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Hi all I am trying to get good clear images of the Foucault test images of my mirrors. The image of the source at the image plane is about 3 to 4mm across. I purchased a 12mm lens for my pc900 webcam to operate at infinity focus but it can't reach it - the lens unscrews. So I tried to get an image without a lens and it was really clear - but also really large . This is taken about 3mm outside of focus using a ronchi grating on my 12" F/2.7 cassegrain paraboloid primary, hence the hole in the middle. What do others do ?. I don't have enough space for access with a DSLR lens. regards Mike