Define new unit in siunitx with angles and spaces

Question

I would like to define a unit to render

\SI{5}{\degTTh}

as

So far, I've been able to define a new unit

\DeclareSIUnit{\degTTh}{%
 \text{$^\circ\ 2\theta$}%
}

but there is a space between the numeral and the degree sign. Using the option \SI[space-before-unit=false]{5}{\degTTh} doesn't resolve the issue.

I've also tried declaring

\DeclareSIUnit{\TTh}{%
 \text{$\ 2\theta$}%
}

and then using \SI[space-before-unit=false]{5}{\degree\TTh}, but this also results in a space between the numeral and degree symbol.

.

How can I define such a unit?

Answer 1

When creating a new unit with siunitx, you can override any key-value pair option by supplying it in the <options> field in

\DeclareSIUnit[<options>]{<unit>}{<definition>}

The symbol between the number and the unit is set with number-unit-product. In your case, you want to have no symbol between number and unit, i.e. {}. The definition of your unit then becomes

\DeclareSIUnit[number-unit-product={}]{\degTTh}{\SIUnitSymbolDegree\ensuremath{~2\theta}}

Additionally, I replaced the ^\circ with the symbol \SIUnitSymbolDegree, which is defined by siunitx. Finally, I replaced the $ ... $ with \ensuremath, as suggested in the package documentation.

\documentclass{article}
\usepackage{siunitx}

\setlength{\parindent}{0pt}    % only to make all examples aligned.

\DeclareSIUnit{\degTThold}{\text{$^\circ\ 2\theta$}}
\DeclareSIUnit[number-unit-product={}]{\degTTh}{\SIUnitSymbolDegree\ensuremath{~2\theta}}

\begin{document}

\SI{5}{\degTThold} \\
\SI{5}{\degTTh} \\
$x = \SI{5}{\degTThold}$ \\
$x = \SI{5}{\degTTh}$

\end{document}