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DIY Spherometer/Mirror Calculator

beginner ATM DIY mirror making lens making
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#1 Zardiw

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Posted 19 November 2019 - 08:51 AM

I made an Online Calculator for the DIY Spherometer from Gordon Waite, and other mirror calculations for Focal Length, Sagitta, Focal Ratio, etc.

 

https://ddamanda.com/Sphero.php

 

Great Forum!

 

z

 



#2 JohnH

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Posted 19 November 2019 - 03:30 PM

Like it, though adding error bars to these numbers would help.

 

 

There is also a spreadsheet where you can plug in the numbers and get a tabulated set of results



#3 MKV

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Posted 19 November 2019 - 06:16 PM

Spherometers must be calibrated for internal or repeating ±error. Otherwise the results are a guesstimate, which is fine for Newotnians, but not for refractors or Maksutovs..

 

The source of ±error is in the measuring equipment used to determine contact separation, and in the apparatus used to measure the sagitta depth. Usually manufacturers provide the data of their product's ±error. These errors are then integrated mathematically to yield the uncertainty factor of the measurements, and thus establish the error tolerance envelope. 

 

Raw numbers can be misleading. Just because a dial indicator reads 2.4568" doesn't mean it's actually 2.4568". A dial indicator may a scale showing 0.0001" resolution but its internal error may be ±0.005".  Thus, when your scale says a sagitta = 2.4568" it's really 2.4568±0.005".

 

This also limits the number of decimal places you can trust, and that will affect the accuracy of the spherometer.

 

For a better understanding of this subject, read William S. Browne's section "Accuracy of Spherometers", in Advanced Telescope Making Techniques -- Mechanical, by Willmann-Bell, p. 109.



#4 Zardiw

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Posted 19 November 2019 - 08:18 PM

I'm using Gordon Waite's equations, and the results match.

 

But I've found other formulas for the arc radius that differ from his.

 

Still working on it...

 

It's good for getting rough numbers though......

 

z




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