Over the years I've polished a few mirrors and the subject of fine grinding and ensuring you have close to a spherical figure before polishing has perplexed me a lot.
I have a bit of a theory about to get it right and basically when I get down to 25 micron down I fine grind for around an hour then do a test with a permanent marker.
I draw a grid pattern and then apply some 25 micron paste and move the mirror in different directions to work out of the figure is spherical.
If its not in contact in the middle the solution is to use the tool on top and test after some work and then work out if its closer to a sphere.
I then look for pits using the sun. I need glasses nowadays to do this correctly but this very effectively shows up any pits from a previous grade of abrasive. These must be removed before I go to the next stage.
Once I have good contact and I'm free of previous pits I go to 9 micron. This is a big step and I usually do 2 hours with this repeating the process above. I also use the sun or a bright filament of a lightbuld on an angle to see if the surface if even from centre to the edge.
I've noticed for example if the centre if not in good contact as shown by the sharpie I can easily see that the image of the filament fades out in the centre compared to the rest of the surface.
Good contact,checking the reflection on an angle and looking for pits always means I know I can confidently go to the next grade. I usually finish with 3 micron and have successfully gone from 9 to 3 using this technique.
At 3 micron If I have good contact no previous pits and the view on an angle is good I can be confident polishing will go well.
In the past I've seen things like greyness in the centre or edge and its because I havent had good contact. Grinding removes glass a lot faster than polishing and sometimes its better to bite the bullet and regrind. I find that I usually have to go back to 15 micron or 25 to ensure I have ground out any greyness and then follow the process of ensuring I have a sphere described above.
I'd be interested in any thoughts about this and if others agree or not. Presently I'm doing a crown element for a refractor and on 5 micron on R2.
I have a bit of a theory about to get it right and basically when I get down to 25 micron down I fine grind for around an hour then do a test with a permanent marker.
I draw a grid pattern and then apply some 25 micron paste and move the mirror in different directions to work out of the figure is spherical.
If its not in contact in the middle the solution is to use the tool on top and test after some work and then work out if its closer to a sphere.
I then look for pits using the sun. I need glasses nowadays to do this correctly but this very effectively shows up any pits from a previous grade of abrasive. These must be removed before I go to the next stage.
Once I have good contact and I'm free of previous pits I go to 9 micron. This is a big step and I usually do 2 hours with this repeating the process above. I also use the sun or a bright filament of a lightbuld on an angle to see if the surface if even from centre to the edge.
I've noticed for example if the centre if not in good contact as shown by the sharpie I can easily see that the image of the filament fades out in the centre compared to the rest of the surface.
Good contact,checking the reflection on an angle and looking for pits always means I know I can confidently go to the next grade. I usually finish with 3 micron and have successfully gone from 9 to 3 using this technique.
At 3 micron If I have good contact no previous pits and the view on an angle is good I can be confident polishing will go well.
In the past I've seen things like greyness in the centre or edge and its because I havent had good contact. Grinding removes glass a lot faster than polishing and sometimes its better to bite the bullet and regrind. I find that I usually have to go back to 15 micron or 25 to ensure I have ground out any greyness and then follow the process of ensuring I have a sphere described above.
I'd be interested in any thoughts about this and if others agree or not. Presently I'm doing a crown element for a refractor and on 5 micron on R2.