Messier then switched to an "achromatic telescope
of V-h feet [focal length] with a triple objective, made in London by Dollond," to time the Io eclipse (it began at 5:53 a.m.).
In 1824 the world's largest achromatic telescope
of that time was installed and Friedrich von Struve started further observations from Tartu.
five' telescope, a 2%-inch achromatic telescope
with stand; one terrestrial eyepiece and three astronomical eyepieces: 'Annual report of the Council', MNRAS 37(4), (1877 February).