Eason's mirror getting there maybe

Eason, Dale (deason@msmsmtp.StPaul.NCR.COM)
Wed, 26 Apr 95 15:36:00 CDT

The overcorrection on my mirror is history. I used the center over center stroke and got the mirror back to smooth sphere except for a shallow concentric groove 3/4 inch wide about 1 inch from the outside. I tried to porabolize since the foucault image looked like one in Texereau that was an undercorrected elipse. That wasn't the smart think to do. The groove was too deep. At first I thought I had a faulty lap. The outside squares where about 20% bigger than the center ones. So I cast a new lap. I'm getting good a casting laps. This is my 4th for this mirror. I now use Dobson's method of pouring the hot pitch on the tool and channeling with a wooden dowel. It is the easyist for me of all I tried.

At the end of the first session the trough was smaller but then it started to get deeper after more pollishing. Something must be making the lap bad. I think I know what it is.

The mirror has two flaws that I have never mentioned and I think these are the cause of the grooves. This mirror was stored for 20 years face up in a garage with only an oak board on top of it for protection. I think two drops of something mabe water landed on the mirror and etch two small javalin shaped radial grooves about 1/16 inch wide by 3/4 inch long 3/4 inch from the edge. They are just a little too deep to polish out.

Since they wouldn't degrade the optics too much I had not worried about them. I bet they make a raised face on the lap durring pressing which makes a groove on the mirror. I will try to place them over the lap channels during the next pressing.

Still Pushing Dale Eason