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Re: [ATM] confusing results
Saquib,
> i have calculated the focal length to be no more than 300mm.
Please give us details of your calculations which resulted in 300 mm.
Or you by mistake used the word "calculated" instead of "measured"?
If the sagita is s and radius of the mirror is r than its
ROC (Radius Of Curvature) is
ROC = ( r^2 + s^2)/ 2 s
6" mirror has r = 76.2 mm
if sagita s = 1 mm than
ROC = 2904 mm.
Since focal length is half of the ROC
F = 1452 mm , as you have correctly calculated by yourself.
Big difference to 300 mm, therefore please do the
so called "sanity check". In this case take your mirror out
on the sunny day, direct it toward the Sun and look for the
Sun image made by the mirror on the peace of paper.
You will quickly find out what went wrong.
>is short focal length due to bad reflective coating?
No way.
>or due to less thickness-diameter ratio ?
>From the formula you can see that mirror thickness does not
enter calculations. But extremely thin mirror can be deformed
mechanically to give varying focal length measurements.
But not in your case, and especially not in that amount.
Accurate measurement of sagita is essential, as you can see from
Ukyo's calculations.
Best regards
Vladimir.
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