what's the xtal frequency? 1.6384 MHz?
for "50Hz" it's 3.27680MHz, for "60Hz" it's 3.932160Mhz.
I put the 50Hz and 60Hz in quotes because the actual oscillator frequency is double that - an LED only gives one light pulse per AC cycle, whereas a conventional or fluorescent lamp gives two (hence you need double the frequency for an LED).
Both crystal types I have found in catalogues - so they aren't difficult to get - it's just 'my' usual store doesnt keep them.
Put a mark at every correct angle. At the end, you will have a mismatch. Thats ok!
This is true, although all my strobe discs built into platter mats or cut into platter edges are integer numbers of bars, and hence are not accurate.
60Hz lighting CAN give absolutely accurate results since it requires an integer number of bars for 33.3 and 45 rpm.
As for ON pulse duration, the shorter the on pulse, the better the contrast of the strobe bars when viewed. It also allows a much higher LED current than the LED can normally handle (hence brighter light). Shortening the on-pulse is the function of the 555 Ic in my circuit. (which I have just realized can instead be done by more intelligent use of the divider's otherwise unused outputs)
Normal AC lighting provides a rather nebulous on duration, but it works well enought, so short on pulse are just a luxury really.
If one wanted to go to extremes, a xenon flash tube could be used, as is done on the strobe lights used for auto-ignition adjustment.