Quote:
Originally Posted by Lognic04
About 4mm now. (did you pass on a 10 inch f6 tool? if so, i am using it now!  )
I am at 2.1mm sag right now btw. Is there a way to check the FL other than physically and without the sun? |
Yes that sounds like it could be mine. I used it 20 years ago to make a 10 inch F6. I would need to see a photo to be sure.
You can test the focal length approximately with a torch. Place the mirror on a stand. Hold a torch parallel with your eyes. Wet the mirror. Locate the torch reflection in the mirror and bob your head up and down. When the torch reflection moves in the same direction in the mirror you are inside the radius of curvature. When the reflection moves in the opposite direction, you are outside the radius of curvature. The point at which the torch reflection does not move is the radius of curvature. Halve that and you get the focal length.
Some people do a couple of wets with 120 grit and then 240. This helps give a clearer reflection and the water seems to stay on the surface longer.
Hope that helps.
Rod