Finished hogging and start going down the grit scale

(day 2)

We last left off with a sagita about half way to the desired depth. So back to grinding with cordial strokes, mirrors on top. After 3 hours of grinding and stopping when I got tired to measure the sagita along the way, I reached my target sagita of 58 thousands of an inch. All Excited I wet the mirror, grab a tape measure and run outside to measure the actual focal length by reflecting sunlight off the mirror and focusing it onto the back side of my wood fence then measuring the distance from the mirror surface to the fence. To my utter surprise I was nearly 10 inches shorter than I wanted to be! My goal was a 64-inch focal length and I was measuring only 54 inches. The good news is each mirror was almost identically wrong.

So, I went back and re-read how to calculate the my Sagita, and It would appear that I had done it correctly. Now I’m a little confused and I sit there staring at my mirrors and admiring the curve that’s in them. Out of curiosity I caressed the surface of the mirror with my hand, and that’s how I found my problem!

I had wrongly assumed that the curve generated by cordial strokes would propagate evenly across the entire surface of the mirror, instead (as evidenced by the following pics) it starts in the center growing larger in diameter as you grind. This leaves the outer edge of your mirror frosty, but basically flat.

As you can see in the pictures the outer half inch of my mirror was still flat, and although I had the depth in the center of the mirror correctly, the diameter of the curve was an inch shorter than my mirror diameter. The result being a shorter focal length because you have a steeper curve! It all makes sense now!

To fix my newly discovered problem I started grinding again, but this time put the mirrors on the bottom and the tool on top. After another three hours of this I had hit my mark! (Happy Dance!) Both mirrors now appeared to have roughly a 64-inch focal length and it was time to start spherearizing and moving down the grit scale.

To make sure I ended up with a sphere on each mirror, I switched strokes from cordial to a center over center stroke. The length of the stoke allowed about one third of the diameter of the mirror, or tool, to over hang the other on each stroke. Again I did 20 strokes per position of the barrel, eight positions around the barrel to make one lap, and two laps per wet. Between moving positions around the barrel I would rotate the mirror or tool one-quarter turn opposite the direction of travel around the barrel (I did this during the cordial strokes as well, but forgot to mention it….)

Another trick unique to grinding mirrors for my bino scope was the need to keep the focal length the same while grinding both mirrors. To do this I came up with the following grinding scheme.

1. Mirror A with tool on top, one wet, two laps around the barrel

2. Mirror A With Mirror on Top, One wet, two laps around the barrel

3. Mirror B With Mirror on Top, One wet two laps around the barrel

4. Mirror B with tool on top, one wet, two laps around the barrel

Just to make things a little more complicated I would repeat the sequence, but backwards. The complete sequence looked like this:

ATOT, AMOT, BMOT, BTOT, BTOT, BMOT, AMOT, ATOT

I did an hour of this with 80 grit, then moved onto 120 grit for about 2 hours. Checked the surface for pits from the previous grit level, finding none I then checked the focal length (just to make sure!) everything being fine, I moved onto 220 grit. I did 220 grit for another 2 hours, inspected for pits and checked the focal length, and moved onto 25 micron, did this for an hour and called it a night.

End of day 2.