: I'm testing my first mirror (8 in. f/4.5) and can't get the edge to shadow test. The right edge goes into the heavy shadow but the left side stays lit (Knife enters from the right) Do I need to re-adjust the lateral as I test the outer zones or leave it alone? A Ronchi test looks picture book from inside to outside and the surface looks smooth. I'm also having a problem with the edges of the silt showing up in the mirror during testing. If I widen the slit the problem goes away but then what's the fuss about narrow slits and tint pinholes in Texereau. Out to the 70% zone things are as expected. The problem may be your foucault testers light source or slit,your light should have a piece of fine ground glass in front of the bulb to diffuse the light, and this should be parallel to the knife edge, The slit width varies with the distance from the mirror, the greater the distance from the mirror the wider the slit needs to be. Use your ronchi screen to determine the approx imate shape,move the knife away from the mirror until you see 3 or 4 lines,the lines should curve slightly outward from the center to edge,if you have t.d.e the lines will curve abruptly outwards at the edge,(at the radius of curvature)and inwards if turned up-edge is present,if you determine you have t.d.e you should take the mirror partially back to a sphere ,with the mirror on top of the lap,press for about 10-minutes then use a short center over center stroke extending the mirror about 1-inch over the edge of lap,do this for 2-minutes then press for 2-minutes with no weight on top of mirror,repeat this process 4 or 5 times then test again(use very slow strokes with light pressure). good luck.