ATM FIGURING FOR THE FIRST TIME

[HGLyall@aol.com]

Hallo! I have just joined your group.  I have no idea how many there are of 
you.  I am in Gloucestershire, England.  I am trying to complete my mirror, 
which I bought as a kit about 30 years ago, and which has been in the roof 
(attic) for most of that time.  It is 8.5 in diameter, with a radius of 
curvature (Foucault test) of 103 +/- 1/8th inches. When, recently,  I had 
finished polishing, the Foucault test suggested it was spherical, but the 
Ronchi test showed the edge to be high.  I then appeared to over-correct 
(TOT, short strokes) because the Ronchi test show the lines to be very close 
to what is needed for a parabola, except that they are too straight in the 
centre of the mirror.  So I appear to have gone through the spherical stage.  
With six lines showing, the ones at the edge appear as shown in the 
instruction manual, but the two nearest the centre are nearly parallel, 
except at the top and the bottom.  I have removed pitch from the lap (most at 
the edge, none removed from the centre) and done two lots of five minutes, 
mirror on top, 1/3rd strokes, but nothing seems to change.  Does the Ronchi 
pattern mean that I should remove more from the centre of the mirror?   
Should I remove more pitch from the edge of the lap, in a manner which 
reduces radially inwards.  How close am I to a parabola, in terms if 
wavelengths?  I have no idea.  I dont know what a Couder test is.  I dont 
know what TEX is

    Thanks very much, in anticipation, for any advice.

    Harry Lyall.