In 1875, Engelhardt settled in Dresden and built himself an observatory with his own money, where he worked alone without assistants until 1897. His main work was researching comets, asteroids, nebulae, and star clusters. In 1870 he completed the observation of 50 comets and 70 asteroids. Turning to star clusters, in 1883 he completed a catalog of more than 400 nebulae. Beginning in 1886, he observed 829 stars in Bradley's directory to determine if they had companion stars. In the late 1890s, deteriorating health forced Engelhardt to give up personal observing, and he gave all his equipment to Kazan University, of which his friend and fellow astronomer Dmitri Dubyago was rector. The university built an observatory to house the equipment, which was opened in 1901 and named for Engelhardt from 1903 until 1931. Until the end of his life, Engelhardt took an active part in the construction and organization of the new observatory, and in his will gave Kazan University all his money and property, to be used for the development and maintenance of the observatory. Engelhardt was awarded an honorary doctorate by Kazan University in 1889, and in 1890 he was made a corresponding member of the Russian Academy of Sciences.
For most of his life, Engelhardt took an active interest in Russian history, gathering materials and donating them to Russian collections. Engelhardt was a close friend of the composer Mikhail Glinka. When Glinka died in Berlin in 1857, Engelhardt arranged the transfer of his remains home to Tikhvin. Engelhardt also sent Glinka's collection of manuscripts to St. Petersburg, where the Glinka Public Library was founded, and published the scores of Glinka's operas and symphonic works. Later, at the request of the critic Vladimir Stasov, Engelhardt published his memoirs of Glinka and the composer Alexander Dargomyzhsky. In the late 1890s, moving away from astronomical research, Engelhardt began a collection of material on Suvorov's campaign in Switzerland, which he donated to the Suvorov Museum in St. Petersburg in commemoration of the 100th anniversary of the 1812 war. Engelhardt's efforts made a significant contribution to perpetuating the memory of the Russian campaign in Switzerland. Engelhardt's correspondence with leading cultural figures is considered of historical value.
