I purchased a screw-on 150 degree lens for my 290MM mini guide camera so I can use it to help collimate my Newtonian telescope. I have seen several tutorials where folks have done this using the reticle feature on several different software packages, including ASI Studio.
Today I tried it for the first time and noticed something strange. First, let me say that I used a well collimated laser to ensure that the focuser was collimated. But even if it wasn't it shouldn't make a difference with what I found. I pulled the laser out of the 1.25" EP adapter just after checking that it was straight, and put in the 290 Mini. The photo below is what I was seeing in live view with ASI Studio. The reticle feature of ASI studio puts the reticle dead center of the frame, however as you can see, the circles generated by the reticle feature are not concentric with the focuser tube (outermost ring). The camera body is exactly concentric with the ring, as it is exactly parallel with the tube. When you rotate the camera, the point that the crosshairs are pointing to stays consistent, and the rest of the FOV rotates around that spot.
I don't think the CMOS chip is perpendicular with the camera body, which would explain why I have badly elongated guide stars. I'm using the camera with an OAG, and the stars are terrible. I have until now figured it was because I had the spacing wrong on my field flattener, but now I'm thinking that the chip not being square is the primary contributor to my issues. I have asked what I can do to ZWO support, but I was wondering if anyone here has had a similar issue, and how they dealt with it. Thanks.