Do you have this issue with your camera? Moiré patterns.

The interferogram may look fine on this camera display but if I zoom in, things are not well as I actually have 309 very thin fringes. It's caused by the camera's firmware and hopefully not all cameras do this.

[@yobb89]

Did you 3d print a mirror holder? Have any stl files for an 8inch?

[@pollux7525]

First of all, shoot RAW format, it will help and you can do some post editting before going to DFTFringe.
Then, don't try to have so many fringes. Your fringe spacing must be at least 3 or 4 pixels if you want to resolve them.
And last, maybe change the lens to a longer focal length. I tend to choose a lens that gives me an FOV such as the igram occupies 1/2 to 2/3 of the frame. That gives me more pixels per fringe spacing and allows to have more fringes without having any moiré or resolution problems.