An apparatus comprising: a support structure; a first large shaft pivotably and slidably coupled to said support structure; a first flexible coupling attached to one end of said first large shaft; a first transducer holder attached to said first flexible coupling; a first transducer array held by said first transducer holder; and characterised by: a first centering mechanism attached to said first transducer holder, wherein said first centering mechanism comprises: first and second small shafts supported at opposite ends thereof by said first transducer holder; first and second pivot/slide mechanisms slidably coupled to said first and second small shafts respectively; and first and second four-link (94a, 94b, 96a, 96b) centering guide assemblies pivotably coupled to said first and second pivot/slide mechanisms respectively; wherein each of said first and second four-link (94a, 94b, 96a, 96b) centering guide assemblies comprises first and second upper centering guides (94a, 94b) pivotably coupled to said first and second pivot/slide mechanisms respectively and first and second lower centering guides (96a, 96b) respectively pivotably coupled to said first and second upper centering guides (94a, 94b) and pivotably coupled to each other. a support structure; a first large shaft pivotably and slidably coupled to said support structure; a first flexible coupling attached to one end of said first large shaft; a first transducer holder attached to said first flexible coupling; a first transducer array held by said first transducer holder; and characterised by: a first centering mechanism attached to said first transducer holder, wherein said first centering mechanism comprises: first and second small shafts supported at opposite ends thereof by said first transducer holder; first and second pivot/slide mechanisms slidably coupled to said first and second small shafts respectively; and first and second four-link (94a, 94b, 96a, 96b) centering guide assemblies pivotably coupled to said first and second pivot/slide mechanisms respectively; wherein each of said first and second four-link (94a, 94b, 96a, 96b) centering guide assemblies comprises first and second upper centering guides (94a, 94b) pivotably coupled to said first and second pivot/slide mechanisms respectively and first and second lower centering guides (96a, 96b) respectively pivotably coupled to said first and second upper centering guides (94a, 94b) and pivotably coupled to each other. first and second small shafts supported at opposite ends thereof by said first transducer holder; first and second pivot/slide mechanisms slidably coupled to said first and second small shafts respectively; and first and second four-link (94a, 94b, 96a, 96b) centering guide assemblies pivotably coupled to said first and second pivot/slide mechanisms respectively; wherein each of said first and second four-link (94a, 94b, 96a, 96b) centering guide assemblies comprises first and second upper centering guides (94a, 94b) pivotably coupled to said first and second pivot/slide mechanisms respectively and first and second lower centering guides (96a, 96b) respectively pivotably coupled to said first and second upper centering guides (94a, 94b) and pivotably coupled to each other.