ATM : What is a TDE

["robert allen" <rallen@iland.net>]

        I notice what apears to be some confusion as to what a
        TDE is.

        For what it is worth I'll put my 2 cents worth in.

        It seems to me that three distinct conditions are called
        "TDE" on this web page. #1 A very narrow part of the
        edge that causes the difraction ring on the side of the
        mirror the knife edge enters the light cone to be dimmer
        or even invisable. Sometimes this error can be painted or
        masked out. [sometimes this hides something worse]

        #2 The "Rolled" edge. The outside zone has so much
        "slope" that you can see the curve in the surface area
        of the "window" in your Couder mask. This makes the
        edge zone impossible to measure. [this REALLY sticks
        out with the Ronchi test]

        #3 The edge zone reads longer than it should. I've seen
        mirrors with two spherical surfaces. [the edge longer by
        quite a bit]

        All three are curable by the same method. You must use
        strokes that remove glass from the inside of the edge. [not
        from edge or center] These are short strokes MOT and 
        long strokes TOT. Both of these will drive a sphere to
        oblate or a parabolid to an elipse [also reduce an over
        corrected figure] BUT - this will "lift" the edge and give you
        a chance to make it right the next try. 

                Good luck, Keep polishing.  Bob Allen A.S.K.C.