We have created a custom epi-fluorescence microscope using the delta stage main body and a custom made filter cube wheel. The distance between the picamera and the achromatic lens is exactly the same and the objective has been moved further away. We believe there is stray light bouncing around between the dichroic lens, the emission lens and the camera. When imaging with the filter lenses there is a green blob/circle appearing in the middle of the image. However, when there are no filter lenses between the camera and objective no such blob appears. Any advice on how to fix this problem would be greatly appreciated!