Very interesting; but I think I can better help you if I've got more information. For instance when your grandfather is born and where. The name Jehannes or Johannes is quite common in the Rienks family, and almost all of them are born in the province of Fryslân. But when you tell about telescopes that rings a bell. Most certainly you are referring to Sieds Jehannes Rienks, who lived between 1770 and 1845. But to me it seems quite impossible that he is your grandfather. You should be born in the 1850's than. But this S.J. Rienks had 9 children, amongst them 5 boys and one of them is also called Jehannes. I followed the leads in the book but were not yet able to find anyone that could be your grandfather. Unfortunately I haven't got much time at the moment, since I will be going on a small vacation tomorrrow. I will be back be the beginning of June; if you could give me some more info by then, I will be very glad to help you!