[Rod771 (Rod)]

I was looking at getting one last year for my C11 Edge HD but decided against it for one reason. Celestron over looked one thing in their design of the OAG. The SCT adapter that comes with the OAG has a 2 inch opening/thread, which is perfectly fine when using it in the F10 configuration, it simply screws onto the visual back adapter of the OTA. If however you want to use it with the celestron .7x HD focal reducer, you discover the design flaw.

The reducer is 3.25 inch in diameter with an "exit" lens 60 mm in diameter. So, Celestron have a reducer 1.25 inches bigger than their OAG SCT adapter. I pointed this out and they advised me to remove the visual back adapter off the OTA and screw it to the back of the reducer, then attach the OAG to the adapter. Problem is the visual back adapter has an aperture 37.7mm and screwing it onto the reducer which has a 60mm lens acts as an optical bottle neck creating major vignetting. This was tested in a thread on the Team Celestron website.

The fix - A fellow TeamCelestron member had a custom part made for the OAG which had a 3.25 thread. The only problem with this I noted, was that the custom part cost more than the OAG itself.

The OAG has great features I believe - Helical focuser , rotating prism and camera orientation etc It also maintains 146mm back focus of the Edge HD OTA . Why they cripple it with the narrow bore SCT attachment , I just dont understand.

I've attached some pics of the vignetting and the OAG set up with Celestrons recommendations (on the left) and with the custom part (right)