RE: ATM Rough Focal Length Testing Question

[BOB PRABOWO <BOB@bahana.co.id>]

Hi Quiz,

You can use the "ruler test". This test will measure the depth (sagitta)
of the mirror and then using the depth, you can measure the focal length
of the mirror. 
Place a straight edge ruler on the mirror, and slip a drill bit which
size is known under the ruler, not just drill bit you can use other
thing like a screwdriver.
Note the size of the bit that slips inder the rule. Use this number to
measure the focal length using the following equation: 

F=r^2/4D
F= focal lenth, r= radius of the mirror and D= depth (sagitta) or in
your case the diameter of the drill bit that slips under the ruler.
If the bit size is 0.18 cm and your mirror radius is 10 cm (mirror dia.
20 cm), then the focal length is:
10^2/4 (0.18)  = 138. The focal length is 138 cm.

If you want to know the depth that has to be reached for a desired focal
length, then use this equation:

D= r^2/4F
D= Depth/sagitta, r= radius of your mirror, F= focal length

For instance, if you want the focal length to be 100 cm (F/5), then the
depth should be:
10^2/4 (100) =0.25 cm.

When you know the depth you have to reach, just keep on grinding until
you reach the depth. Be sure to measure the depth from time to time
(using the ruler test). I measured the depth after each wet.
Hope this helps,
Good luck
Bob P. Sumitro
Jakarta, Indonesia
the new land of the free

"To be able to express your opinion freely, free of fear, ain't that
great!" 

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