Two days ago 
I play the compositions of Buxtehude as I come along them in manuscripts. And this one I had not yet encountered before so I was unfamiliar with it. As it struck me as a beautiful composition and I wanted to try my hands on it, I looked for a suitable manuscript source.
The only source available for this composition is the Frankenberger manuscript, in the posession of "Het Nederlands Muziekinstituut", the national library of The Netherlands. The manuscript is named after one of the former posessors and not after the scribe. The scribe of the manuscript was Johann Gottfried Walther. He wrote several of these compilations and the are generally of very high quality. Not only the music contained in them, but also the transcription work of Walther. These sources are for the largest part considered to be very reliable.
Walther did add little ornaments like trills and pralls as he saw fit. So even though perhaps not all ornamentations in this composition trace back to a now lost original by Buxtehude, it does not make the version in the Frankenberger manuscript less authentic.
The prelude is in the typical Buxtehude style. The choral melody is played by the right hand, ornamented with free figurations. The left hand andf feet plays an accompaniment build from elements and motives of the choral melody.
The recording was done with the Hauptwerk software and the sampleset, made by Sonus Paradisi, of the Schittger organ in the St. Martini-kerk, Groningen (http://www.sonusparadisi.cz/en/organs/netherlands/groningen-st-martini.html).
Score available here: http://partitura.org/index.php/dietrich-buxtehude-ach-herr-mich-armen-sunder-buxwv-178
