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Re: [ATM] Figuring question
Jim Burrows wrote:
> At 2006-05-31 10:42 -0400, lance clarke wrote:
>
>> Here are the numbers:
>>
>> zone radius reading
>> 1 1.032 0.006
>> 2 2.225 0.029
>> 3 2.844 0.043
>> 4 3.574 0.071
>> 5 3.990 0.079
>> 6 4.505 0.095
>> 7 4.863 0.106
>> 8 5.271 0.123
>> 9 5.600 0.132
>> 10 5.939 0.144
It's a small point, but when I ran the numbers in Sixtests, in order to
match Jim Burrows' results, I had to leave the numbers exactly as
presented and put in the ROC as 124 inches.
1. How did the zone 1 reading get to be 0.006? What is it referenced
to? Shouldn't that 0.006 be subtracted off all the readings?
2. It's not hard, if you can dragoon someone in your household to hold
one end of the measuring tape, to measure the ROC to 1/8 or even 1/16
inch precision. Wrap a piece of masking tape around the end of the
measuring tape so it won't scratch the mirror, and have your assistant
hold the end of the tape just touching the center of the mirror.
(First, do the the Foucault test and leave the knife edge set at the
position for the innermost zone.) Then read the distance to midway
between your knife edge and light source. (If using the common moving
source setup, the knife edge and source will be at the same distance.)
You and your assistant may need to gently support the tape with your
free hands in order to minimize the effect of sagging tape. One of
those big fat steel tapes that tend to support themselves is a help here.
The Foucault analysis results are not strongly affected by small ROC
errors for typical atm mirrors, but it is easy to get better than one
inch precision, so why not do so and not worry about the effect?
(Bigger and faster mirrors are probably less tolerant of ROC errors in
the analysis.)
--
Mark Holm
mdholm@telerama.com
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