Hello! I've started to use a star lap on my 10" f3 mirror. I've not used a star lap before, and the readings seem strange.
I generated the star lap using Mel Bartel's program for a 7-petal lap with Mirror on Top and 1/3 CoC strokes.
I had thought that the center would deepen, so when I use my 9-zone Coude' mask and zero on zone 1 then the outer zones should be relatively longer in ROC when compared to zone 1. The opposite seems to be occurring.
I started with a sub-diameter square tool about 2-1/2" x 2-1/2" using CoC strokes in order to get initial figuring.
With the star lap, after the last three sessions (each session is 60 minutes long with an initial cold press and then 10-minute press after each 20 minutes of polishing), the readings went from
Zone1 0 0 0 0
Zone2 29 21 0 -4
Zone3 39 32 0 -8
Zone4 38 28 -8 -12
Zone5 38 26 -10 -13
Zone6 33 21 -11 -14
Zone7 21 17 -16 -16
Zone8 22 17 -15 -16
Zone9 35 40 -5 -10
I use 1/4" x 20 carriage bolt and dial divided into 50 divisions, so a "1" represents a movement of 0.001" in ROC.
This almost looks like the star lap is acting like a full sized lap and taking the mirror back to a sphere. Part of that is good - when I started, zones 6 to 9 were way too short. There was/is a TDE, which is much better. Still, the readings are opposite what I was anticipating, and I am not sure how to use the star lap in the future.
Thank you for your help and best regards.
Edited by msheald, 30 November 2020 - 08:27 AM.