ghatfield There are so many issues in imaging with the 0.7x reducer for the C8 Edge HD that I'm going to try imaging at f/10. Not only is the image circle relatively small for the reducer, but it also causes problems in using and OAG (distorted stars).
Although this is only tangentially related to your original question, here are some things related to reducers and flatteners.
1) when it comes to aperture vignetting, don't expect the image circle of a telescope to improve with the use of a reducer. 99% of the time, the optical design will still vignette with the same image circle (in arc seconds) as the original. The Starizona 0.63x reducer for example, only has an image circle of 27 mm; pretty much still need to crop an APS-C sensor frame. This is equivalent to an original image circle of about 27/0.63 = 43 mm.
Reducers are not used to get a larger FOV, they are used to get a wider f-number. You are simply pushing more photons into each pixel of the camera. So, extended sources like a nebula will need less exposure time. If the original plate scale is such that a star already fits inside a pixel, for example, the star will not get any brighter.
I.e., reducers don't usually give you a larger FOV. This is what many people do not realize.
The other reason to use a reducer is that they are often used to turn cheap non-astrographs into a halfway usable astrograph (optical systems meant for cameras and not visual) -- i.e., spot diagram (point spread function) of stars away from the optical axis are corrected to be a smaller circle.
2) because of that last paragraph, it is super important to get the correct reducer to sensor spacing to match the design back-focus. If the separation is different from the back focus distance, you will get still get distorted stars away from the optical axis (and sometimes even worse). You can use the figures about 40% down this following web page to judge if the distance of your camera sensor is too near or too far from the metal back of your reducer:
https://optcorp.com/blogs/deep-sky-imaging/how-to-set-the-correct-back-focus
3) OAG, by necessity, works with the stars that are away from the optical axis; otherwise their prisms will cast a shadow on the main camera. Estimating the centroid of stars that are distorted will of course make guiding less precise. It is one of the compromises that you must make with a long focal length telescope, where a separate guider will require a much stiffer (to avoid differential flexure and mirror movements) mechanical and optical mount.
Chen