Fine Grinding The Mirror
Making a 4.25 Inch Dobsonian Reflector Telescope
Choosing the Grits
If you purchased your grits in a kit, you don't have to choose the sizes. Ordinarily with each smaller size, you want to lower the grit size by a factor of two. There is not much literature on how you decide when to switch to a lower grit. Usually you will read about looking for pits. I did not find this particularly useful. Once you get below 120 Grit, it is hard to see what is going on without magnification. It is also important to realize that no matter what you do, you will always find pits that are about twice the size of the current grit. When you only see a few of these, then it is time to go to the next grit.
As you move through the grits, you will notice the surface will be smoother both visually and to the touch. It will never end up smooth like glass. For that you will need to wait for the polishing stage. If you are using a glass tool, you may find the blank and the tool may freeze together. If this happens, put both into a bucket of warm water. This will usually free the two.
Keeping things Clean
Of course when you switch grits, you will want to clean up your grinding area thoroughly. If you grind outside in the backyard, you can spray your mirror blank and equipment with a hose. If your clothing has attracted some grit, you will want to change it.
What to Look For?
I choose to take as scientific approach as possible when deciding when to switch grits. This was after a first attempt in which I was in a hurry, and when I got down to a very small grit size, I found I had pits that would not grind out. So after a session of grinding, I would clean off the blank and look at it under a low power microscope. After convincing myself that only a few large pits were left, I would take a photo of the largest one I could find on the surface and then go to a lower grit size. In the pictures below you will see a scale I made on a 600dpi laser printer. The smallest increments are in hundredths of an inch. Note that the lighting is not consistent in the photos. Just keep an eye on the mottled pattern. The circle you see is not the disk of the blank, but rather the view port in the microscope.
After 80 Grit
After 120 Grit
After 220 Grit
After 320 Grit
Notice that below 320 micros the size names change from grits per inch to microns (thousandths of an inch).
After 20 Microns
After 15 Microns
After 9 Microns
After 5 Microns