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Re: [ATM] Focal Length Accuracy
Anthony Anconetani <Anthony.Anconetani@genband.com> wrote:
I now measure the approximate focal length with sun at 33.5 inches.
I've measured the approximate center of curvature with my Foucault
tester. The result was approximately 67 inches +/- 1 inch. I believe
this should have been 72 inches.
67 inches would be correct for a focal length of 33.5 inches which you measured.
72 inches would be correct for your target of 36 inches.
I've measured the sagitta with feeler gauges. The result is about .066
inches. I believe this should have measured between .062 and .063
inches.
It is very difficult to maintain this amount of precision with feeler guages as they are flat and the curve is not. You are doing well to be getting the results you are. The feeler guages will get you in the ballpark, from there you should measure with a more reliable (accurate) method.
So, from the above data I believe the focal length is a little short. It
also doesn't seem like I'm accomplishing much correction.
Yes, it is a bit short of your target. It takes a while longer to make the focal length longer because there is more area around the outside of a circle. A year or so ago someone ran the numbers and I believe he reported there was about a 3 to 1 ratio of work to reverse the focal length by a given amount. Might have something to do with Pi but I don't remember the details.
Questions:
---------------
1) Is now the time to get the focal length as close to 36 inches as
possible?
YES and NO ! It's not really critical but the larger grit sizes will adjust the curve faster. You are very close and most ATM's will finish with their mirrors a bit short of thir target due to the fact that from a sphere, Parabolizing (normal strokes) will deepen the center and shorten the focal length. If you want to finish closer to your target value, I'd recommend grinding a while with Tool on Top, shooting for around 38 inches with rough grinding (testing with the sun) and figure that the difference will be made with fine grinding bringing the mirror closer to 37.5 inches or so, Parabolizing should bring it another 1/2 inch or so shorter (depending on how hard you make it).
Your actual mileage (inchage?) may vary.
Anyway, The only one that would know whether you were exactly on target would be the one that measures the mirror, Pride would drive one to make it as close as possible but there are diminishing returns and unless you are trying to match an optic to an existing set of circumstances, it's not necessary to be that accurate. If it turns out 30 inches (or 40 inches) long and has a good figure, you'll have a good mirror.
Ken Hunter
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