That sort of thing should work well - provided you have a suitable disc with slots in it of a size that are compatible with the device.
You can also get reflective optical detectors (I use the QRE1113 device) which will detect black or white stripes. Printing a disc on a PC might be more practical than making a slotted disc.
If the Arduino misses steps it will be due to faulty programming or the slots/stripes moving too fast past the detector. I doubt if a telescope would need anything fast.
I use my QRE1113 to detect the speed of a small DC motor and it works perfectly fine with pulses at 300 per second (18,000 RPM) and can probably work much faster.
...R