Are you using an Off Axis Guider (OAG)? If not this could be mirror flop. That said, if you have an EdgeHD and use the mirror locks with some care then it shouldn't be mirror flop (but, YMMV).
Also, the backspacing requirement for the EdgeHD isn't 55mm, but it depends upon where you are taking the measurement and what camera adapter you are using and Celestron does provide the necessary instructions to achieve the needed spacing. Celestron gives the backfocus for the 9.25" EdgeHD as 146.05mm +/-0.5mm. But, yes, Charles is right and if you are using Celeston's T-Adapter for the 9.25" EdgeHD you need to use an additional 55mm of spacing between their adapter and the sensor. Note, the camera adapter for the 8" EdgeHD and their other non-EdgeHD SCTs isn't the same as that for the 9.25".
From Celestron's EdgeHD whitepaper:
The EdgeHD 925, 1100, and 1400 form their best images 5.75 inches (146.05mm) behind the telescope’s rear cell 3.290×16 tpi threaded baffle tube lock nut. For best results, the image sensor should be located within ±0.5mm of this back-focus distance.
It is easy to place a digital SLR (DSLR) camera at the proper distance using the Small T-Adapter (item #93644) for the EdgeHD 800, or the Large T-Adapter (item #93646) for the EdgeHD 925, 1100, and 1400. The small adapter is 78.35mm long while the large adapter adds 91.05mm, in both cases placing the best focus 55mm behind the T-Adapter.
Edited by james7ca, 24 July 2019 - 08:32 AM.