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Model: celestron-off-axis-guider-sctPart Number: 93648
An Off-Axis Guider is an astroimaging accessory considered by many to be the most accurate method of autoguiding.
The Celestron Off-Axis Guider uses a large 12.5mm prism to intercept a small portion of the telescope’s focal plane (outside the field of view of the main imaging camera) to locate a guide star. Any movement seen by the guide star will be the exact same movement seen by the imaging camera. Guiding this way will correct for tracking errors, as well as opto-mechanical errors or flexure, for pinpoint round stars.
Off-Axis Guiders have been around for a long time, but typically pose three main challenges:
Celestron's Off-Axis Guider addresses these challenges by:
NOTE: The Celestron Off-Axis Guider is not compatible for DSLR imaging with the 8" EDGE-HD Focal Reducer.
The following manuals and downloads are available for this product:
Celestron Off Axis Guider Instruction Manual
I bought this OAG primarily for my C8 EdgeHD scope having had little success guiding with a finder-guider in the past (mainly down to the huge FL difference). Being a Celestron produce I expected it to integrate with my C8 without problems - which so far it has.The included manual is reasonably clear but it did take a bit of trail and error to work out which of the numerous adaptor rings I needed to use to get my DSLR and guide camera attached. I disregarded the manual in the end and just used the ring that quite obviously fitted the thread on my T-Adaptor. Getting my QHY5 guide cam connected was a bit more of an issue. The OAG accepts a standard 1.25" nose piece but I found that I needed to add an extension tube to the QHY in order to get focus and that then made the connection to the OAG a bit unstable. I resolved that by adding one of the supplied rings to extend the OAG's guide camera tube. However, doing this means removing the thumb screws from the OAG - meaning you can't then secure the camera. Fortunately I the nose piece adaptor off my finder-guider had a thread that fitted the OAG so I was able to use that. Things would have been far simpler if the OAG's guide camera tube could be extended without losing the thumbscrews. I suspect Celestron want you to use one of their guide cams that conveniently screw on rather than a QHY.I haven't had the opportunity to test it thoroughly get but it seems to work reasonably well (with the C8 anyway). With the mount way out of PA it seemed to to allow PHD to keep it adequately guided - something not possible with my finder-guider. Didn't notice any obvious artefacts or shadows in the test images I took. It wasn't exactly a scientific test as the sun had just gone down and the scope hadn't cooled but what stars there were seemed round after a quick 320s exposure (pretty impressive as mount wasn't aligned at all).I also have a Skywatcher 100ED and I over-optimistically attempted to use the OAG with it also. Fitting it to the thread on the 100ED's field flattener was straight forward enough but then the problems started. I now know why Celestron say it's not compatible with their reducers and/or flatteners. The stars shown in PHD2 were rather 'triangular' in shape with quite an obvious flare. Worse still, the stars in the actual images were horribly flared. I suspect is the fact that the OAG is quite fat an introduces a large gap between the field flattener and the sensor resulting in reflections.In summary: works as advertised with the Celestron C8 (and similar SCT's I suspect). Needed an additional adaptor to get QHY guide cam connected. No good for scope with flattener or reducer fitted.
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