How to find a suitable 'constant'

To arrive at a 'constant' which will make the measured numbers come closer to the calculated (hm^2/R) numbers, try the following:

In this example the hm^2/R values for the four zones were calculated to be as follows:

zone 1 zone 2 zone 3 zone 4
0.0025 0.0229 0.0590 0.1109

The numbers taken from the 4 zones of the mask used during testing gave the following result

zone 1 zone 2 zone 3 zone 4
0.152 0.172 0.209 0.260

Take the reading from the 3rd 'window' in the mask (zone 3 ) )which is usually the most accurate of the readings you will make. (If you have say 5 zones then take the 4th zone as being the most accurately read measurement.)

So you take the third reading number and subtract the hm^2 number calculated for the third window which let us say is 0.059 then subtract it from the reading number three(0.209) this gives you a 'constant' of 0.150 If you now subtract this same number 0.150 from all the readings you will get the following:

zone 1 zone 2 zone 3 zone 4
0.0020 0.0220 0.0590 0.1100
Note that the third reading is exactly the same as the calculated number: (0.059) whilst the other zone numbers are very close to the required value. This makes comparison of the readings with the calculated values much easier. Once the differences become less than about 10 or 12 thousandths of an inch then the accuracy of the mirror's 'figure' becomes very close to the ideal.

As long as you subtract the same 'constant' number from each of the readings - you will still maintain the difference of relative spacing between them, but they become easier to compare with the calculated numbers taken from the hm^2/R value for each window.

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