The question may be trivial, but has eluded me, may be it is more appropriate for mathstack-exchange.


Let $B$, $C$ be boolean algebras and $i:B\to C$ be an homomorphism.
By Stone duality to each such $i$ corresponds a continuos map $\pi:St(C)\to St(B)$ defined by $G\mapsto i^{-1}[G]$ for any $G$ ultrafilter on $B$
(where $St(B)$ is the compact $0$-dimensional space of ultrafilters on $B$ with topology given by the base of clopen sets
$N\_c$ given by ultrafilters to which $c$ belongs, as $c$ ranges in $B$).


It is easy to check that if $\pi$ is open then $i$ is a complete homomorphism (i.e. it preserves arbitrary suprema whenever they exist in $B$).


Does the converse always hold? 


The converse holds in case $\pi$ induces an order preserving
map $\pi^\*:C\to B$ defined by $N\_{\pi^\*(c)}=\pi[N\_c]$ for all $c\in C$,
which occurs for example if $B$ is a complete boolean algebra. 
What if $B$ is not complete and $i$ is complete? In this case I'm stuck.
 Here is a sketch
of the easy implication and of the partial converse:


(1) Assume $i$ is not complete. Let $D$ be a predense subset of $B$
such that $i[D]$ is not predense in $C$. 
Fix $c\in C$ such that $c\wedge i(d)=0$ for all $d\in D$.
Then
$A=\bigcup\_{d\in D} N\_d$
is a dense open subset of $St(B)$ and $\pi[N\_c]\cap A=\emptyset$.
Since $St(B)\setminus A$ is closed nowhere dense,
this gives that $\pi[N\_c]$ is closed (since $St(B)$ is compact Hausdorff) and nowhere dense, 
hence it is not open. 


(2) It suffices to show that $\pi[N\_c]$ is clopen for all $c\in C$.
Let $A=\{b\in B: i(b)\wedge c=0\}$. By completeness of $B$, $\bigvee A=d$ exists, and by completeness of $i$, $i(d)\wedge c=0$. 
It is not hard to check that $\pi[N\_c]=N\_{\neg d}$ (since $G\not\in \pi[N\_c]$ if and only if $G\cap A$ is non-empty).



A curiosity on complete homomorphisms of boolean algebras
An example in which $i$ is a complete homomorphism but $\pi$ is not an open map:
Notation: For any Boolean algebra $A$ and any $a$ in $A$, $S(a)$ is the set of all ultrafilters $F$ such that $a$ is in $F$. If $f:A\to B$ is a homomorphism, then $f^d: \operatorname{ult}(B)\to\operatorname{ult}(A)$ is its dual, assigning to each ultrafilter $F$ on $B$ the inverse image $f^{-1}[F]$. finco($\omega$) is the Boolean algebra of finite and cofinite subsets of $\omega$.


Let $f$ be the inclusion map from finco($\omega$) into P($\omega$).Then f is a complete homomorphism; see the Handbook of Boolean Algebras, pp. 21, 59. Suppose that $f^d$ is an open map. Let $E$ be the set of even integers.Then there is an a in finco($\omega$) such that $f^d[S(E)]=S(a)$. We claim that $E$ is a subset of $a$. Otherwise let $G$ be an ultrafilter on $B$ containing $E$ and the complement of $a$. Then $G$ is a member of $S(E)$, so with $F=f^d(G)$ we have $F$ a member of $S(a)$. So a is a member of $F$, and hence also of $G$, contradiction.


This proves the claim. But $a$ is not equal to $E$, so let $b$ be a nonzero member of finco($\omega$) below both $a$ and the complement of $E$. Let $H$ be an ultrafilter on $A$ such that $b$ is in $H$. Then also a is in H and so there is a $K$ in $S(E)$ such that $f^d(K)=H$. Then $b$ is in $K$, so the complement of $E$ is in $K$, contradiction.


