Sans Serif and Roman Sigma with Source Sans Pro and Charter

Question

Consider the following MWE:

\documentclass{article}

\usepackage[utf8]{inputenc} 
\usepackage[T1]{fontenc}                

%------------------------------
% FONTS
%------------------------------

\usepackage[default]{sourcesanspro}

\usepackage[charter, cal=cmcal]{mathdesign}
\linespread{1.07}
\usepackage{XCharter}

%------------------------------
% DOCUMENT
%------------------------------

\begin{document}

$\mathsf{\Sigma}$ 

$\Sigma$

\end{document}

The $\mathsf{\Sigma}$ does not render properly:

I have many German umlauts in my document, so I would like to keep the T1 font encoding. Furthermore, keeping the Source Sans Pro would be nice, as the sans serif symbols seem to fit with Charter.

The following MWE seems to do the trick:

\documentclass{article}

\usepackage[mathletters]{ucs}
\usepackage[utf8x]{inputenc}
            
%------------------------------
% FONTS
%------------------------------

%\usepackage[default]{sourcesanspro}

\usepackage[charter, cal=cmcal]{mathdesign}
\linespread{1.07}
\usepackage{XCharter}

%------------------------------
% DOCUMENT
%------------------------------

\begin{document}

$\mathsf{Σ}$

$\Sigma$

\end{document}

However, it sacrifices the Source Sans Pro and uses utf8x input encoding (that some users suggest avoiding). Is there a good way to achieve this look (sans serif Sigma and roman Sigma)?

Answer 1

I guess you want to modify \mathsf to point to the default sans serif font (in OT1 encoding).

\documentclass{article}

%\usepackage[utf8]{inputenc}% no longer needed
\usepackage[T1]{fontenc}

% fonts
\usepackage[charter, cal=cmcal]{mathdesign}
\usepackage{XCharter}
\usepackage[default]{sourcesanspro}

% commands
\DeclareMathAlphabet{\mathsf}{OT1}{\sfdefault}{m}{n}

% settings
\linespread{1.07}

\begin{document}

$\mathsf{\Sigma}$

$\Sigma$

\end{document}