Celestron Focal Reducer and mystery black disk

[Alan Conroy]

I have a Celestron 9.25" OTA and have just fitted a Celestron Corrector/Reducer f/6.3 model 94175. A black disc appears in the eyepiece roughly in the centre of view but it shifts from side to side on focusing. There is no disc with the usual visual back in place. I positioned it between the OTA and the visual back. Does anyone see any obvious problem with what I have done or is there some inherent problem with these focal reducers

Thanks

Alan

[Davey-T]

There's a recommended distance between the focal reducer and a camera sensor that produces artefacts and vignetting if wrong, they're generally pretty forgiving though.

Never seen a disc in mine.

Dave

[Xplode]

That is the shadow from your secondary and it moves because of the mirror shift when focusing with an SCT.

[Alan Conroy]

Thank you. As it is a shadow does that mean it will not be visible when in use at night?

[Xplode]

Correct

[Alan Conroy]

Thank you.