\magnification=\magstep1


$$
\eqalign{
\pi \quad & = \quad \arctan \left( {{b}\over{d}} \right)  \cr
\pi \quad & \simeq \quad \left( {{b}\over{d}} \right)  \cr
}
$$

$$
d \quad = \quad \left( {{b}\over{\pi}} \right)
$$

$$
\pi \quad = \quad 100 \pm 20 {\rm\ \  mas}
$$

$$
\pi \quad = \quad 10 \pm 7 {\rm\ \  mas}
$$


$$
{\rm\ fractional\ uncertainty\ in\ parallax\ } \quad = 
                 \quad { {\sigma_\pi} \over {\pi} }
$$

$$
\eqalign{
 \quad { {10{\rm\ mas}} \over {\pi} } \quad > \quad 0.10  
       \quad \rightarrow \quad \pi &> 100 {\rm \ mas}  \cr
       \rightarrow \quad d &< 10 {\rm \ pc} \cr
}
$$

$$
\pi \quad = \quad 2.39 \pm 0.03 {\rm\ \  mas}
$$


$$
{\rm size\ of\ PSF\ \ } \theta \quad \sim \quad {{\lambda}\over{D}}
$$


$$
d \quad = \quad {{1}\over{\pi}}
$$


\bye