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Hi Matt,, consistency is directly proportional to the type of guide mounting and the money thrown at it. OAG's have their place but also have their problems. Again the amount of money thrown at an OAG definitely reflects in the usability and usfulness of an OAG.
Things to consider are the type and size of the pickup prism. it's ability to move around the image and the magnitude of the stars or the magnitude your camera can resolve well. You need to think about the ability to correctly focus your guide camera and your imaging camera at the same time. So the use of a par focal ring on an EP or helical focuser for the guide camera.
Last thing I found a problem with was the amount of in focus travel when using a focal reducer/flattener.
I had a Lumicon 2" and sold it quite quickly as the process of focussing both cameras was a real pain.
In my opinion if flexure is your only concern then bolting a set of fixed rings to the top of your imaging scope and making a simple but strong support for your guide scope focuser and camera is a way easier and way less expensive means of removing flexure. Finding a guide star is always easier with say a 70mm guide scope than an OAG even if it is fixed and bolted down. Save yourself some money and see if you can borrow one for a trial first.
My 2c worth.
Good luck with it all.
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