What product do you recommend to clean the external glass on my optical window?

If you see donut shaped shadows on your flat field images you will likely have small specks of dust somewhere in your optical train. If you rotate the camera and the donut remains in the same position, it’s likely the dust is on the front optical window of the camera. First try cleaning with an air duster or a soft optical brush. More stubborn dust or marks can be removed with a lens pen or cleaning solution such as Baader Planetarium’s Optical Wonder Fluid – this is what we use and strongly recommend, just be careful not to scratch the surface of the window.