He was skilled maker of astronomical instruments and created a number of Moon-scape sketches, and did astronomical observations with two Dollond refractors of 6 and 10 ft. F.L. as well as a Gregorian telescope of 8-ft. He contributed numerous observations to the Astronomisches Jahrbuch, the Monatliche Korrespondenz and the Philosophical Transactions.
From 1772 to 1778, he compiled a catalog of 20 nebulae and clusters, which was published in 1782 in the Astronomisches Jahrbuch for 1784. In 1779 he found two other objects, M59 and M60.
Koehler died in Dresden on September 19, 1801.
Koehler is quoted with the original discoveries of M67 (before 1779), M59 and M60 (both on April 11, 1779), 1 day before Oriani independently found M59, and 4 days before Messier found both (and in addition M58) on April 15, 1779. The latter two were found on the occasion of observations of the comet of 1779. In addition, according to Bode, he found "three other somewhat nebulous stars the area of the northern shoulder of Virgo" on May 5, 1779; these cannot be identified with certainty, but Wolfgang Steinicke speculates these may be M84, M86, and M87.