# Accuracy of a telescope, measurement without systematic error [closed]

I got these values from the measurement:

20.1, 20.2, 19.9, 20, 20.5, 20.5, 20, 19.8, 19.9, 20

I know that the actual distance is 20 km and the error of the measurement is not affected by the systematic error. What is the accuracy of a telescope? [Answer is: σ^2 = 0.061] How to calculate it?

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You can find the variance by the following: $\text{Var}(X)=\text{E}(X^{2})-\text{E}^{2}(X)$. Hopefully this helps you. (Also note that $\sigma^{2}=\text{Var}$) –  Shaktal Jun 3 '12 at 18:34
Should I use the actual distance: 20 ... or the one, which I calculated as a mean of all the measured values, which is 20.1? –  user1111261 Jun 3 '12 at 18:37
Crossposted to stats: stats.stackexchange.com/questions/29743/… –  Qiaochu Yuan Jun 3 '12 at 19:51