Celestron Off Axis Guider (OAG)
Celestron Off Axis Guider (OAG)Celestron Off Axis Guider (OAG)Celestron Off Axis Guider (OAG)
Celestron Off Axis Guider (OAG)Celestron Off Axis Guider (OAG)

Celestron Off Axis Guider (OAG)

  (3 Reviews)
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About this product

Model:  celestron-off-axis-guider-sct
Part Number:  93648-CGL

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:

  • Finding a guide star from the far edges of the telescope’s focal plane can be difficult, considering the small field of view and sometimes less-than-ideal illumination.
  • Getting both imaging camera and autoguider to reach focus with one another is time consuming and sometimes requires experimentation.
  • Additional spacers and adapters are usually required for your particular camera, telescope, and autoguider, which adds to cost and complexity.

Celestron's Off-Axis Guider addresses these challenges by:

  • Providing a large 12.5mm prism that can be adjusted to move closer to the center of the telescope’s focal plane, depending on the size of the imaging camera. This results in brighter guide stars, with the help of the large prism to fully illuminate the autoguider sensor.
  • Featuring a high-quality, fixed-orientation Helical focuser. This provides extremely smooth and accurate focus of the autoguider without changing (or twisting) the autoguider camera orientation.
  • Including all the needed spacers and adapters are included for most common imaging configurations, including DSLR cameras, Nightscape CCD cameras, and more. The following adapters are included: SCT/EdgeHD, female M48, female M42 (T-thread), male M48, male M42 (T-thread), 3 T-thread spacers

NOTE: The Celestron Off-Axis Guider is not compatible for DSLR imaging with the 8" EDGE-HD Focal Reducer. 


Weight 510g
Clear Aperture 48mm (1.89 inches)
Helical Focuser Travel 8mm
Telescope Connection M42 T-thread, M48, and SCT/EdgeHD
Autoguider Connection M42 T-thread, and standard 1.25" adapter
Camera Connection M42 T-thread, and M48

Downloads / Manuals

The following manuals and downloads are available for this product:

Celestron Off Axis Guider Instruction Manual

Customer reviews

Average Rating (3 Reviews):  
Write a Review and share your opinions!

Rating (max 5):  
Good product
15 December 2023  | 

Thanks FLO as usual for excellent service.Merry x mas.
Re OAG,what strikes you out of the box is the build quality.It was fairly trivial to assemble an image train.Found pdf celestron pdf online which explains how much backfocus each component of the oag eats up.I hit my 105mm for my C8/focal reducer using adapters etc.Room for a filter holder or wheel if i want to swap out adapters later.Saw defocused guide stars straight away which was reassuring.I had measured distance between guide cam holder and main sensor before,and i could see that the way the oag had been designed it woud bring the two sensors out to be fairly parfocal.Used the in built focuser to bring them into focus.. The focuser does not rotate the guide star image,big plus.sub arc second guiding.Weight is reasonable..Money well spent.

Rating (max 5):  
27 October 2020  | 

I have this married with a Celestron EdgeHD 800, 0.7 Reducer and Zwo EFW and ZWO 1600MM camera. it is very bulky and other than a 11.25mm spacer (included with OAG) and a 0.3mm spacer i had i am 0.2mm from the 105mm recommended focal length, and i have achieved focus on both the guider and the main scope fine, cannot see any occlusion of the main scope. stars in the guide cam seem very slightly elongated. I did not have any issues at the 1432mm focal length of the scope finding guide stars, however I am however after the first night out experiencing issues when guiding (i have upgraded to this from a 50mm x 9 guide scope) I am stil investigating, however it seems this on my celestron AVX mount is causing the guiding to bounce back and forth on the dec axis which is throwing things out. on the odd sub where this didn't happen the image produced completely round stars. but the failure rate was 80% and most came out blurry. I am going to try and re-balance my scope and i have some ideas for another session tonight on preventing the guiding unwinding the backlash. but either way thought it is worth pointing out that this has exposed issues on my mount that needs some work

Rating (max 5):  
Worked fine on C8 EdgeHD, not so good on 100ED
21 May 2018  | 

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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