Let $\mathbb{T}$ be a geometric theory. Let $\mathrm{Set}[\mathbb{T}]$ be its classifying topos, such that geometric morphisms from any (cocomplete) topos $\mathcal{E}$ into $\mathrm{Set}[\mathbb{T}]$ are in canonical one-to-one correspondence with models of $\mathbb{T}$ in $\mathcal{E}$.

As with any topos, associated to $\mathrm{Set}[\mathbb{T}]$ is its *localic reflection* $L(\mathrm{Set}[\mathbb{T}])$, the locale which has the partially ordered set $\mathrm{Sub}_{\mathrm{Set}[\mathbb{T}]}(1)$ as frame of opens. Like any locale, this locale is the classifying locale of some propositional geometric theory $\mathbb{T}'$.

**Question:** Can the theory $\mathbb{T}'$ be described in terms of $\mathbb{T}$?

For instance, let $\mathbb{T}$ the theory of an object, containing a single sort and no axioms. Then $\mathrm{Set}[\mathbb{T}]$ is the *object classifier*, the copresheaf topos $[\mathrm{FinSet}, \mathrm{Set}]$. Its localic reflection is the one-point locale, so $\mathbb{T}'$ is the empty theory.

My motivation is that I want to find out whether a certain geometric $\mathcal{F} \to \mathcal{E}$ morphism is open. For this, I have to check whether its localic reflection is open (as a locale internal to $\mathcal{E}$); if I knew which theory the localic reflection classifies, I'd have a handle on this.

An obvious place to look for an answer would be Olivia Caramello's beautiful paper on lattices of theories. However, there she discusses only subtoposes, not the localic reflection.