above) that, when they are inserted into the body of the OAG and the three thumbscrews (on each side of the OAG body) are tightened, allow for secure fitment. This dovetail adaptor design also allows full 360-degree rotation of the camera (for framing) and the body of the OAG for finding a suitable guide star.
With all the components securely attached to the OAG body (above), it can now be attached to the OTA (below) ready for use.
With the OAG attached to the telescope, we can have a look at what the view is like looking into the OAG from the imaging camera-side, looking towards the secondary mirror and corrector plate end of the telescope. The image below shows this view and the large pick-off prism can be seen near the centre of the photograph.
The OAG’s pick-off prism is 12.5mm in size and is larger than a number of other OAG’s. It is also multi-coated and has an aluminised backing for greater reflectivity and illumination to the autoguider camera sensor. The prism is attached to an asymmetric shaped hollow bar (prism holder) which allows the light from the prism to reach the guide camera sensor. The asymmteric shape of the prism holder means that should the prism be removed from the OAG (e.g. for inspection or cleaning) there is only one way that it can go back – i.e. the correct way so the prism directs the beam from the telescope to the guide camera! Good thinking Celestron!! The image below (shown with the helical focuser for the guide camera removed) shows the top of the hollow prism holder being hand-held and the opening on the OAG body where the prism holder slides into.
The prism location (height) in the light path can be easily adjusted. You may need to do this for finding a suitable guide star and/or to stop the prism obstructing your imaging camera’s sensor. The prism holder is held in place by a socket head hex-screw located just under the helical focuser on the camera-side of the OAG. To adjust the height of the prism, simply loosen the screw using a 2mm Allen key (not supplied) and slide the prism to the desired position. In the image below, again with the helical focuser unit removed for clarity, the prism is fully inserted into the light path (left) and about half way out in the image on the right. The Allen key for tightening/loosening the screw which holds the prism holder in place can also be seen in the image on the right.
It should be noted that the prism holder is not a captive design, so if the hex-screw is loosed too much the prism holder will simply fall out of the OAG – this may be into the body of the OAG itself or out of the helical focuser to the ground or onto the guide camera sensor area – all depending of course on the orientation of the OAG. The image below shows a side view of the prism and prism holder. A channel can be seen running the length of the holder. If the hex screw is slightly loosened, the end of the hex screw will be inside this channel, allowing the prism height to be adjusted without the prism holder falling out. From top to bottom of the channel there is approximately 16mm of prism height adjustment. Circular marks in the groove show where the prism holder has been held previously.
The image below shows the OAG with the helical focuser attached to the OAG body and a guide camera in place. The helical focuser allows a guide camera with a 1.25″ nosepiece to be simply inserted and then locked in position using two locking screws. The guide camera used here is a Starlight Xpress Lodestar X2 which has a 1.25″ diameter body so it simply slides into the focuser and locked into place. Guide cameras with (female) M42 T-threads can be securely mated using the male T-threads at the top of the focuser. In the photograph below the male T-threads can be seen at the top of the focuser, and one of the two 1.25″ nosepiece locking screws has been removed to show the thread more clearly; the remaining locking screw would be removed if an T-threaded guide camera is used. The large knurled ring at the base of the helical focuser is for focusing (by simply rotating the ring) and the silver screw, facing the camera, located above the focuser ring is a focus locking screw.
The helical focuser allows 8mm of travel to allow the guide camera to reach focus. The side-by-side photograph below shows the fully retracted and extended positions of the focuser.
As mentioned previously, the camera-side and telescope-side adaptors feature a dovetail profile on the side of the adaptor that is inserted into the OAG main body. This allows for both the camera-side adaptor (and camera) and the OAG body (and guide camera) to be fully rotated by 360 degrees. All that is required is to slightly loosen each of the three thumbscrews on the telescope-side of the OAG, whilst holding (supporting) the OAG unit and imaging camera, and then rotate the OAG unit until a suitable star for guiding is found, and then firmly re-tighten the thumbscrews. To rotate your camera for framing, support the camera, slightly loosen the three camera-side thumbscrews, rotate the camera as needed and then re-tighten the three screws. We thought it would be nice to end this blog by showing the OAG body being fully rotated as if it was being used to find a suitable guide star – see the image below.