[Renate]

To get any speed you need to run this in constant voltage, current mode. Non-inverting input to a 2 V reference, output through a high value resistor to diode pulled directly to V+.

In this world of obsolescence you have to lay out some serious money before you get a knob with IR interrupters. Cheesy contacts are good enough. And if it breaks?

I was somewhere in your neighborhood once to see the big refractor telescope and do the usual tourist things.