I've been watching for a long time the "Cloudy Nights".A good part of my ATM knowledges comes from this forum.I reach a point where I am stuck,so this is why I am posting now.
Few months before I decided to start grinding my own mirrors,being unhappy with the quality of the usual commercial ones.I start with an 8"/7.5,a bit unusual f,but I want a planetary tool.
I follow the Mel Bartels path(by hand): rough grinding using a subdiameter ring tool(4"diameter),grit 80 and 180 till reach 5/6 focal lenght,fine grinding using a plaster and tiles tool,half TOT half MOT,grit 320,25,15,9,5 and 3 microns;I checked carefully the focal lenght using a feeler gauge after every grit.I did always the "pencil test" before changing grits.I verified also the grit pits before changing the grit size.
Materials are good quality,also the mirror blank(1",the back was also verified to be flat).1/2" of newspapers was alway placed bellow blank to avoid astigmatism.
Strokes:mostly 1/3 Center-Over-Center;after10 strokes,rotate tool step to the right,mirror to the left and repeat.
Started polishing with a slightly oversize lap(9")to avoid turned down edges.MOT always,1/3 center-over-center first hour.Second hour,trying to lowered the central hill wich seems to be present on the mirror,I shortened the stroke.
My mirror seems to be in a bad shape,away from spherical;I am doing something wrong for sure,but cant figure what.
In the Ronchi pictures,upper line is after 1h polish,the bellow one after 2h polish.80 LPI
Kindly asking for some guidance:)