Some advice please on what is going on with flats

[Spookyer]

Hi there, I think I have an issue with my optical train somewhere. I have been imaging Eta Carina for a few nights and the subs I am getting have looked okay. I went to get some flats after the session the other night and this is what they look like more or less the same for every one of my 7 filters.

Does anyone know what the dark area in the bottom left is all about?

I have a ZWO 7 position filter wheel and a 1600pro MM camera if that helps. I have never seen a flat like this before but I don't think I have done flats before before with this camera. Scope is a FSQ106 with Orion OAG.

cheers
Brett

[Nikolas]

issue with the filter wheel

Try recalibrating the filter wheel and retake the flats

[Spookyer]

I am guessing if it was like this for the light frames I would have noticed. Seems weird it has just done this for the flats.

[Spookyer]

I just recalibrated the filter wheel and took some flats. Same result with the shadowing. I tried rotating the filter wheel and camera and took some flats.
The shadow moved more towards the bottom of the frame but was still prominant. This makes me think it is the Celestron OAG. It must be shading the sensor. Does that make sense?

[Spookyer]

Okay, I have changed the angle of the OAG with reference to the camera sensor so it is lined up with the long edge of the sensor and I have moved the prism back out of the light path a bit. I now barely have a dark edge visible on my flat. On the weekend I will test to see if the guide camera can still pick up enough light to work.

[The_bluester]

I have found in the past that an OAG will work properly a surprisingly long away off the center of the light path, the stars will usually look ugly in the guider images but that does not really matter.

That is one thing I like about the newer ZWO, bolted down filter wheels and OAG instead of the threaded ones, the prism is put in a much better position in terms of obstructing the light path.